September 2009 Archives

In marked contrast to the relatively quiescent weather regime that dominated weather across much of North America during September, the October atmosphere has become strongly energized. It's part of the annual cycle of the seasons during which the atmosphere makes a turbulent transition from summer to winter. And now, computer models indicate that we can expect a parade of vigorous storm systems in the next few weeks. The first system brings rain and plenty of it to Chicago and the Midwest from Thursday into Saturday. That's good news for Wisconsin, which has been struggling through several months of far subnormal precipitation that has built into a drought situation across much of the state. This system's rainfall is likely to double Chicago's full September total.
 
And here we go again

A second vigorous storm system heads into the Midwest quickly on the heels of the first. And like the first rainy spell, it too will be attended by abundant moisture and copious rainfall. Preliminary estimates suggest this storm has the potential to deliver one to four inches of rain to the Midwest -- on top of one to four inches from the previous system.
 

Dear Tom,
What exactly does it mean when the forecast says there is a 50 percent chance it will rain for a given area? Does it mean there is a 50 chance it will rain in that entire area, or that 50 percent of that area will definitely get rain
James Shaw, Northrop Grumman Corp.

Dear James,

Neither of your statements is precisely correct. In weather forecasts, the probability of rain is a so-called "point probability," the chance of measurable precipitation (at least 0.01 inch) at a specific location, such as at your residence. Regarding a 50-percent forecast, the forecaster may believe the entire area will experience rain if it rains, but his confidence that rain will actually arrive is only 50 percent. Or he may believe scattered showers will definitely occur, but affect only 50 percent of the area. Regardless of the weather situation, the meaning for you is always the same: a 50 percent chance of rain on your head

 

Dear Tom,
I was curious how many days in the 139 years of Chicago weather records have had the same high and low temperatures on a given day.
Cole Turgeon, Brookfield

Dear Cole,
A computer sweep of Chicago's entire official temperature data base, from Nov.1, 1870, through Sept. 30, 2009, reveals only three days on which temperatures remained absolutely steady through the entire 24-hour day: March 13, 1878, when the city's official thermometer registered a constant 44 degrees; March 24, 1891, with 32 degrees; and Feb. 6, 1942, with 35 degrees.

On an additional 24 days, the range between the city's maximum and minimum temperatures was only one degree. The most recent occurrence of that event was about six and one-half years ago -- April 8, 2003, when the high and low temperatures were 32 and 31 degrees, respectively.

WGN's Duffy Atkins visited Queen of the Rosary in Elk Grove Village, spending some time with the school's 6th through 8th graders. Duffy had some company, too, bringing along her pet dogs Stormy Weather and Blue Sky!

Following a lengthy period of sub-normal rainfall that resulted in Chicago's fifth driest September on record (Midway data, 1928-2008), October is set to arrive dripping wet. Two powerful storm systems, each attended by abundant moisture, are targeting Chicago. The first occurs Thursday into Friday; the second, Monday into Tuesday. And in the immediate future: lake-effect sprinkles today, mainly across northwest Indiana, but possibly on Chicago's shoreline.
 
Sub-normal temperatures to continue

Chicago's normal high temperature now is 69 degrees, but, as weather-wise Chicagoans know, the city's usual weather consists of big swings above and below the normal values. We'll be experiencing a below-normal swing this week with daytime readings 5 to 12 degrees below normal. Chilly temperatures become increasingly likely as the days slide toward winter and, on average, the chance of a day whose high temperature remains below 60 degrees is 19 percent during early October, 70 percent in early November, 96 percent in early December, 99 percent in early January.
 
 

Dear Tom,
What are the effects of El Nino on Chicago's winters?
Mario Ortiz

Dear Mario,

El Nino, a widespread warming of the surface waters of the equatorial Pacific Ocean, occurs every few years. An El Nino that began in July is likely to persist well into 2010. When it is in progress, El Nino causes changes in atmospheric wind patterns that have worldwide weather
ramifications.

Past El Ninos have had little effect on Chicago summers, but they have dramatically altered our winters. Chicago's "El Nino winters" tend to be milder, drier and "quieter" than normal: fewer occurrences of bitterly cold arctic air and fewer big storms. Snowfall in 17 El Nino winters since 1950 has averaged a few inches below our seasonal normal of 40 inches, but snow totals in any given winter have ranged from well below to far above normal.

Jackie Evans, a student at Valparaiso University sent along these photos of high wave hitting the beach in Porter County, Indiana Monday. Peak wind gusts there reached 52 m.p.h. Thanks for sharing these photos with us Jackie.

 Bill Snyder, WGN Weather Producer

 

 

 Photos by Jackie Evans

Canadian air that has settled across Chicago in the wake of Sunday's severe thunderstorms and Monday's blustery winds is likely to make its presence felt through the week. The air mass isn't horribly cold as things sometimes go at this time of the year, but we will have daytime temperatures running 6 to 12 degrees below normal through the weekend. Tuesday's expected high temperature, 58 degrees, will be 9 degrees below the climatological normal maximum for the day, and it will be the city's coolest day since 55 degrees was logged on April 29. It's nowhere near a record: The city's high temperature on this date in 1899 was a wintry 44 degrees.

At O'Hare International Airport, Monday's highest measured wind gust was 40 mph, the strongest wind at that location since a 44-mph gust on July 28.

Rains in the offing

Computer models are in agreement that the atmosphere is priming itself to deliver a multiday rain event. Models indicate as much as 2 inches of rain at Chicago from late Thursday through Saturday.

Dear Tom,
Kendall, my six-year-old son, asks if there is any place on Earth where blizzards, tornadoes and hurricanes never occur. He wants to live there!
Lisa Gerhold-Dirks and Kendall Dirks, Elmhurst

Dear Lisa and Kendall,

Such places do exist, but "never" is a powerful word. The reality of the world's weather is such that locations blessed with a benign climate will on very rare occasions nonetheless experience one or more of the three phenomena that you wish to avoid.

Blizzards and occasionally harsh winter weather threaten most places outside tropical and subtropical areas, hurricanes threaten the subtropics (except for locations far inland) and tornadoes and severe thunderstorms constitute a worldwide menace outside polar regions. Taking it all into consideration, only the world's harshest desert regions, such as Death Valley or the Sahara Desert, meet your criteria, but other aspects of the climate in those places are certainly undesirable.

Thanks to Kyle VanderMeer for sharing this shot he took last Saturday over Huntley, Illinois. Kyle and his friend Arnie we flying in a retired US Air Force T34 Beechcraft.  Kyle reported that it was a bit hazy with the ceiling around 2600 feet. The shot was taken just below the clouds at about 2500 feet.

Gusty west and northwest winds will make it really feel like fall for the next few days.  According to our WeatherBug network, several locations have reported gusts close to 50 mph since midnight.

 

Here are some of the stronger gusts through 6 AM today:

Gust          Location

49 mph      Chicago Heights

48 mph      Naperville

48 mph      Burlington

48 mph      Geneva

A wind advisory is in effect for most of us from 10 AM until 7 PM today.  A lakeshore flood warning is in effect for Lake and Porter county Indiana from 10 AM until 1 PM today.  Strong winds could produce waves 12 to 15 feet high.  The National Weather Service warns, "DOCKS...PIERS...AND BREAK WALLS WILL BE DANGEROUS PLACES TO BE
DURING THE HIGH WAVES AND SHOULD BE AVOIDED."

The calendar says September, but, yes, severe weather is still possible in the region. Case in point: Severe thunderstorms rolled across portions of northern Illinois and southern Wisconsin Sunday, heralding the arrival of autumn's first genuinely chilly blast of Canadian air. Preliminary reports of severe weather Sunday evening included hail in Madison, Wis., that stripped so many leaves from trees that sewers were plugged and city streets briefly flooded; high thunderstorm winds that tore branches from trees in Aurora, and downed trees that blocked several roads.
Cold air and blustery winds across the area today and Tuesday will depart by midweek and we'll enjoy a sunny and pleasant interlude before another storm system arrives Friday. Thursday's temperatures are forecast to climb into the 70s, but Thursday's warmth will be short-lived. Gusty winds and rain return Friday, and once here this next storm system will be slow to move out. Computer models suggest Friday's dreary weather is likely to drag through the weekend and Chicagoans will face the prospect of a weekend washout. 

Dear Tom,
What happens when cold and warm fronts collide?
John Padna, Chicago
Dear John,
"Collision" implies a violence inappropriate to the situation. The collision of solid, fast-moving objects like automobiles is violent, but in the atmosphere we're dealing with air, a medium so fluid that it readily gives way. Air density always determines the outcome when fronts collide. In the cold frontal situation, advancing cool air, heavier and more dense than warm air, pushes under and lifts warm air; advancing warm air (the warm frontal situation), lighter and less dense, slides over cooler air. A typical frontal collision situation involves a cold front overtaking a warm front. When this occurs, the cold front always "wins" in the sense that the cool air slides under and bodily lifts the warm frontal zone. The surface warm front disappears (it's no longer a ground-level feature) and the advancing cold front is now called an occluded front.

Today's burst of 80-degree warmth brings Chicago its highest temperatures in nearly two weeks, but it won't last. A strong cold front -- the leading edge of much cooler air -- arrives late Sunday night, preceded by showers and thunderstorms that have the potential to produce strong and locally damaging winds. Temperatures crash after frontal passage and readings plunge into the lower 50s by daybreak Monday. The chilliest air of the newly-arrived autumn season dominates the area on Monday and Tuesday, and afternoon temperatures on both days will struggle to reach 60 degrees. Autumn's first chilly outbreak is always an eye-opening reality check because we're still acclimated to the warmth of summer, but it'll be even worse this time: The chilly air arrives on gale-force winds that will gust above 40 mph on Monday. Mariners take note: Those winds are likely to generate towering 12-foot waves on Lake Michigan.
 
A bit of noise in the tropics

A poorly organized tropical depression has developed about 600 miles west of the Cape Verde Islands. It's no threat to land, but the storm's 35-mph winds may briefly strengthen to produce Tropical Storm Grace before the weakening and dissipating as it encounters cooler water and unfavorable upper winds.

 

Dear Tom,
In 1960 when I was in the Navy, our ship was in Chesapeake Bay and was caught in Hurricane Donna. I think it was a Category 5 storm? Can you provide details?
--Bob Phillip

Dear Bob,
Hurricane Donna was a blockbuster storm that wreaked havoc on the entire East Coast and remains the only hurricane to produce hurricane force winds (74 mph or higher) from Florida to New England. At its peak intensity, Donna was indeed a Category 5 storm with top winds of 160 mph, but that was early on as it approached the Virgin Islands. When you encountered the storm on Sept. 12, 1960, it was a Category 2 storm with peak winds around 100 mph. However, much higher gusts were recorded offshore, at 138 mph on the Chesapeake Lightship located 17 miles off of Virginia Beach.

Chicago's multiday run of hazy days, light winds and nighttime fog is coming to an end and a new pattern of active, changeable weather is beginning. It's almost as if the city's weather is taking a cue from the calendar, because the pattern change occurs with the transition from summer to autumn. Warmer air sweeps into the area this weekend, borne by gusty southwest winds that may send afternoon temperatures to 80 degrees on Sunday. But a surge of Canadian air follows quickly and Monday's temperatures will struggle to reach 60 degrees. Powerful northwest winds gusting near 40 mph will add a real punch to the chill.

Thar she blows!
A whale of a storm is set to stir up Lake Michigan later this weekend. An intensifying low pressure system expected to pass just north of Lake Superior on Sunday will generate gale force winds (39 to 54 mph) across Lake Michigan. The Chicago National Weather Service has issued a gale watch for the entire lake from Sunday afternoon through Tuesday afternoon.

Dear Tom,
Has it snowed in all 50 states?
Maggie Drummond
Dear Maggie,
All 50 states, including Hawaii and Florida, have recorded snow. Most states get snowfall every winter as part of their normal cold-season weather, especially in the higher elevations of the warmer states. In Florida, some snow occurs at least every couple of years across the far north, but even southern portions of mainland Florida have experienced flurries on rare occasions, though snow has never been reported in the Keys. During an exceptionally strong arctic outbreak in January 1977, snow fell at Homestead, Fla., south of Miami. In Hawaii snowfall is usually confined to the highest elevations of Mauna Loa and Mauna Kea on the Big Island and Haleakala on Maui, but it has fallen on some of the other islands as low as the 4,000 foot level during cold snaps.

Here are two photos taken in northern Wisconsin on Wednesday by John H. Kloch, who notes that the tree colors there are starting to change a little. Thanks John for the great photos!

Autumn weather around here tends toward two extremes: relatively long periods of stagnant and unchanging weather on the one hand and turbulent, abruptly changing weather on the other. Chicagoans wondering if hazy days and foggy nights have become a new reality in the city's climate -- it's been that way for upwards of three weeks -- will have their answer by this weekend. Higher temperature punctuated by rainy spells sweep into the area Friday afternoon through Saturday into Sunday. A blustery 20-degree temperature drop follows on Monday, then windy and sharply higher readings arrive by next Wednesday.
 
 
Dust storm smothers Australia

It's received little attention here, but Australia continues to suffer through a multi-year, desiccating drought. And now, the worst dust storm in seven decades is compounding the nation's misery. High winds buffeted much of central and southeast Australia, especially New South Wales, on Tuesday and Wednesday, raising gigantic clouds of dust that played havoc with transportation and most aspects of normal life. The storm prompted the Sydney Morning Herald to comment that it was "the day the country blew into town." Winds subsided and skies cleared Thursday, leaving Australians to ponder to widespread wind erosion damage to the rural landscape.

Dear Tom,
In a January article you wrote that on perihelion day the Earth is 93.1 million miles from the sun and "the Earth actually passes closer to the sun during winter and swings further from it during summer." Is this correct? Text material at my son's school says the opposite: "The summer sun is closer and the winter sun is further."
Tilly Dushman

Dear Tilly,
The school text material that you have quoted is incorrect. Triton College astronomer Dan Joyce tells us perihelion, the point of the Earth's closest passage to the sun, occurs on or about Jan. 3 and aphelion, the Earth's farthest passage, occurs on or about July 4. Those are winter and summer dates in the Northern Hemisphere.

The average Earth-sun distance, 93.0 million miles, diminishes to 91.3 million miles at perihelion and increases to 94.4 million miles at aphelion.

The colorful autumn season is getting underway across the Chicago area as evidenced by these shots taken in Centennial Park in Orland Park by Lynne Hanes.

Chicago weather historian Frank Wachowski informs us that the city logged 84 percent of possible sunshine during the first 19 days of September -- among the sunniest periods ever to occur at this time of the year -- but an abrupt pattern change has delivered cloudy, hazy and foggy weather since then. The city has experienced only 28 percent of possible sun since Sept. 20. Chicago's first cool surge of the autumn season is set to arrive in the city on Monday (Sept. 28), and it promises to bring the chilliest readings in the 16 weeks since back-to-back high temperatures of 60 and 61 degrees were recorded here on June 2-3. Computer models indicate Chicago's next rain might occur in two stages: late Friday into Saturday and another Sunday night preceding the arrival of Monday's chill. 
 
Desert conditions in coastal California

Easterly winds blowing into the Los Angeles Basin, warming and drying as the air descends from the interior highlands of southern California, have generated desert-like conditions across metropolitan Los Angeles. At 3 p.m. on Wednesday, the temperature at Long Beach hit 101 degrees with a relative humidity of 8 percent, and in downtown Los Angeles it was 100 degrees with a humidity of 10 percent.

Dear Tom,
Vesuvius is considered to be one of the world's most dangerous volcanoes. Are that mountain's eruptions in any way weather-related, and should I be concerned for the safety of friends who live in Naples?
John Liska

Dear John
Indeed, the storied and sometimes catastrophic eruptions of Mount Vesuvius rank it among the world's most dangerous volcanoes, but its eruptions (and the eruptions of volcanoes worldwide) are determined by forces deep within the Earth and are in no way weather-related.
 
Italian authorities are gravely concerned that, in the event of a sudden eruption of Vesuvius, it might be impossible to evacuate the three million people who would be threatened. Vesuvius is quiet now, but Giuseppe Mastrolorenzo of the Vesuvius Observatory in Naples says, "It could remain in this condition for centuries, or even erupt next year."
 

Actually, it isn't always sunny in Seattle but it will be on Sunday.  It might not always be sunny in Seattle but it doesn't always rain either.  Seattle gets a bad rap.  New York City has more annual rainfall that Seattle.  Chicago only gets two more inches of rain annually than Seattle. 

None of that matters for the Bear's game at Qwest Field on Sunday.  It should be sunny and close to 70.  Light breezes may blow from the north end zone to the south end zone so overall, weather should not be a factor for the game.

Noise could be a factor though.  While Qwest is an open stadium, the roofs along each sideline hang over the upper deck and cover about 70% of the 67,000 seats.  That tends to trap the noise inside the structure.  This can cause false start penalties for the visiting teams.

Weather is just one thing to consider when comparing NFL stadiums.  There is tradition to consider along with fan support, parking, and food.  Soldier field got three "wows" ouf of five in a recent ESPN blog that ranked stadiums and Qwest stadium got 4 "wows".

Go Bears!!!!

 

Chicagoans experienced the city's most humid air of the past six weeks Tuesday. The moisture fueled morning fog formation---a process which was predicted to occur again Tuesday night into Wednesday morning. By afternoon, dew points ---a measure of the air's water content---nearly matched those found along the Gulf Coast. Weather balloon data determined 1.80 inches of moisture was evaporated in Chicago's atmosphere---a reservoir of humid air which led to shower development in parts of the area. While the rains bypassed Chicago proper, areas northwest and south weren't as lucky. A late morning shower-cluster doused Rockford with 1.19 inches before lunchtime---then lifted into southern Wisconsin where even heavier rains occurred. The downpours which deluged Madison and nearby Middleton were triple Rockford's---reaching 3.63 inches and 3.10 respectively. That cloudburst represented the state capital's heaviest single-day September rain tally in 139 years of weather records. 
  
Talk about contrasts: 100+ in Oregon; local 24 inches mountain snows in Colorado
 
U.S. weather farther west was truly bizarre in its variation. While sections of West Coast states broiled in record and near record triple digit heat---reaching 105-degrees at Ontario and Paso Robles in California and 101-degrees at Medford, Oregon----snow was the big weather story in Colorado's mountains. Winter storm warnings were hoisted west of Denver where snowfall above the 7,000 ft. level was heavy. By late Tuesday, 14.5 inches had accumulated at the 9,000 ft. level a few miles northwest of Conifer, Colorado--a community southwest of Denver.  At least one computer snowfall projections suggested the highest elevations of Colorado's San Juan mountains might see at least some totals exceeding 24 inches.

Dear Tom,
Has Chicago or any other place ever established a record high and a record
low on the same day?
Douglas H. Hanbury East Peoria Ill.

Dear Douglas,
Setting a record low and a record high on the same day is an extremely rare event; one that has never occurred in Chicago. Accomplishing this feat normally means a large range between the day's low and high, a condition that almost always requires a dry atmosphere. Other contributing factors would be unlimited sunshine to send daytime temperatures soaring and clear
skies and light winds at night to allow the mercury to plummet. Such conditions were present in August 2002 when a rash of same-day record highs and lows were set in western United States. Park City, Utah broke or tied its record high/low three times in one week (Aug. 11,13,15) and Alamosa, Colo. did it on consecutive days (Aug. 25-26).

It becomes official at 4:18 p.m. this afternoon---Autumn 2009 gets underway. That's the moment the sun's most direct rays, which have been shifting south the past three months, fall on the equator. The southward shift continues another three months, an astronomical evolution which will bring summer to the southern hemisphere as the northern hemisphere sinks into the short days and weak sunlight of winter.
 
Monday's persistent cloud cover, which didn't break until late afternoon, limited Chicago's high to 67-degrees--the city's coolest in 23 days. But the overall atmospheric pattern for the area is a warm one with high humidities and plenty of haze likely to thicken into fog in cooler late night and morning hours. A northward expansion of the warm humid air mass in which temperatures surged to 84-degrees in downstate Cairo, Lawrenceville and Carbondale, is likely to boost Chicago area highs. Mixed sunshine Tuesday, expected as morning fog lifts, begins a warming process likely to generate a build-up of showers and thunderstorms over sections of the metro area this afternoon and evening.
 
 
Mega-rains hit metro Atlanta area hit by 7 to 14 inches; Tulsa, Oklahoma measured 7+inches
 
  The Chicago area downpours which snapped a 22 day dry streak and drenched sections of the region with an inch or more of rain late Sunday and Sunday night, resulted from a huge pattern change which has seen powerful jet stream winds dive from Canada into the Lower 48 with cool air in tow out West. It's a development which has activated the once sanguine U.S. September weather pattern, producing some extraordinary downpours over sections of the country Monday. Sections of Atlanta metro area were swamped by more than 6 inches of rain (6.63 at one observation site in the city proper) while 14.02 fell at nearby Stone Mountain to the northeast of the city---this in an area in which reservoirs were once so low, there was concern about the region's water supply. And north Georgia wasn't alone with big rains. A squall line from Texas into Oklahoma, northwest Arkansas and Missouri unleashed a 7.18-inch deluge on Tulsa, Oklahoma producing flooding.
 
Meantime, Colorado's mountains were whitened by snow. At Bailey, Colo. 5.5 inches of snow had fallen by nightfall while 5 inches fell at Evergreen.  Computer model projections put possible mountaintop snowfalls in sections and New Mexico in coming days as high as 10 to 20 inches.

Dear Tom,
Many East Coast hurricanes seem to develop off of the west coast of Africa. Where do the hurricanes that hit Baja California form?
Ann Marie Perkins Naperville
 
Dear Ann Marie,
The hurricanes that roam the eastern Pacific tend to develop in the warm tropical waters off the west coast of southern Mexico, with some of them forming even further south off Guatemala. In general, the storms tend to track east and northeast and eventually dissipate as they encounter the colder waters in the central Pacific. However, when steered by favorable winds, these tropical cyclones can move north affecting not only Baja California, but also areas along the west coast of northern Mexico. On a few occasions, a storm has moved north through the Gulf of California bringing heavy rainfall to portions of the southwestern U.S.

Our thanks to the Aspen Skiing Company for these shots to Monday's snow in the Colorado Rockies---specifically in the Aspen/Snowmass area. In her e-mail to us, Melissa Rhines tells us it's "been snowing and 'snaining' (snowy rain) all day" with more in the forecast. Our in-house RPM (Rapid Precision Mesoscale) model confirms this and suggests accumulating snow is not only likely to whiten mountaintops across Colorado by in sections of Wyoming---but some spots in Utah as well!  THANKS for the photos Melissa!
 
Tom Skilling
 

 

It's been a long time coming, but rain moved into northeast Illinois, southern Wisconsin and northwest Indiana Sunday afternoon, ending a dry period that started in late August. Rainfall totals by Sunday evening ranged from a quarter to a half inch over the metro area. Earlier on Sunday, heavy rains downstate caused flooding of creeks, streams and susceptible roadways. There were numerous reports of one and two inch rainfall, and several locations in central Illinois received well over three inches. Jacksonville, just west of Springfield, recorded 4.76 inches. More flooding and 3-inch-plus rains occurred along the Ohio River in northern Kentucky and southern Indiana. All this rain was actually the northern edge of the record rainfalls that have been occurring across the Gulf Coast states as well as Missouri, Arkansas and Tennessee.

Cloudiness and showers to dominate forecast into next weekend
A strong low pressure system to the lee of Rockies over the Central Plains is forecast to remain essentially stationary much of the week ahead, finally ejecting to the northeast next weekend. This will mean an almost continuous flow of warm, moist air north up the Mississippi Valley, occasionally reaching northern Illinois and resulting in extensive cloudiness and periods of showers/thunderstorms, rather high humidity, and mild temperatures for the Chicago area.

Dear Tom,
I enjoy how you indicate the cities with the highest and lowest temperatures of the day. Has Chicago ever been host to either extreme?
Laurence K. Marks
Dear Laurence,
The National Weather Service releases the daily list of the nation's highest and lowest temperatures, but Chicago has never appeared there. Many would say that's fortunate because, more often than not, those temperatures are, at the very least, uncomfortably hot or cold.
On the very rare occasions when Chicago's temperatures are either so hot or so cold that they might qualify for the national extremes on any given day, some other Midwestern city invariably registers a more extreme value.
One relevant fact: The daily tabulation of the nation's highest and lowest temperatures is limited to cities and locations in the 48 contiguous states; Alaskan and Hawaiian cities are excluded.

It's been two weeks since September's only rainfall was recorded at O'Hare International Airport, Chicago's official observing site. It won't take much rain to exceed our entire monthly total of 0.03 inch. The high pressure regime that has produced an almost steady east wind along with the dry conditions and mild temperatures is finally breaking down. A low pressure system is set to establish itself over the Rockies and influence weather over the Midwest this week. A southerly flow up the Mississippi River Valley should feed warm, moisture-laden air into Illinois, resulting in considerable cloudiness. Even though showers or thunderstorms will probably not occur every day, it will be difficult for meteorologists to leave rain out of the daily forecasts.

Eastern U.S: A land of contrasts
The upper-air blocking pattern has persistently held the center of cold high pressure over the Northeast. Sunday morning frost and freeze advisories were posted for most New England states as well as northern lower Michigan. Meanwhile, persistent low pressure to the south has resulted in record heavy rains and flooding over the Gulf Coast the past week. Atlanta was among the latest, reporting a record daily rainfall of 3.7 inches Saturday.

Dear Tom,
We once noticed the full moon low on the eastern horizon glowing orange. It was beautiful, and as it rose it became yellower until finally it was white. What caused this?
Joanne Prusik, Chicago
Dear Joanne,
The spectacle of the moon's color change as it gradually rose from the horizon was caused by the effect of the Earth's atmosphere on moonlight. Moonlight (actually, reflected sunlight which is white when pure) is massively scattered when the moon is on the horizon, and its light must pass through a great distance of dense air. Only the longer wavelength components of light, shades of red and orange, arrive at our eyes.
As the moon climbs, its light makes a shorter passage through dense air, and less scattering occurs. Shorter wavelength colors, like yellow, appear and finally the full white spectrum.

Thanks to Benjamin Stillwell for passing along this great shot of a waterspout that developed over the Gulf of Mexico off of Madeira Beach, Florida.  

Atmospheric blocking patterns -- periods in which the normal progression of weather systems slows or comes to a halt, often for an extended period of time -- produce weather winners and losers, and Chicago has been on the winning end of this most recent block. Saturday becomes the 22nd day without significant rain, the city's longest such spell in four years. The trace amount of rain for the month at Midway Airport makes this the driest September in half a century.
 The weather story has been a much different one across the South. There, waves of thundery rainfalls have hit repeatedly for weeks. The past week has seen rainfalls in excess of a foot across sections of Arkansas -- including 12.98 inches at Pine Ridge and 12.52 inches at Mena.
With the blocking pattern breaking down in the days ahead, the humid air within which those southern rains fell heads north. Its arrival in Chicago next week sets the stage for hazier, more humid weather Monday through Wednesday. Moist air retains warmth, and coupled with southerly winds and some period of mixed sun, Monday and Tuesday are likely to host temperatures that flirt with or exceed 80 degrees. The changes in the now three-week-old pattern are under way will become more evident Sunday as an overcast assembles and thickens, threatening scattered showers in the afternoon and more widespread rainfall -- even a few possible thunderstorms -- Sunday night into Monday morning.
 
Second very different block next week keeps Chicago mild and dodging waves of rain
North America will see one blocking pattern replaced by another with characteristics very different than those which have dominated since late August. The development of a cut-off low across the nation's mid-section threatens waves of rainfall which could generate rainfall totaling 0.50" to as much as 2" in the coming week.

Dear Tom,
In a recent column you mentioned that is has been as cold as 15 degrees at Death Valley, Calif. Has it ever snowed there?
Neil Walker
Dear Neil,
Once again we invoked the services of our Death Valley expert, Paul Kubecka, and his answer was a resounding "yes." Snow has been recorded on two occasions at the official weather station there, located at 168 to 194 feet below sea level. Traces of snow fell on Jan. 9 and 11, 1949, and again on Jan. 4 and 5, 1974. In addition, ice pellets (sleet) fell there on Jan. 2, 1949. A more significant snowfall of 4 inches was observed at the National Park Service headquarters at an unofficial weather station located at Cow Creek, at an elevation of 151 feet below sea level on Jan. 12, 1949.
Kubecka also found reference to a mention of 0.5 inch snowfall on Jan. 29, 1922, but could not verify the validity of that report.

Sister Bonnie Boilini sends us this intriguing photo and asks, "Why is it when you are out West, the streaks in the clouds showing rain are sometimes black and white?"  I certainly see why you ask the question after looking at this photo. I've shown it to my colleagues here in the office and, without being there to witness this scene first hand, we can only speculate.

But our best guess, Sister Boilini, is that what we're looking at here are a series of rainshafts--several of which may involve microbursts which hit the ground and kick dust into the area. The ones which appear white are being illuminated by light. You see this sort of phenomenon on the Front Range of the Rockies in eastern Colorado with some frequency.  Rainshafts in the shadows appear darker.

It's worth noting that when you see white streaks emanating from the base of a thunderstorm, it's not uncommon for them to be hail shafts--columns of hail pouring out of the thunderhead above. Sister Boilini tells us the picture was taken near Antelope Canyon in Page, Arizona, and that she had just gone through a terrible storm. Great beautiful photo, Sister Boilini--thanks for sharing it with us!



Photo courtesy of Sister Bonnie Boilini

When it comes to beautiful photography in the United Kingdom, Mark Vogan never disappoints us. Check out these beautiful shots taken on a recent two-day trip during which Mark and his wife Karen toured Wales. Mark tells us:

"Karen and I took a brief trip down to Wales for two nights, one night in the tiny village of Dale on the Pembrokeshire coast (south coast) and the other on the north coast. We drove up along the west coast and also through Sanowdonia National Park. The guest house owner said we had the best weather since June!"

Spectacular shots, Mark!  THANKS as always for keeping us abreast of the UK's fascinating geography and weather--and all the best to you and Karen!









Photos courtesy of Mark Vogan, Glasgow, Scotland

Timing is everything.  O'Hare has only picked up a paltry .05" of rain during the the last three weeks.  Late Sunday we have our first decent chance of showers in almost three weeks.  It doesn't figure to have a big impact on the Bears game though.  The showers would be scattered and late in the day.  If the rain should arrive earlier and be heavier than expected, that could be another story.  Soldier field was re-sodded this week in the wake of the U2 concert there last Saturday night.   

In an article in the Chicago Tribune this week, officials at Soldier Field said the new sod should hold.  The replacement strips of sod are 3 and a 1/2 feet wide and 30 feet long weighing in at 2,000 pounds a roll.  Go Bears!

It's been a remarkable run of dry weather here. Friday marks the 21st day without significant precipitation. Only a trace of September rain is on the books to date at Midway Airport---the least of any September in half a century and the longest period without measurable precipitation (0.01-inches or more) in the 4 years since a stretch of 24 days of dry weather in 2005.

There are signs the current dry pattern is growing tired. The first rains since late August---part of a system drifting north after drenching parts of the Deep South with as much nine inches of rain in recent days. Hardest hit have been sections of Arkansas and Tennessee. On Thursday alone, Camden and El Dorado---both in Arkansas---recorded rainfalls of 6.25 inches and 3.84 inches. At present, there are no indications rainfall of that intensity is on the way, but, clouds from that disturbance are to invade Chicago's airspace Sunday---and showers and even some possible thunderstorms could reach the area by late in the day.

The pattern developing across the central U.S. next week remains extremely complex. It has the potential of introducing haze and humidity to Chicago while holding mild air over the area ---rather than the truly chilly air some models predicted earlier in the week might spill into the city as early as Tuesday or Wednesday. The timing, location and extent of rainfall is likely to depend on precisely how next week's pattern unfolds and that in turn will depend on how the remnants of Super-Typhoon Choi-Wan influences the North American jet stream pattern. Current projections suggest the jet may buckle into a huge ridge over western North America which could send jet stream winds over the top of the wet upper low pressure expected to spin up across the nation's Heartland. That would lead to a system which would linger for days across the area.
 
Year's longest above normal temperature spell ends with Thursday's 72/50 temperature extremes
 
Thursday marked to first day to post a temperature deficit for first time in 12 days ending the longest spell of above normal temperatures here in 2009.
 

Dear Tom,
It seems that September and October usually receive a great deal of sunshine. Have either of these months ever received 100 percent of possible sun?
William J. Ooms Jr., Alsip

Dear William,
September with an average of 62 percent and October with 58 percent of possible sunshine, are usually the city's last sunny months before the dark, dreary days of late autumn and early winter set in. We checked with Chicago climatologist Frank Wachowski and found that dating back to 1893 the two sunniest Septembers (percentage of possible sunshine) were in 2004 (90 percent) and 1895 (86 percent.) The city's sunniest October was in 1924 (84 percent) followed by 1938 (79 percent.) Wachowski noted that Chicago never has and probably never will log a totally sunny month. The sunniest month on record here was July 1916 with 95 percent of the possible sun.

Freddy Cahue was biking along Lake Michigan Wednesday and photographed one of the waves kicked up by the day's powerful 30 m.p.h. northeasterly wind gusts. THANKS Freddy---GREAT SHOT!
 
Tom Skilling
 

Photo courtesy Freddy Cahue, Chicago
 

September's spectacular weather continues Thursday. Only  a trace of rain has fallen this month at Midway Airport. That's the site's least rain at this point in a September in 50 years (since 1959)---and only the second time the month has produced so little rain through Sept.17 in the 81 years in weather observations there.

Thursday's uninterrupted sunshine occurs in a much calmer environment than on Wednesday, when winds gusted to 30 m.p.h. Winds gently stroll across the area Thursday at velocities no greater than 8 m.p.h. This allows temperatures to surge well into the 70s---a nice recovery from the mid to upper 40s which occurred overnight in the coolest outlying areas.

The month has been drenched in sun.Sunshine to date in Chicago is double that of a year ago---83 percent  versus 44 percent---and the month to date rainfall comparison with 2008 is stunning.  While O'Hare has recorded just 0.03 inches this month, more than a foot of rain (12.61 inches) had fallen by this time a year ago and a number of area rivers were in flood.
 
Chicago appears headed for significantly wetter weather if at least one longer range global forecast model is correct. The change wouldn't begin taking place until late this weekend. For the past few days, the European Center's global forecast model has been consistent in predicting the development of a new atmospheric blocking pattern next week---only this one could lead to a deep, wet upper low stalling over the nation's Heartland. It's a scenario, which if true, could bring 2-inch or greater ten-day rain tallies here beginning with showers starting late Sunday. Interestingly, the outcome of that forecast could be affected by Super Typhoon Choi-Wan---the 2009's strongest to date in the western Pacific. The storm is to pass 300 miles east of  Tokyo, Japan Friday. It was producing 160 m.p.h. sustained winds and nearly 200 m.p.h. gusts late Wednesday.  Typhoons can play a huge role in how jet streams buckle as they lose tropical characteristics, and this could have an impact on North America's upper air pattern next week.
 
Same blocking pattern responsible for dry weather here drenched Arkansas with nearly 9 inches of rain Wednesday
 
While Chicago moves into an 18th day of comparatively dry weather thanks to a persistent blocking pattern this month, Arkansas is among the areas which has been doused with repeated rains. Wednesday rainfalls hit 8.20 inches at Mt. Vernon, Ark.
 

Dear Tom,

You've told  us that Chicago has twice recorded a trace of snow in September. What kind of snow season followed those early flakes?
Mark Keenan
 
Dear Mark,
You're correct that Chicago has twice recorded a trace of snow in September since snowfall records began in 1884. Both instances occurred on Sept. 25, first in 1928 and again in 1942 following the passage of strong cold fronts. The winter of 1928 turned out to be a subpar snow season with just 30.1 inches compared to around 40 inches in a typical Chicago winter, while the 1942-43 season was snowier with 45.2 inches. The typical date for the arrival of the season's first snow flurries is around Halloween, though they have occurred as late as Dec. 5 in 1999. Last year the city's first snowflakes fell on Oct. 26.

 

Our thanks to Debra Prisby of Lowell, Indiana who snapped this beautiful sunset photo from the deck of the Lighthouse Restaurant in Cedar Lake, Indiana. Debra tells us she was enjoying dinner with he fiancée and his son, who is a Staff Sergeant with the U.S. Air Force and was home on leave!  Debra says dinner was great and that she enjoyed the view!  So are we through your photos, Debra!  MANY THANKS!
 
Tom Skilling
 

Photos courtesy of Debra Prisby, Lowell, Indiana
 

Check out these shots of central Iowa ground fog which come to us from Douglas H. Hanbury.  They were taken between 6:30 and 7 a.m. this past Monday morning (September 14, 2009) outside Tama, Iowa--located 52 miles west of Cedar Rapids of about 65 miles northeast of Des Moines.  The highway shots, Douglas tells us, were taken within 10 miles of Tama mainly on US 63.   These are beautiful shots, Douglas!! THANKS so much for sharing them with us!

Tom Skilling






Photos courtesy of Douglas H. Hanbury, East Peoria, Illinois

Chicago area residents aren't alone with the gusty northeast winds churning up whitecaps on Lake Michigan and pounding the city's  shoreline Wednesday with impressive waves, some in excess of 5 feet. Over more than 1,000 miles from New England and the Mid-Atlantic west to Oklahoma, sharply varied barometric pressures beneath the southern flank of a sprawling Canadian high pressure have activated nature's vast wind machine. Winds blow as part of nature's effort to balance barometric pressure inequities. Gusts topping 25 m.p.h. are likely to be frequent visitors Wednesday up and down Lake Michigan's Illinois and Indiana coastlines---including the Chicago metro area.

The cooler air here Wednesday has origins north of Lake Huron in Canada. That's where it was situated yesterday. Its arrival brings a string of back to back 80-degree temperatures to an end. O'Hare's 84-degrees Monday and 82 Tuesday.
 
Longest spell of above normal temps since June winding down
 
The area has recorded 10 consecutive days of above normal temperature---the longest such spell since June and the second longest of 2009.  September 2009's opening half has been a meteorological joy compared to the washout the area experienced during the same period a year ago. 83 percent of Chicago's possible sunshine has occurred this year---compared to last year's lackluster 42 percent--and the monthly tally of 0.03 inches or rain was dwarfed by last September's 12.61 inches to date.

Dear Tom,
How rare is it for Chicago to experience a 90-degree day in October?
Megan Samuels

Dear Megan,
October 90s are extremely rare in Chicago with only six on the books since 1870. The combination of shortening daylight and lowering sun angle make 90s unlikely so late in the year.

The last October 90s recorded in the city took place in 1971 when back-to-back 90s were logged on Oct. 1 (92) and 2 (91). Both of these readings established new record highs for the day.

The latest occurrence for a 90-degree day here is Oct. 6, 1963 when the mercury peaked at 94 degrees---not only breaking the record high for the day, but set the all-time high for the month.

Since 2000, the highest October temperature recorded here has been 87 degrees; peaking at that value on Oct. 4, 2005 and for three straight days during a mini-late-season heat wave from Oct. 6-8, in 2007, which affected the Chicago Marathon.

Flying through a "hole in the wall" of thunderstorms at 8,000 ft. over Springfield, Illinois this past Thursday
 
Our friend and ace pilot Anson Mount sends us this photo he took last Thursday flying over Springfield at the 8,000 ft. level last.  He writes:
 
"I was actually in controlled airspace and was being directed right into this hole! It was a wall of cumulonimbus building along a wave downstate that afternoon. I like the fact that you can see the clouds in the distance!"
 
Very cool shot! THANKS Anson----ALWAYS great hearing from you! By the way, Anson asks we remind you that the Waukegan Air Show takes place this weekend at the Waukegan Regional Airport. Hope the turnout is great!
 
Tom Skilling
 
 
Photo courtesy of Anson A. Mount III, Algonquin, IL
 
 

It's not your imagination--those ARE palm trees on Oak Street Beach. These pictures come to us from David W. Skyzyniarz who tells us in his e-mail that he considers riding a bike through Northerly Island--especially in the morning time of the year----an amazing place to take in the beauty and sounds of our Chicago Lakefront.  Can't agree with you more, David. By the way, regarding the palm trees------Zvez Kubat, spokesperson for the Chicago Park District, tells me they were planted by the folks who operate Oak Street Beachstro--that's the restaurant on the beach. The palm trees are obviously not native to our area---but Zvez tells me they are quite able to survive our winter weather. In fact, they were planted last year.  Thanks to David Skrzyniarz for the pics and Zvez at the Park District!
 
Tom Skilling


 



 Photos courtesy of David W. Skrzyniarz, First Analysis Securities Corp, Chicago

Corrina De Maria sends us this fascinating shot taken Monday in downtown Chicago. In it, you can see a shadow being cast by the Willis Tower (formerly the Sears Tower).  She writes:
 
"I couldn't quite figure out why or how this happened, but while I was riding by the Sears Tower I saw it casting a shadow upon the sky! The sun was behind the building. I have my theories, but I was wondering what you thought."
 
 
Corrina, what's happening here is the backlit Willis Building is casting its shadow on the low "scud clouds" making their way across the Loop. You've caught the whole thing magnificently in your photo!  THANKS for sharing it with us!
 
Tom Skilling

 
Photo courtesy of Corrina S. DeMaria, Chicago

French photographers have painstakingly stitched together 1200 digital images to put together an amazing panoramic view of the Milky Way galaxy.

The image was produced by the European Southern Observatory's GigaGalaxy Zoom project.  The original image contains about 800 million pixels.  A zoomable image can be found by clicking here.  It has a measly 18 million pixels.

For you photographers, a Nikon D3 digital camera (50 mm lens open at f5.6) was used for the photos used to piece together this awesome view.

 

It felt like summer Monday. The 84-degree highs at O'Hare International and Midway Airports were Chicago's warmest readings since an 88-degree high Aug. 16. Even the 73-degree water temperature along the lakefront tied the season's highest to date. But the march continues toward autumn's astronomical open at 4:18 p.m. Sept. 22 when the sun's most direct rays cross the equator and head south.

 An incredibly stubborn atmospheric blocking pattern, responsible for slowing U.S. weather movement to a crawl since late August, moved into its 16th day Monday. Areas like Chicago that are in the midst of dry weather remain dry while regions trapped beneath wet weather can be subjected to extraordinary amounts of rain. That was the case Monday in Pensacola, Fla., where the Naval Air Station recorded 7.23 inches of rain. The ongoing block has deflected rain-producing systems away from the Chicago area. The paltry 0.05 inches of rain recorded in the past 2 1/2 weeks at O'Hare is 2 percent of the normal amount and renders the span from Aug. 29 through Sept. 14 the driest in this area in 22 years.

Dear Mr. Skilling,
Does lightning come up from the ground or down from the sky?
Sarah

Dear Sarah,
In answer to your question, lightning researcher Dr. Martin Uman of the University of Florida says, "In a sense it does both. The usual lightning flash between cloud and ground begins with a visually-undetected downward-moving traveling spark called the stepped leader. On the other hand, when the stepped leader reaches ground (or a spark jumping up from the ground), the leader channel first becomes highly luminous at the ground and then at higher and higher altitudes. This is the return stroke."
 
It is therefore fair to say that visible lightning moves from ground to cloud but the bolt's initial development occurs from cloud to ground.
 

Jake Krippelz from Chicago sent us this picture of his palm tree. He tells us:
 
"I have included a pic of my palm tree that I have pool side at my house, they're King Palms you can't get north of Orlando Fla. I transported them here 6 years ago and they do great here in Chicago during the summer months out by my pool, I keep the in my Gazebo during the winter you can see the moon rise in the back round"
 

Thanks for the shot Jake!

This is quite a shot from Margaret Espiritu, a former Chicagoan who writes to tell me she watches our weather programs as often as she can from her new home in San Diego. Having seen a shot we showed from Arizona the other night, she says she thought we might like THIS one taken over San Diego! YOU ARE SO RIGHT, Margaret!  What a spectacular shot!  Thanks for sharing it with us--and great hearing from you!

Tom Skilling

Many thanks for Kevin Rooker who shares this August 31 photo of cumulus clouds near Chicago's lakefront taken at 7:30 a.m. near Foster and Sheridan Road. 
 
Tom Skilling
 

Photo courtesy of Kevin Rooker, Chicago
 

Michael Kellems sends us this beautiful shot of the masts of sailboats silhouetted against Sunday's beautiful sky just beyond sunset in Michigan City, Indiana!  It's quite a photo, Michael!  MANY THANKS for sharing it with us!
 
--Tom Skilling


 
Photo courtesy of Michael Kellems, Michigan City, Indiana
 

Matt and Maggie Karns from Sugar Grove send us this shot of "Charlotte''s web". They tell us:
 
My wife and I watch you every night on the nine o'clock news.  We are always intrigued by the neat photographs our local Chicagoans take in their neighborhoods, so we decided to take this picture of the most amazing spider web ever spun!  It measured 22 inches by 24 inches and it was hanging from our garage gutter.  The coolest part about this spider web photo is that if you look closely in the top right corner, you will see the spider all curled up!
 
This is a great shot Matt and Maggie!  THANKS for sharing this with us!
 
Tom Skilling
 


Photo courtesy of Matt and Maggie Karns, Sugar Grove, Illinois

 

With high pressure locked in place across the Chicago area for more than two weeks, rainfall has been a scarce commodity.  Officially, 0.03 inches fell at O'Hare Airport on Sept. 6, but much of the area has been rain-free the entire month as the city has enjoyed a seemingly endless string of delightful weather -- marred only by repeat episodes of early-morning dense fog.
Winds off the lake have also been a daily occurrence, and today's lake breezes will blow for the 16th consecutive day, a record for this time of the year. The old record of 15 straight days of lake winds was established Aug. 29-Sept. 12 back in 1950.  
The week ahead portends more of the same with mainly sunny days, more easterly winds, and only slim chances for precipitation.
 
Fred may not be dead yet
In the tropical eastern Atlantic, once a powerful Category 3 hurricane, Fred, now reduced to a remnant low pressure system, is showing signs of possible regeneration and could regain tropical storm status later this week.

Dear Tom,
Severe storms swept the Chicago area on Aug. 23, 2007. Was that storm a
derecho or a microburst?
Steve Plainfield
Dear Steve,
The storm you are referring to was a large derecho, a fast-moving squall line that swept the entire Chicago area during the afternoon of Aug. 23, 2007. It raced across the Chicago area at speeds approaching 60 m.p.h. The thunderstorms, some of which towered to nearly 65,000 feet, packed top winds of 80 m.p.h. that were measured at south suburban Manhattan; but almost all of the metropolitan region reported damaging winds of 60-70 m.p.h. That being said, there were likely small pockets of intense damage superimposed in the overall swath of wind-damage path that was caused by microbursts. The thunderstorm complex leveled countless trees and knocked out power to many areas.

Tom Sidor of Algonquin shares these pictures from his bike ride Sunday morning. He described the morning as "eerie" and slightly dangerous for road biking, but the pictures he sent us are definitely amazing. Thanks Tom!

Since Aug. 30, rainfall in the city has been a virtual no-show, with only a scant 0.03 inches on the books. As persistent high pressure continues to grip the area, Chicagoans can look forward to another week of mainly sunny and seasonably warm weather with patchy early-morning fog and a daily occurrence of winds off the lake. The stable weather patterns have repeatedly thwarted advancing precipitation systems and this may well be the case again this week with two minor rain threats: Tuesday with showers developing along a weak cold front, and Friday as low pressure passes south of the city.
 
What a difference

Just a year ago the entire metropolitan area was battering major flooding as record deluges fueled by copious amounts of moisture from Pacific Tropical Storm Lowell and Atlantic Hurricane Ike targeted the city. The cloudbursts of Sept. 12-14 set the stage for the city's second-wettest September on record with 13.63 inches---surpassed only by 14.17 inches in 1961---and helped make 2008 the wettest year on record here with a yearly tally of 50.86 inches.
 

Dear Tom,
We only hear about the blistering highs at Death Valley, Calif. What is their highest overnight low? On the cold side, what is the site's record low and lowest daytime maximum?
--Dale Kalina,  Naperville

Dear Dale,
We asked retired Chicago weatherman Paul Kubecka, an expert on Death Valley climatology. The site's record high of 134 on July 10, 1913, is well-known, but Kubecka filled us in on some lesser publicized records. Death Valley is one of the few places on this planet where low temperatures have remained above 100. Kubecka cited lows of 110 on July 5, 1918, and 105 on July 22, 1917, as two of the highest. Despite the scorching summers, it does get cold there in winter. Death Valley's all-time record low of 15 was set Jan. 18, 1913, and the lowest maximum was achilly 38 recorded on Christmas Eve1990. 

Latest forecast models now indicate the dry high pressure air mass will persist well into next week. Rain may hold off until next Friday. Accordingly, this weekend could be the sunniest in 2009 according to Chicago's veteran weather observer Frank Wachowski. Frank says the sunniest weekend this year was back on March 14-15 when 100 percent and 91 percent sunshine was recorded at his station near Midway Airport. The only possible fly in the ointment could be early morning fog that may limit sunlight at the start of the day, otherwise we may well experience 100 percent sunshine both Saturday and Sunday.

Record heat along the West Coast and flooding in Texas
Friday was another hot day along the West Coast with several record highs including 93 degrees in Portland, 87 in Seattle, and 95 at Mt. Shasta, Calif. Jarrell, Texas, recorded 12.90 inches of rain with extensive flooding of roads reported in many areas of south-central Texas.

Dear Tom,
Was that beautiful full moon that I saw rising on Sept. 4 the Harvest Moon?
Brandy DeWitt
Dear Brandy,
The Sept. 4 full moon was indeed beautiful, but it was not the Harvest Moon. It carried a variety of other names, among them the Fruit, Barley and Corn full moon. According to folklore, the Harvest Moon is defined as the full moon closest to the autumnal equinox, the astronomical beginning of fall. However, this year the equinox occurs on Sept. 22, 18 days after the September full moon but only 12 days before the one on Oct. 4 which is designated the Harvest Moon. Having the Harvest Moon fall in October is a bit unusual and won't happen again until 2017. The full moon following the Harvest Moon is traditionally called the Hunters' Moon and will occur on Nov. 2.

If you thought this was a cool summer, you would be right.  It has been cool not only for us here in Illinois and Indiana, but for most of the country. According to a new report from the National Weather Service, for only the second time this decade, the United States recorded a summer that was cooler than average.  Michigan, Minnesota and Wisconsin were among several states that saw their tenth coldest summer on record.  It was the 11th coolest summer for Illinois and the 13th coolest summer for Indiana.

The cool summer weather spilled over into fall.  Yesterday's 81 at O'Hare was the first time we hit 80 in 15 days going back to August 25th.

A high temperature of 81-degrees was registered at Chicago's official observing site at O'Hare International Airport Thursday. This marked the first time since Aug. 25 that the thermometer reached or exceeded 80-degrees. The string of days failing to hit 80 degrees finally ended at 15---the city's longest since a 28-day run between Aug. 24 and Sept. 20, 1885.

Sunny days will continue right through the weekend expanding on an already very sunny start to September, a marked contrast to the previous 6 months which all registered sub-normal sunshine tallies. Veteran weather observer, Frank Wachowski, who logs Chicago's official sunshine data, tells us that the opening ten days this month have registered 79 percent of possible sunshine, well above the 62 percent considered normal and the 57 percent observed by this date a year ago.

Dry weather could end early next week

An upper air pattern change could be evolving which may bring Chicago it's next chance for rain by this coming Tuesday or Wednesday. Low pressure passing to the south should spread cloudiness north into southern Wisconsin with Chicago possibly resting on the northern fringe of the associated rain area.

Dear Tom,
My barometer shows that air pressure fluctuates up and down, but can the extremes of air pressure be a threat to us?
James Walker, Chicago

Dear James,

Air pressure is a measure of the "weight" of the column of air above the surface of the Earth. A barometer determines the height of a column of mercury in a tube that is required to exactly balance the weight of the air column. Average atmospheric pressure, 29.92 inches of mercury, presses on us with a weight of about 14.7 pounds per square inch at sea level.

Surprisingly, air pressure is one of the most benign of weather phenomena, and its effects on the human body are minor, even at the extremes. The difference between Chicago's air pressure extremes (30.98 inches on Feb. 16, 1989, and 28.70 inches on March 12, 1923) is 2.28 inches; this represents only about one pound per square inch, or a change of 2,160 feet in elevation.

Computer models have backed off predictions of weekend rainfall here---a development which appears to assure the ongoing dry weather will, by late Sunday, make this the driest late August/early September spell in 22 years. The past 13 days have managed just 0.05 inches of rain---far short of the 1.50 inches of precipitation the period normally produces and a fraction of the 4.16 inches which fell during the period a year ago.

The haze of recent days is the product of rising atmospheric moisture levels and a gradual accumulation of particulates. Chris Price, meteorologist with the Illinois EPA, reports air quality is moderate and that the weaker September sun has helped prevent a surge in ozone levels. Ozone forms as a by-product of reactions between air pollutants which occurs in strong sunlight--and the September sun which delivers only 70 percent the energy which reaches the surface in June and July.

Hurricane Fred, which is churning through an area of the far eastern Atlantic is making news of its own----even though it represents no threat to land. The major storm with 115 m.p.h. top winds is the strongest on record to occur so far south and east in the Atlantic. It's one of only three major Atlantic hurricanes which have occurred east of 35-degrees west longitude---just off Africa.

 

Dear Tom
Have we ever gone through an entire September without recording an 80?
Brian Beecher
 
Dear Brian,
It has happened, but only once in 139 years of weather records. The year was 1876 when the city's official thermometer was located downtown near Lake Michigan. The highest temperature that September was just 78 degrees,recorded on the Sept.1 and again on the Sept.19. Since then, the closest the city has come to an 80-less September was 16 years ago in 1993, when only one was recorded: an even high of 80 degrees on the Sept.12. The month was cloudy with only 52 percent of possible sunshine and quite cool, averaging 5.2 degrees below normal. Besides the pronounced lack of 80s, it featured five days with highs in the 50s and the month ended with an early-in-the-season freeze with the low temperature reaching 31 degrees on the 30th.
 

Gary Wojton from Scottsdale, Arizona sent us this beautiful picture of monsoon clouds.
Thanks for the great shot Gary!

Kathy Nadura from Bensenville, Ill. sent us this great snapshot. She tells us:

"I thought you would enjoy this picture which I took on Labor Day morning in my back yard.  It really is a weather bug!"

Thanks for the shot Kathy!


Photo courtesy of Kathy Nadura, Bensenville, Ill.

Thought you'd get a kick out of these shots of our visit Tuesday on board the Environmental Protection Agency's Lake Guardian research ship.  On board, scientists are keeping a close eye on the Great Lakes---monitoring everything from invasive species which are literally changing the food chain and character of these treasured bodies of water but also looking at toxins which flow into them. We went out onto Lake Michigan aboard the Lake Guardian and had a chance to see the equipment used in these Great Lakes studies up close. We report on our trip and introduce you to these scientists on Wednesday evening's (9/9/2009) WGN News at Nine!  Hope you have a chance to check our report out tonight at 9 P.M. and online!
 
Tom Skilling



 

NOAA has just completed the final phase of a nine-year, $180 million plan to upgrade the computer system that powers our nation's forecasts for the National Weather Service.  These aren't your parent's computers.  These IBM supercomputers would fill about half of a tennis court and are 34 times more powerful than the most powerful supercomputer that existed just 10 years ago.  According to NOAA, it would take someone with a calculator 3 million years to match the number of calculations the new "Stratus" supercomputer system can whip out in one second.

All this extra power should enable the use of more complex weather models that should improve accuracy and lead to more lead time for severe weather warnings.

Check out NOAA's article for more information.

A testament to just how cool the 2009 summer has been is the number of days that temperatures have reached at least 80 degrees at O'Hare International Airport. The city has not recorded an official high of 80 degrees since Aug. 25, a  string not seen here in nearly 125 years, when readings were taken downtown near Lake Michigan's cooling breezes. To date, there have been just 51 days of 80 degrees or higher in Chicago this year, the fewest on record here in the 50 years since records began at O'Hare in 1959.  In contrast to this summer's paltry count of 51, the city logged 103 warm days in 2005 and 101 in 2007.
 
The weather has also been dry with only 0.03 inches of rain in the last 10 days.  A few  showers may dampen the Indiana and southern suburbs Wednesday but a more general- coverage rain pattern should begin to evolve by this weekend into next week as a  frontal system approaches.
 
Cloudbursts hit Kansas-High winds in Texas

It was anything but dry in portions of south-central Kansas Tuesday as heavy thunderstorms swamped the area. Eureka measured 6.50 inches while nearby Benton recorded 5.66 inches.  In the Texas Panhandle thunderstorm wind gusts reached 67 m.p.h. at Childress and 61 m.p.h. at Lubbock. 



 

Dear Tom,
Our recent seven day dry spell seemed long. Was that anything close to a
record?
Barry Bishop

Dear Barry,
Not by a long shot. Though seven straight dry days was a nice break from
precipitation after the heavy rainfall of late August, it was far from a
record.

Back in 1979 after a brief shower brought 0.01 inches of rain to Chicago on
Sept. 1, not a drop fell in the city for 22 days, the city's longest period
of absolutely dry weather. The streak was broken by some sprinkles on Sept.
23 but the month closed with no more measurable rain and remains the city's
all-time driest month. Chicago's longest span between measurable
precipitation (0.01 inches or more) was 30 days established back in 1919
from Jan. 7-Feb. 5, followed by two 29 day periods, one from Dec. 7,
1943-Jan. 4, 1944 and the other in the above mentioned month from Sept.
2-30, 1979.

It's been an extraordinary run of late summer weather the last 10 days--and it's not finished yet. The same high pressure that has greeted Chicagoans so many recent mornings produces light easterly lake breezes a 10th consecutive day on Tuesday. These breezes were responsible for a near-12-degree east-west spread in high temperatures on Labor Day across the Chicago area, with readings that ranged from 68 degrees at the University of Chicago and Highland Park to 81 degrees in Elgin and 80 degrees in Burr Ridge, Algonquin and Lake Geneva, Wis.
Not once in the last week and a half have average daily wind velocities exceeded 9 m.p.h. The paltry 0.03 inches of rain over the period make it the driest Aug. 30 through Sept. 8 since 2000. A comparable period a year ago had rains that amounted to 2.93 inches.
Though 80s have occurred in west suburban locations this month, not once has a daytime high reached 80 degrees at O'Hare International Airport--the city's official weather observation site. That makes this the first September in 16 years not to have produced an 80-degree high by Sept. 8.

Dear Tom,
Ocean currents rotate clockwise in the Northern Hemisphere, but there are also counterclockwise winds. Is the difference caused by the Coriolis force?
Sam Olinger
Dear Sam,
The terms "clockwise" and "counterclockwise" refer to the sense of rotation around a center, when viewed from above. Do not confuse those terms with the Coriolis deflection which, in the Northern Hemisphere, is always directed to the right of the motion -- for water, air and for anything that moves.
When air blows inward toward a central point (toward a low pressure center, for example), the rightward Coriolis deflection causes a counterclockwise inward spiral. When air blows out from a central point (from a high pressure center), the rightward Coriolis deflection causes a clockwise spiral outward.
Ocean currents, driven by wind and density differences, experience the same deflections.

In recent days, some of our blog readers have sent us some pretty amazing photos of morning fog as seen across the Chicago area. We'd like to share a few of them with you:

Nicholas Andrews sent us this image of fog draping Lake Point Tower (near Navy Pier) on Sunday.



Mike and Candi Straub sent these beautiful images from their farm in Harvard. The pictures were taken last Friday morning between 6 and 6:20 a.m.





Jeremy Draftz sent this picture of fog seen over the Lake in the Hills Airport early Friday morning.



Finally, Lisa Histrom of Hebron took this picture while driving in Lake Geneva area, also on Friday morning.



Thanks to everyone for sharing their pictures with us!

The upper-air pattern remains relatively unchanged with the stronger northern jet stream winds well to the north in Canada and the southern jet running from the Gulf of Mexico up the eastern seaboard. The light winds aloft leave little opportunity for any significant air mass changes here in the Midwest or western Great Lakes. As a result, the light easterly flow continues to allow fine particulate concentration to slowly increase, and hazy conditions will persist. In Wisconsin the Department of Natural Resources has continued an Air Quality Watch through Monday over southern and central portions of that state with concern that particle pollution and ozone concentrations might reach unhealthy levels for people in sensitive groups.

Rain/cooler conditions possible next weekend
Computer models indicate a cold front accompanied by showers and thunderstorms could bring cooler Canadian air into northern Illinois next weekend. Chicagoans may be in store for highs in the 60s, more like late September and early October.

Dear Tom,
Why is it that we get storms when a cold front arrives but not when a warm front arrives?
Steven Bloome
Dear Steven,
It's true that the weather in advance of a cold front is more turbulent and more likely to erupt into thunderstorms than warm frontal weather, but warm fronts do sometimes generate precipitation.
A front is the boundary between air masses of differing characteristics, most notably temperature. We call it a cold front when cooler air is advancing, and similarly for a warm front.
Stormy weather develops when air is lifted, but cold fronts are far better lifting mechanisms than warm fronts. Advancing cold air, heavier and more dense, turbulently forces its way under warmer air, thrusting it up and thereby triggering stormy conditions. Advancing warm air, lighter and less dense, glides gently over cool air, usually generating little more than clouds and steady rain.

Under the influence of cool dry high pressure, It's been a week (Aug. 29) since rain was last measured at the official O'Hare airport observation site. The rainless streak could well continue at O'Hare Sunday, but showers are becoming more and more likely, especially in southern portions of the metro area and in northwest Indiana. Low pressure has moved into western Illinois. Showers that fell over central and southern sections of the state Saturday and Saturday night have been working their way east and north, ever closer to Interstate-80. The Chicago area will again on the borderline for showers Labor Day into Tuesday.

Warm-up Wednesday, then turning cloudy/cooler

As the low moves east and weakens over Indiana, clouds will thin allowing the sun to warm readings into the lower 80s Wednesday. However a cold front will approach from the northwest and even though it's movement through the Midwest will be slow, associated clouds and showers/thunderstorms along with cooler air behind the front will lead to a return to sub-normal temperatures later in the week.

Dear Tom,
My son, age 7, asked a seemingly simple question that has me stumped: How fast do raindrops fall?
--Ellen Sadowski

Dear Ellen,

It might seem that raindrops tumble from the sky at great speeds as they zip past our eyes, but photographic measurements show that raindrops actually fall at speeds ranging from about 5 m.p.h. to 20 m.p.h. Larger raindrops fall faster than smaller ones. German physicist Dr. Philipp Lenard made accurate measurements of raindrop sizes, shapes and fall speeds more than 100 years ago (between 1898 and 1904). He found that raindrops can attain sizes no greater than one-fifth of an inch in diameter (they become unstable and shatter at larger diameters) and the largest drops fall at about 20 m.p.h. Tiny, misty droplets of drizzle (0.02 inch in diameter or less) descend at 1 to 4 m.p.h.  

Haze and some patches of ground fog mark the opening of the Labor Day holiday weekend Saturday. A similar situation Friday provided city high-rise residents and workers spectacular views of fog bank which shrouded sections of the city, extending to altitudes no higher than 300 feet above ground level.  That meant anyone above the 30th floor of many buildings was able to gaze down on the fog and clouds below.
The stagnating high pressure behind the generous sunshine expected to dominate Saturday once fog patches lift has been in the area eight consecutive days, and will by the end of Saturday have produced easterly lake breezes in seven of them. Incredibly light winds are present through a huge swath of the atmosphere -- from ground level aloft to 36,000 feet. Air movement is minimal thanks to wind velocities no higher than 10 m.p.h. It's a situation which has allowed the air mass to become increasingly dirty -- a situation which has prompted the Wisconsin Department of Natural Resources to issue an air quality alert. 
Those who flock to Lake Michigan will find shoreline water temperatures 7 degrees lower than a year ago: 68 degrees vs. the reading of 75 on this date in 2009 -- and lake levels 10" higher than a year ago.

Kansas and Missouri whacked by thundery downpours, some exceeding 3"
A compact disturbance bearing clusters of downpour-generating thunderstorms has been creeping across the west and southern Midwest, dousing sections of Missouri and Kansas. Gypsum, Kan., was hit with rains totaling 4.75" while Bennington registered 3.60".  Doppler radar scans put cloud tops up to 45,000 feet.

Dear Tom,
Could the deep solar minimum be responsible for our cool, cloudy summer?
Michael Leach
Dear Michael,
The answer to your question varies in scientific circles. Storms on the face of the sun -- so-called sunspots -- have all but disappeared in recent years. On Sept. 3, NASA reported: "The sun is in the pits of the deepest solar minimum in nearly a century. Weeks and sometimes whole months go by without even a single tiny sunspot. The quiet has dragged out for more than two years." Astronomer Dan Joyce reports the last Earth-sized sunspot -- something which occurs with some frequency in typical years -- occurred in December 2006.
Some researchers believe lengthy solar minima triggers global cooling. Others dismiss solar cycles as having little more than a modest effect on the planet's climate. Global temperatures continue to run above normal, but there's no question subnormal summer temperatures have occurred  over much of the U.S., though the West and Southwest U.S. have baked in record warmth.

Mike Tannura sends us this great "before and after" sequence of clouds rolling in off the lake Friday morning as seen from his lakeside condo. Tannura explains: "This morning has been particularly neat -- sunny, then foggy, then clouds whizzing around the building, then sunny." Thanks Mike for the great pictures!

Virginia Drozek of Elgin sends us these pictures of a foggy yet beautiful sunrise in Sugar Grove in the past few days. Thanks Virginia for sharing these wonderful images with us!

They are amazing, spectacular and awesome but were not discovered until just 8 years ago.  Scientists have captured photographs of upward shooting lightning that starts at the top of thunderstorms and stretches all the way to the outter edges of our atmosphere.  They are called "gigantic jets" and they transfer charged particles from the top of thunderstorms to the top layer of our atmosphere, the ionosphere.

As amazing as they are, they were discovered by accident.  Check out this article from LiveScience for more information.

Signs of autumn are beginning to appear. Little surprise---the past two weeks have been extraordinarily cool. Chicago's mean temperature of 63.8-degrees from Aug. 21 through yesterday (Sept. 3) was a stunning 7.6-degrees below the 139 year average---chilly enough to rank 2nd coolest for the period. The cool air has apparently convinced some trees, especially in the west and northwest suburbs, to put on a bit of autumn color early. But cool as it's been, warm weather is hardly history just yet. Nearly a quarter of the area's 70-degree-plus days occur beyond this date. And a delightful Labor Day weekend appears to lie ahead---the one wildcard being the amount of Atlantic moisture which rides southeast winds into the area from the Carolinas late toward Monday. While sunshine is to be abundant Saturday and fairly widespread Sunday---when mixed clouds become a bit more extensive ---several computer forecast models suggest atmospheric moisture levels are to approach saturation from just above the surface up to 6,000 ft. Monday. It's a forecast which suggests significantly increased cloudiness at that time. Such a setup would be capable of producing  sprinkles in spots--- possibly even a shower or two. In advance of Monday's clouds, a bit of instability---i.e. a set-up in which temperatures decline quickly with altitude---reaches the south and far western suburbs Sunday afternoon. In combination with daytime heating, this could set off an isolated shower or thunderstorm---but dry weather would likely dominate most of the area.
 
Sunlight's down as days shorten
 
 Sunlight shines down on Chicago 2.2 fewer hours than on June 20 when summer began. Not only will another 1.6-hours of daylight disappear as days continue to shorten over the coming month, the sun will continue to trek across the sky at a lower angle.

Dear Tom,
Hurricane Bill made a landfall in Newfoundland last month. How rare is that?
Brian Rohde, Tinley Park

Dear Brian,
A hurricane affecting Newfoundland is not as rare as you might think. Its North Atlantic location puts it directly in the path of northward moving tropical cyclones that skirt the East Coast of the U.S.

According to Environment Canada, Newfoundland leads all Canada with a tropical cyclone impact every 1.4 years followed by Nova Scotia with an encounter every 2.4 years. Most of the storms hitting Newfoundland are in a decaying mode, a result of the surrounding cold water. However, one of the deadliest Atlantic hurricanes on record hit eastern Newfoundland on Sept. 9, 1775. More than 4,000 sailors perished in the storm that brought 20-30 foot seas to the area.

Thursday's easterly winds carried these low-level "scud" clouds off Lake Michigan into the downtown area Thursday---a process which Bob Winkelmann captures so beautifully in this shot taken looking south down Lakeshore Drive toward the Michigan Avenue Exit.  Often, the downtown buildings produce an "orographic lift" effect, much as mountains do.  That what happens when incoming winds encounter a feature which causes the air to rise and cool.  When the air being lifted is near saturation, the cooling induced by lifting the air is just enough to saturate the air and produce the water droplets of a cloud. It's one reason clouds can form over the downtown area while surrounding skies remain clear.  This is a terrific shot, Bob!  THANKS so much for sharing it with us!
 
Tom Skilling

Damon Frugoli shares these shots of a shallow layer of ground fog which hovered just above the wetlands behind his home in northwest suburban Gilbert, Illinois---a bit northwest of Elgin. It's a scene that has no doubt been repeated many times in recent cool mornings given the calm which has prevailed within the high pressure which has been a fixture of our weather the past six days!  Great shots, Damon!  THANKS very much!!

Tom

Signs the El Nino underway in the equatorial Pacific may already be having an impact northern hemispheric weather are growing. While difficult to establish a direct link between the cool temperatures which have dominated the Chicago area over the past few weeks and the developing El Nino between South America and the Australia, it's worth noting cooler than normal September through November periods have been El Nino hallmarks here.

An in-house analysis of 17 El Ninos since 1950 indicates 14 of them have featured below normal temperatures during the three month meteorological fall period. In the past, falls have also displayed a modest tendency toward wetter than normal weather. Each month from September through November has produced precipitation tallies above the long term average during El Ninos. It will be interesting to see if a wetter, more humid weather pattern predicted to begin taking hold late in the upcoming holiday weekend turns out to be the opening salvo of a wetter autumn weather regime.

Other signs that El Nino may already be at work include the limited number of tropical storms and hurricanes which have occurred in the Atlantic at the same time the eastern Pacific hurricane season has been on overdrive. Powerhouse Hurricane Jimena is just the latest tropical cyclone to sweep that region.  The storm weakened dramatically as it encountered cool waters off Mexico's Baja Peninsula where it made landfall during the day. Wednesday's 73-degree high was up one degree from the day before. It was only the fourth time since June 1 Chicago has received 100 percent of its possible daily sunshine.

Dear Tom,
Did this summer crack the top ten of coolest summers in Chicago?
Tim Guimond
 
Dear Tim,
This summer was cool, with just four official days in the 90s and about triple the typical number of sub-70 days; but not only did it not crack the top 10, it didn't even make the top 20. The summer of 2009 with its average temperature of 69.2 degrees goes into the record books as the city's 21st coolest on record and the coolest since 2004 (68.6 degrees). Chicago recorded its all-time coolest summer way back in 1875 (66.6 degrees) at a time when the official thermometer was located downtown near Lake Michigan. In fact, 16 of the 20 cooler summers were recorded before the official thermometer was moved to a warmer inland location in 1942. Since that time, only the summers of 1982,1985, 1992 and 2004 have been chillier than this past summer.
 

Check these out! These incredibly timely shots of California's devastating wildfires come to us from Mary Novak of Flossmoor who took them on a flight back to Chicago from Burbank Tuesday. In her e-mail accompanying the shots, Mary tells us:
 
"As you can see, the California fires are causing much smoke and some unusual cloud-like formations."
 
These are terrific, Mary--THANKS very much for sharing them with us!
 
Tom Skilling
 









Photos courtesy of Mary Novak, Flossmoor, Illinois

There have been some fascinating images of fire tornadoes or fire whirls inside the California wildfires that have burned over 120,000 acres.  These swirling columns of intense heat are an additional hazard for the nearly 6000 firefighters are these swirling columns of intense heat.  Click on the picture below for video of an impressive fire vortex.

 

The surface in wildfire areas can exceed 2000 degrees.  This causes columns of air that rise quickly upward, commonly called updraft columns.  Some of these columns begin to rotate and results are spectacular but extremely dangerous.  The 1923 Great Kanto earthquake in Japan sparked a huge fire whirl that killed 38,000 people in fifteen minutes.

 

For some amazing pictures from the California fires, check out this gallery from the Los Angeles Times.

Chicago enters a fifth day in the same slowly stagnating air mass. The strongest upper steering winds continue circumventing it---traveling across Canada and the U.S. Deep South, bypassing the Midwest. This provides the sprawling fair weather system little impetus to move. It sits in place, warming a few degrees each day in the September sun. The atmospheric setup is to remain locked in place into the coming weekend assuring a generous supply of sunshine is to keep coming. The air mass is going to become hazier as particulates and other pollutants accumulate over coming days. Moisture begins seeping into the area over the weekend as Gulf and Atlantic moisture creeps north. It's a situation  likely to increase prospects several showers and thunderstorms may bubble up later in the Labor Day Weekend.

Recent mornings have been extraordinarily cool away from the city's heat island---the dome of warm air which hugs urban areas at night. While city lows held to 60-degrees at Northerly Island early Tuesday and in the mid to upper 50s across much of the city, the bottom dropped out west of the area. Huntley recorded 39 degrees early Tuesday after a 38-degree low the morning before.

September is a month of shorter days and declining temperatures. Normal highs retreat from 78 degrees Sept. 1 to 69-degrees on Sept. 30. Days shorten 80 minutes and the three month meteorological autumn period (September through November) will see 3.2 hours of daylight disappear.  Normal high temperatures slide from 78-degrees Sept. 1 to just 40 on Nov. 30. But, Chicago's warm weather isn't entirely over.  Nearly a quarter of the city's 70-degree and higher temperatures have historically occurred beyond Sept.2.
   

El Nino impacting 2009 Atlantic and eastern Pacific hurricane season--but in very different ways
 
Wind shear---the change in wind speed and direction with height--- over the Atlantic Basin during El Ninos, like the one currently underway, can disrupt tropical storm and hurricane formation. Such shear along the East Coast is likely to make it difficult for Tropical Storm Erika, which formed in the Atlantic Tuesday,  to seriously impact the East Coast. By contrast,  eastern Pacific tropical activity often soars in El Nino years---Hurricane Jimena, with 115 m.p.h. top winds late Tuesday as it churned into Mexico's Baja peninsula is this year's latest example.

Dear Tom,
I have heard that an  El Nino is now occurring. What are the likely weather consequences?
David Liu

Dear David,
El Nino, a warming of tropical Pacific Ocean waters, causes changes in planetary wind patterns that have major effects on global weather. When strong El Nino events occur, as in 1997-98, temporary climatic disruptions are widespread and, in some locations, the consequences are devastating.
 
Climatologists estimate the 1997-98 El Nino brought abnormal weather to 80 percent of the Earth. At Chicago, the "El Nino winter" of 1997-98 was exceptionally mild and relatively storm-free, but elsewhere there were weather disasters. A few examples: Floods swamped California, Peru and central Africa; droughts ravaged southeast Asia, Australia and Brazil. Worldwide, the 1997-98 El Nino was responsible for 2,100 deaths and $33 billion in property damage.