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.
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.
Thunderstorms exploded to life along a narrow 20 mile wide corridor from Cook County southeastward into northwest Indiana midday Wednesday. An inland-moving lake breeze front was the meteorological culprit. It ushered easterly winds off Lake Michigan into a hot, humid air mass dripping with 1.50 inches of evaporated water. The inland racing flow encountered a windless tropical air mass, producing a pile-up of air which was forced to rise and cool. Thunderstorms soon resulted and towered above 50,000 feet unleashing torrential downpours. As much as 2.12 inches was measured at the Weather Bug rain gauge at Frazier Magnet School in Chicago as 63 m.p.h. winds raked Midway Airport and floodwaters submerged streets and basements in Cicero. The storms erupted over the same corridor repeatedly between 1 and 3 p.m.---a process referred to as "training." They generated more than 1,700 cloud to ground lightning strokes in just over two hours.
Heat enters third day Thursday; more storms due
Three consecutive 90-degrees have occurred at this early stage of the summer season in only one of three years at Midway Airport since 1928.
The mercury reached 90 degrees in Chicago for the first time in 2009 at 12:09 p.m. Tuesday on its way to the warmest June high temperature in the city in four years.
The 94-degree high at O'Hare International Airport equaled last summer's hottest temperature.
This year the heat is late to arrive in Chicago. Since 1928 at Midway Airport, an average of five highs of 90 degrees or warmer have occurred by this date.
Heat fuels Iowa storms
Near triple-digit heat in Iowa fueled powerful thunderstorms with cloud tops that towered to 57,000 feet and sent temperatures diving more than 20 degrees while producing 60 m.p.h. wind gusts and numerous reports of large hail.
The storm cluster began Monday evening in Colorado and is the latest in a series to rotate around the periphery of a dome of hot air in what meteorologists refer to as a "ring of fire" pattern.
Tuesday's 67 degree high made it the coolest June 9 in 11 years, a reading more typical of early May and 11 degrees below normal.
Temperatures through the opening nine days of June are running 11.5 degrees below the same period a year ago, ranking June's opening among the 20 coolest here in 139 years of weather records. Myriad factors drive weather patterns, but it appears the persistence of wet weather through spring--it was the wettest spring on record at Midway Airport--on the heels of the wettest year on record in 2008 has produced a huge swath of wet soil across the Midwest. Wet soil inhibits warming by contributing to increased cloud cover and rain. Chicagoans have seen only 40 percent of the area's possible sunshine--well below the 68 percent considered normal.
Twisters and hail
Thunderstorms slammed sections of Kansas and Missouri producing seven twisters, more than 3 inches of rain and bombarding some locations with windshield-shattering hail. Sleeper, Mo., was hit with 2.75-inch diameter hail.
Monday's thunderstorms hit in several waves. Downpours in pre-dawn
storms totaled more than 2 inches (heaviest at DeKalb, 2.20 inches and
Dixon 2.05 inches), prompting flood warnings in the west and northwest
suburbs.
Afternoon storms hit hardest in northern counties and
areas of southern Wisconsin. The strongest storms developed in
southeast Wisconsin towering to heights of 45,000 feet. Ping-Pong-ball
size hail struck near Mukwonago and high winds toppled trees. Several
funnel clouds were sighted in the area. Blinding downpours also
accompanied the Wisconsin storms, with nearly an inch of rain flooding
streets in the Janesville area.
The storms formed on the heels
of the area's most humid air mass since last September. Dew points, a
measure of atmospheric moisture approached 70, readings typical along
the Gulf coast.
Cooler weather will dominate the rest of the
week, with another round of rain and thunderstorms on Wednesday. Warmer
weather will return by the weekend, with highs returning to around 80
degrees.
Friday's bright sunshine, aided by gusty west winds that overwhelmed any attempt by the cool air hovering over the lake to surge ashore, produced the city's warmest temperature readings of June. Temperatures peaked at 80 degrees at Midway Airport for only the sixth time this year-equal to the number of 80-degree days by this date a year ago, but only half the 12 considered average.
Alaska has had more warm weather this June than the Chicago area. Fairbanks has recorded highs of 70 degrees or higher every day so far this month while Chicago has only been able to muster two.
Frost advisories for the North Woods
Temperatures will drop into the 30s Saturday morning across portions of the upper Midwest from northern Minnesota to far northern Wisconsin and Upper Michigan, where rare June frost and freeze advisories were issued. Another chilly night is expected Saturday night with lows near 40 degrees.
It hasn't been as warm as Friday's predicted 76 degrees in nearly a week.
The
break in this week's chilly weather is welcome after one of the seven
coolest June 1-4 periods here in the last half century. But, the warmth
wanes Friday night and Saturday with a cold front passage and wind
shift to the northeast. And the potential for downpour-generating
thunderstorms for least a portion of Sunday calls into question just
how much warm air may return to the area that day.
Weekend
rains won't be continuous. But when precipitation does occur, it may
arrive within clusters of thunderstorms capable of downpours,
especially Sunday. By the time the developing wet pattern exits
Tuesday, 2 or more inches of rain may have fallen here.
Storms erupt in record Pacific Northwest heat
Seattle
baked at 91 degrees Thursday--a record. Heat in the region produced
uncharacteristic severe thunderstorms, with 70 m.p.h. gusts and 0.71
inches of rain in 20 minutes near Harrisburg, Ore. Fairbanks, Alaska,
hit 83 degrees.
Chicagoans can bid farewell to the area's coolest early-June weather in
eight years Thursday. Not since 2001 has a June opened here with
back-to-back readings as cool as Tuesday's 60 degrees and Wednesday's 61
degrees. Wednesday's high made it one of the six coolest June 3 since
1928.
Unlimited sunshine is to boost Thursday afternoon readings by 7 degrees
with far more modest northeast winds likely to limit the reach of lake
cooling to areas within a mile or two of the lake. Southwest winds
Friday are predicted to send temperatures into the mid-70s-warmth likely
to spread to the lakefront.
80-degree days ahead
Though warm temperatures have been in short supply of late, warm weather
enthusiasts should take heart. Weather records reveal a near 100 percent
chance that the coming month will bring five or more 80 degree or higher
temperatures. The first may occur as early as Sunday if thunderstorms
don't impede the northward movement of a warm front.
It's the third day of meteorological summer-but it certainly doesn't
feel that way. Wednesday's predicted low 60s would be right at home in
mid-April. Highs as chilly as Tuesday's 60 degrees at O'Hare
International Airport and 61 at Midway Airport have occurred this late
in the season only 29 of the past 81 years (or, on average, about a
third of the time).
Though it's far too early to write off summer heat, many Chicagoans
sense warm seasons haven't been as hot as they once were. It's a
proposition we investigated-and there's more than a shred of truth to
it. While Chicago's overall June-through-August average temperatures
have risen slowly the past five decades (from 72.5 degrees in the 1960s
to 73.9 degrees in the 1990s), the number of truly hot 90+-degree days
has waned. Since 2000, the annual average of 18.3 days at or above 90
degrees at Midway is down from 23.2 days in the 1980s and 21.1 days in
the 1990s. Summer days have cooled an average of 0.46 degrees since the
1990s while summer nights have warmed 1.64 degrees during the same
period.
The weather of two seasons visited Chicago on Monday, setting the stage for downpour-generating thunderstorms that prompted a late-day weather watch.
Waves of thunderstorms towering 44,000 feet into the atmosphere unleashed local rains of 1 to 2 inches and flooding was reported in areas near Rockford and Belvidere, where 4 to 6 inches of water covered some roads late Monday.
Meantime, severe thunderstorms hit east central Illinois, producing a reported tornado touchdown near Hoopeston.
The wet pattern that dominated the just-completed meteorological spring is to continue. The past two weeks have seen 2.23 inches of rain fall at O'Hare--almost a half inch more than normal for the period. A suite of computer models suggest about the same amount of rain the next two weeks.
As the weather system moves off to the east, showers will gradually diminish Wednesday morning. Cooler drier air will feed into northeast Illinois and persist the remainder of the workweek.
The next chance for significant rains occurs on the weekend.
