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.
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.
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.
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.
