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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
