A sunny and warm Sunday afternoon turned showery late as rains developed along a wind-shift line moving southeast out of Wisconsin. Though the showers were low-topped and produced little thunder and lightning, they did spawn numerous funnel clouds west of the city along a corridor between Interstate Highway 39 and the Fox Valley. These funnels were "cold air-type funnels," the kind not associated with severe thunderstorms that almost always dissipate without producing any damage.
After a chilly 4th of July, temperatures rebounded into the comfortably warm lower 80s Sunday, a level expected to repeat here through midweek. Hot weather has been noticeably absent from the city since a streak of 90s in late June, but the latest suite of computer forecasts hints at a brief surge of hot weather that should reach the city by Friday. The downside to the expected warm-up will be a threat of showers and thunderstorms that will continue to add to the city's growing 2009 precipitation total that has now reached 26 inches -- nearly three-quarters of the city's normal annual total of 36.27 inches.
Chicago's 4th of July weather was a bummer: cool and rainy. The official high of 69 degrees at O'Hare marked the first time since 1997 (66 degrees) that the holiday failed to break 70, and the 0.20 inches of rain was the most since 1.72 inches fell in 1995. The only positive note was that most of the rain ended during the early evening, a few hours before scheduled holiday fireworks. Warmer weather is on the horizon as a persistent dome of hot air that has been baking the central and southern Plains and lower Mississippi Valley with record-breaking heat appears to be making a move toward the area. Triple-digit heat shattered 4th of July temperature records in Texas Saturday, led by a 107-degree high at McAllen. Chicago-area temperatures are expected to surge into the 90s by Thursday and again on Friday for the first time in two weeks. Gulf-level dew points in the 70s will accompany the heat, assuring that recently silent air conditioners will be humming again. The heat will also bring another round of thunderstorms -- adding to area rain totals that are nearly 9 inches above normal for the year.
Eastbound low pressure, which first developed late Thursday with an eruption of thunderstorms across the western Plains, is behind Saturday's gray, cool weather so reminiscent of much of this area's spring and early summer. Blazingly hot air charged with a huge supply of tropical moisture is fueling Saturday's cloud and shower-producer here. But, its heaviest downpours are to drench Missouri, Downstate Illinois and Indiana with thundery deluges that may deposit local rains of 4 inches or more. More conservative totals appear a good bet in the Chicago area. An average of 30 widely varied computer rainfall estimates suggests precipitation here may average 0.61 inches -- though individual projections range from as little as 0.02 inches to as much as 2.61 inches. Summer rains are fickle and often widely varied, which supports the huge spread in projected rain totals. With clouds expected to limit temperatures to levels more than 10 degrees below normal and winds off the lake likely to limit shoreline highs to the 60s, Saturday may end up the area's coolest in 12 years.
Friday afternoon's predicted 80-degree temperatures over much of the metro area will mark the first time this week it's truly felt like summer. Many Chicagoans have openly voiced disappointment over the recent succession of lackluster daytime highs which have been more reminiscent of May than late June and early July. But increased sunshine Friday is to allow temperatures to surge. It's a move that finally returns temperatures here to seasonable levels.
Storm clouds loom Friday night into Saturday over sections of the Midwest. Thunderstorms that flared late Thursday over the Plains (with cloud heights towering as high as 57,000 feet and prompting a series of severe weather watches) are targeting sections of Iowa, Missouri, Downstate Illinois and Indiana.
A suite of computer projections has shifted these storms progressively farther north in recent days. Individual projections of potential Chicago rainfall late Friday night into Saturday varies widely across 15 models. The average of these forecasts calls for a total of 0.52 inches. A consensus of these forecasts places the axis of potentially heaviest rainfall across central Illinois and Indiana.
Much improved weather is due Sunday with sunshine allowing temperatures to surge back into the 80s in all immediate Lake Michigan shoreline locations where upper 70s are likely. Evidence that hot weather is preparing to stage a comeback later next week continues to mount. A dome of hot air is predicted to become established over the nation's mid-section by Thursday---a development which may well produce the Chicago area's next round of 90-degree temperatures. The heat could lead to increased rainfall. Rainfall estimates in the 1-2 week range here passed two inches in several computer projections, evidence a sporadically stormy "ring of fire" pattern could take shape, sending a succession of thunderstorm clusters running along the northern flank of the predicted dome of hot air across the Chicago area with some regularity.
What has happened to summer? That was the question from many area residents Wednesday amid May-level 60-degree temperatures. The day's high of 65-degrees marked the chilliest open to a July here since 1930 and was one of the three coolest July 1 readings on the books in 139 years of weather records since 1871. Summer temperatures at that level are truly rare. Of 7,452 meteorological summer (June through August) highs on the books since 1928 at Midway, only 184 of them--just 2 percent---have registered temperature as cool or cooler.
Scattered lake-enhanced rain showers amid the Wednesday's chill lowered cloud bases in the downtown area, obscuring the tops of skyscrapers while producing periods of upper 50-degree temperatures.
While Chicago missed the July 1 record low maximum of 61-degrees set 1904 and 1924, record low daytime highs occurred at Rockford (65-degrees) and downstate at Lincoln where the high was just 70-degrees.
Thundery downpours could be part of the holiday weekend downstate
Thunderstorms expected to erupt in the Plains later Friday threaten to track east/southeastward into sections of downstate Illinois and Indiana where they may produce downpours totaling 2 or more inches for a portion of the upcoming July 4 holiday weekend.
The coolest late June air mass in decades spills over into Wednesday, but it is to loosen its grip on the area later this week in time for the 4th of July holiday weekend. The abnormally cool 65-degree high predicted Wednesday would deliver the Chicago area its coolest July opening in 79-years--since a 65-degree reading in 1930. A temperature at that level at this point in the season is more typical of early May and 18 degrees below normal for the date. It comes on the heels of the coolest June close in 23 years. Clouds and northwest winds were so pervasive that highs struggled to reach 66 at O'Hare International Airport. Northern Wisconsin and Upper Michigan highs failed to make it out the 50s. Rockford's 65 degree high Tuesday broke a record for the coolest June 30.
Cloudiest June in 4 decades
June 2009, which ended at midnight, closed as the Chicago area's cloudiest in the four decades since 1969, having only 51 percent of its possible sunshine. A typical June sees 68 percent.
A mass of cool air more typical of May than late June is in firm control of Chicago's weather and appears to have the area headed for one the coolest July openings in 79 years. While temperatures struggle to a high of 70 Tuesday, the predicted 67-degree high Wednesday would make it the city's coolest July 1 since a 65-degree reading in 1930.
The temperature pullback is in dramatic contrast to last week's heat and humidity, which featured a string of three 94-degree highs. The cool pool of air is of Canadian origin, with a faster than normal vertical temperature decline. Such air masses are said to be "unstable" because milder air remains buoyant as it ascends into the chilly air aloft. It's a process that may produce widely scattered light showers Tuesday and Wednesday.
Quiet to the north
Not a single report of severe weather occurred in May and June across northeast Wisconsin north into Upper Michigan. It's the first time that's happened since severe weather records started there 30 years ago.
Much drier and cooler air follows the thunderstorm-producing cold
front that passed through
Chicago overnight. Severe thunderstorm
warnings were issued for counties west of the metro
area with a 70
m.p.h gust reported in Franklin Grove, Lee County and 60+ m.p.h. gusts
reported at Ottawa, Mendota, and Earlville in LaSalle County. A strong
low pressure center is
forecast to develop and intensify over the
eastern Great Lakes early this week with the
resulting northwest flow
pulling much colder air into the western Great lakes and Midwest.
Clouds associated with the low will spread out far to the west,
covering most of northern
Illinois as well as a good portion of
Wisconsin, Michigan and Indiana. Showers, perhaps even
a few
thunderstorms are possible both Monday and Tuesday under this
unseasonable setup
which should see high temperatures some 10 to 15
degrees below normal.
Warmer, closer-to-normal readings later in the week
As the low pressure system slowly moves northeast, clouds will break
and temperatures are
expected to slowly modify later in the workweek.
Friday's break in the three-day string of 94-degree highs Chicago area residents sweltered through Tuesday, Wednesday and Thursday, ends briefly Saturday afternoon and evening when a resurgence of heat is expected to result in the week's fourth 90-degree-plus high--and set the stage for potentially powerful thunderstorms expected to hit with a cold front Saturday night.
Friday's official 86-degree high at O'Hare International Airport occurred at 11:29 a.m. before readings declined slowly through the afternoon as northeast winds delivered cooler air.
Perhaps even more significant was Friday's humidity pullback. Dew points--a measure of atmospheric moisture--dropped 15 degrees from the low 70s Thursday to the mid-50s late Friday.
Changes ahead
Big weather changes loom--including the threat of potentially severe storms Saturday night--as
temperatures fall to the low to mid-80s Sunday then the low to mid-70s Monday and Tuesday.
The first break in the heat and humidity of the past three days rides into the city on northeast winds Friday.
The air's moisture content retreats from levels more typical of the Gulf of Mexico to those more more closely identified with the Midwest's North Woods region.
Thursday's 94-degree high was the third in as many days, making the three-day spell Chicago's hottest in nearly three years.Only two other occasions since temperature readings have been taken at O'Hare International Airport have highs as warm or warmer than those observed the past three days occurred this early in the summer.
State of the lake
Lake temperatures have warmed to 2009's warmest levels. Satellite estimates of Lake Michigan's average surface temperature now stand at 68 degrees. And the U.S. Army Corps of Engineers reported Thursday that lake water levels are 10 inches higher than a year ago.
Thursday evening threat fizzles under "nose" of powerful jet stream
Heat and humidity as high as Thursday's typically facilitate thunderstorm formation. But area residents caught a break Thursday. Thunderstorms to the city's north and west--including one with 62 m.p.h. gusts at Whitewater, Wisconsin, dissipated as they moved in Chicago's direction, once they encountered storm-extinguishing sinking air beneath the nose of a pocket of
powerful jet stream winds.
