EXPLAINER: May 2008 Archives

Today is the first day of meteorological summer (June-August), and it is expected to
usher in a week of generally above-normal temperatures that will probably extend well
into the following week. A west-to-east jet stream flow aloft over the United States is a
good indicator that colder Canadian air will be held well to the north. Illinois will rest on
the northern flank of warm air centered over the southern United States. Expect brief
clearing intervals between extensive periods of cloudiness and showers and
thunderstorms associated with frequent low pressure systems.

SEVERE STORMS TARGET THE MIDWEST

The National Weather Service Storm Prediction Center has indicated several outbreaks of
severe storms are possible across the Midwest this week. Chicago is included in projected
potential severe storm areas Tuesday and again later Friday.
--By Paul Dailey, WGN Weather Center Meteorologist

Wild weather hammered sections of the Chicago area in stages Friday. The day began
with a seiche along Lake Michigan. Seiches occur when lake levels gyrate rapidly in
short periods of time, and they are produced when fast-moving squall lines push
domes of water into the Michigan shoreline. The waves are reflected back to Chicago
and when this happens, produces the oscillating lake levels. Friday's seiche, which
occurred between 6 and 7 a.m., involved 26-inch variations in just minutes -- changes
amplified in spots by the shape of the shoreline.
Then, powerful winds gushed out of collapsing thunderstorms late morning and
midday. Hardest hit were sections of Lake and McHenry Counties where winds gusted
above 60 m.p.h., snapping trees and power lines and flipping a semi on Interstate
Highway 90 in McHenry County.
The day's third wave of storms bombarded the far southern suburbs near Kankakee with
huge hail the size of tennis balls. The hail fell so prolifically from 57,000-foot-tall
storms, it covered the ground. Torrential rainfall at Milford in Iroquois County -- 60
miles south of Chicago -- generated more than 6 inches in just two hours. A series of
tornado touchdowns Downstate on Friday included one in Springfield.
--Tom Skilling, Chief Meteorologist, WGN-TV/Chicago Tribune

Towering thunderstorms bombarded the Plains and western Midwest on Thursday,
spawning dozens of reports of twisters in Nebraska and Kansas and unleashing
drenching rains and tennis-ball-size hail as far east as Iowa. A powerful new eastbound
spring storm was behind the latest severe weather outbreak. The system threatens the
Chicago area with its own share of weather woes Friday.

Evening Doppler radar scans in the region put the blockbuster storm's cloud tops at
60,000 feet -- nearly twice the cruising altitude of jetliners. Rainfall hit 4 inches
northwest of Des Moines at Jefferson, Iowa, and topped 3 inches at O'Neill, Neb. By late
evening, 10 states from eastern New Mexico to Iowa were under tornado, thunderstorm
and flash-flood watches.

ONLY 2 DAYS LEFT IN COOLEST METEOROLOGICAL SPRING OF LAST 6

With the clock ticking on the March through May meteorological spring season,
Chicago temperatures are more than a degree below normal and more than 4 degrees
cooler than a year ago. It's the coolest spring here in six years.

--Tom Skilling, Chief Meteorologist, WGN-TV/Chicago Tribune

It's unlikely daytime temperatures are going to flirt again with 50 degrees -- as they
did Tuesday -- anytime soon. Chicago readings rebounded to 62 degrees Wednesday
and are predicted to surge to 73 degrees Thursday and 80 degrees Friday. The big
atmospheric changes behind the warm-up show no sign of abating in the coming two
weeks -- a period in which nearly all daily average temperatures are likely to finish near
or warmer than normal. It's a welcome change which more nearly parallels the warmth
observed here a year ago when the high reach 88 degrees.

The three month "meteorological summer" season begins as June arrives Sunday --
and not a moment too soon. Three of four days this month have been cooler than
normal.

Storm development is accompanying the warm-up to Chicago's west. In New Mexico,
baseball-size hail (2.75 inches in diameter) bombarded Conchas beneath
54,000-foot-tall thunderstorms.

SUNNY WEDNESDAY ARRIVES ON HEELS OF 6 CLOUDIER THAN NORMAL MONTHS

Wednesday hosted 100 percent of its possible sunshine -- only the second time this
month skies have been completely clear. Sunshine has fallen short of historic norms
each of the past six months.

--Tom Skilling, Chief Meteorologist, WGN-TV/Chicago Tribune

If Tuesday's jarring temperature pullback seemed harsh even by often finicky May
temperature standards, weather records confirm your suspicion. Not only was the day's
50-degree high the coolest to occur this late in a season since measurements at O'Hare
International Airport began nearly half a century ago (since 1959), the 31-degree
plunge between Monday and Tuesday was one for the books too. A one day high
temperature drop of that magnitude has occurred at O'Hare in May only two times
before. The month is a whopping 7.8-degrees behind the same period a year ago and
ranks 35th of the past 138 Mays placing it among the 25 percent coolest on record here
since 1871. Only eight Mays in the last 50 years have been cooler.

DOWNSTATE FARMERS FORCED TO REPLANT IN SWAMPED FIELDS

Unwelcome rain fell in Downstate Illinois on Tuesday where many farmers have
struggled to plant the year's corn crop and face replanting due to flooded fields.
Goreville and Creal Springs -- both near Marion -- recorded 1.97 inches. The region
has seen a foot more rain this spring than a year ago.

--Tom Skilling, Chief Meteorologist, WGN-TV/Chicago Tribune

The chill has returned. Gone are Monday's well-timed 80s which, in combination with
generous sunshine and southwest winds, pushed the warmth out past the Lake Michigan
shore, where temperatures hit 83 degrees in Lincoln Park. The sunshine and warmth
provided ideal conditions for Memorial Day barbecuing.
Midway Airport's 84-degree high was the city's warmest thus far in 2008 and only the
third 80-degree-plus high of the year. Over the past half century, just four years have had
fewer 80s by this date. The warmest area highs included 85 degrees at southwest
suburban New Lenox and 84 at both Wheaton and Orland Park.
Sinking air in the center of the continent beneath two merging jet streams is behind the
development of a sprawling high pressure responsible for Tuesday's strong northeast
winds, and is likely to keep cool temperatures in place through midweek.
--Tom Skilling, Chief Meteorologist, WGN-TV/Chicago Tribune

A potent weather system that will be moving through the Chicago area Monday blasted
areas to the west with numerous tornadoes and large hail late Sunday.
Several twisters swarmed northeast Iowa, north and east of Waterloo, damaging homes in
the area. At least one person was killed when a twister leveled at least 50 homes in Hugo,
Minn., just northeast of Minneapolis. Hail as large as baseballs accompanied the tornadic
storms.
The storm system, feeding on warm, humid air sweeping north into the Mississippi Valley,
produced nearly 400 severe weather reports through 9:15 p.m. Sunday, including at least
35 twisters.
SEVERE T-STORM THREAT MONDAY
Tornado watches were posted for northwest Illinois Sunday evening, and the remnants of
these storms will pass through northeast Illinois early this morning. More storms should
develop in today's warmth and humidity and could become severe, especially south and
east of the city.
--By Steve Kahn, WGN Weather Center Meteorologist

There has been a dearth of 80-degree days in Chicago this year, with only two on the
books as compared to 13 by this time last year. However, all that is about to change as
gusty southerly winds send temperatures soaring into the lower 80s across the
metropolitan area Sunday -- just in time for the Memorial Day holiday.

After a week dominated by chilly northeast winds and sub-60 degree highs, the city is
about to enjoy a warm period well suited for outdoor activities. A storm system slowly
approaching from the Plains will feed the warmth along with increasing humidity,
setting the stage for gusty, possibly severe thunderstorms on Memorial Day, though a
few storms could arrive as early as Sunday evening.

Highs should top out around 80 degrees again on Memorial Day, though readings will
drop in the afternoon after the rain arrives.

BRIEF MIDWEEK COOLDOWN, THEN ANOTHER WARM WEEKEND

Cooler weather will settle in midweek with readings in the 60s as northeast winds
return. However, temperatures will rebound by next weekend as southerly winds bring
another round of temperatures in the 80s and the possibility of showers and
thunderstorms.
--By Steve Kahn, WGN Weather Center Meteorologist

Gargantuan thunderstorms -- some 66,000 feet tall -- unleashed swarms of tornadoes
on the western Plains for a second consecutive evening Friday. By nightfall, NOAA's
Storm Prediction Center had tallied nearly three dozen reports of touchdowns. At one
point, a large tornado was reported by storm chasers on the ground just 9 miles
southwest of Greensburg, Kan. -- a community demolished by a twister last May.
Northbound hot air -- which sent readings in Texas soaring to 107 degrees at Laredo,
102 at McAllen, and 100 at Midland and Austin -- energized the storm outbreak, and is
to send Midwest temperatures surging Sunday. If thunderstorms remain north and west
of Chicago on Sunday, the year's highest temperature is within reach -- and the warmth
is to extend into Memorial Day, though powerful thunderstorms may erupt in the
afternoon and evening.
MAY'S CHILLY REPUTATION GROWS
Friday marked the 9th time this month Chicago failed to reach 60 degrees. The
57-degree high was the coolest for a May 23 in 21 years and was in stark contrast to
the 89-degree high a year earlier. The month is now about 8 degrees below a year ago.
Only four Mays since 1980 have been cooler.
--Tom Skilling, Chief Meteorologist, WGN-TV/Chicago Tribune

By Sunday, when south winds bring 80s to the Chicago area, the chilly easterly winds
predicted to produce the month's ninth high below 60 degrees Friday will be a memory.
Only five other Mays over the last half century (since 1958) have produced as many or
more days this cool.
When it comes to early season warm weather, easterly winds are truly the bane of May
here, blowing 50 percent of the time off Lake Michigan's chilly waters. There's been
only one 80-degree high this month back on May 6. Only 11 years since 1928 in the
city have had so few 80s.
Dramatic changes loom here. From early Saturday's isolated mid 30s in the coolest
outlying locations -- temperatures low enough to produce a bit of patchy frost toward
the Fox Valley and areas far west -- warming begins in Saturday's nearly unlimited
sunshine. And by Sunday, powerful south winds and surging humidities should push
temperatures well into the 80s.

PLAINS HIT BY DEADLY SEVERE WEATHER; MORE ON THE WAY THERE
The western Plains were punished by waves of powerhouse thunderstorms responsible
for at least three dozen reports of twisters. At least one turned deadly at Windsor, Colo.
--Tom Skilling, Chief Meteorologist, WGN-TV/Chicago Tribune

Chicago's beaches open Friday but cool temperatures riding northeast winds will
hardly have area residents dashing for chilly lake waters. Lake Michigan is 1 degree
cooler than a year ago and at its chilliest levels in four years. But huge changes are
under way that have air masses across North America on the move. Powerful winds--
clocked as high 105 m.p.h. at the 4,900-foot level of Yucca Mountain and 63 m.p.h.
at White Sands, N.M., sent record triple-digit heat in the Southwest packing
Wednesday, whipping visibility-reducing dust into the air. Visibilities near El Paso
dropped to near zero in blowing dust while 50 to 60 m.p.h. gusts at San Simon, Ariz.
made it impossible to see much more than 100 feet.

SUNDAY'S HIGH TEMPERATURES HEADED WELL INTO THE 80s

A sharp temperature rebound, driven by 30 m.p.h. southerly winds and
compressional warming produced as air sinks and compresses beneath the nose
(leading edge) of a powerful jet stream set the stage for a stunning turnaround
Sunday. Readings surge to 85 degrees—the warmest of 2008 and the highest here
since 87 degrees Oct. 8.

Temperatures appear headed for the 80s Sunday and Monday—a dramatic turnaround
from May’s stubbornly chilly opening three weeks The month is nearly 2-degrees below
normal and 7 degrees behind the same period a year ago. To date, it ranks among the
eight chilliest Mays here since 1980. Lake Michigan’s average surface temperature, as
estimated by National Oceanic and Atmospheric Administration satellites, has crept 10
degrees higher in the past two months—yet only averages 41 degrees. A predicted
windshift to the northeast later Wednesday night is to tap the cool air which sits above
the lake surface and keep it coming ashore through week’s end. This will limit shoreline
highs to late March levels of low 50s even as inland areas flirt with the low and mid 60s.

Record-breaking heat in the Southwest is to come to an unceremonious end in the
next two days as unseasonably cool air dives into the region. Temperatures are to
plunge 30 degrees.

IT’S BEEN WARMER BY NOW 90% OF TIME

Chicago highest temperature so far this year has been just 82 degrees, back in April; 72
of the past 80 years—90 percent of them—have recorded warmer readings by this date.

Monday marked the second day temperatures failed to escape the 50s at Midway
Airport--something that has only happened this late in the season an average of one
year in five at the Southwest Side site since 1928. O'Hare's 60-degree high was a
little better. The month's opening 19 days ranks among the coolest 40 percent on
record with 13 of them posting deficits. The remainder of the work week is to remain
unseasonably cool as northwest winds keep resupplying the area with bursts of chilly
air off a stubborn pool of cool air draped across much of eastern Canada. A blocking
pattern in the arctic has locked that air mass in place for more than a month. But
potentially well-timed changes loom over the upcoming Memorial Day holiday
weekend. Eastbound warm air could push temperatures close to this year's highest
level to date by Memorial Day.

BLISTERING HEAT ON THE MOVE OUT WEST; DEATH VALLEY SIZZLES AT 120 DEGREES, PHOENIX AT 110 DEGREES

Even by Southwest desert standards, Monday's searing heat was one for the books.
Death Valley, Calif., sizzled at 120 degrees while residents of Phoenix were treated to
the year's first high of 110 degrees. Las Vegas wasn't far behind with a peak reading
of 108 degrees.

With strong low pressure to the northeast of Lake Huron and cold high pressure
sinking south out of Canada into Minnesota, north to northwest flow brings
unseasonably cool air into northern Illinois today. Very cold air aloft makes the air
mass unstable, with only a small amount of solar heating possibly triggering widely
scattered brief showers this afternoon. The most unstable conditions that produced
some 31 severe weather reports in Minnesota, Wisconsin and Michigan Saturday is
positioned farther north and east today.

BLOCKING PATTERN ALOFT CHANGES LATER THIS WEEK

The strong high pressure ridge that has brought record warmth to the west coast —
dozens of record highs were set from California to Washington Saturday—and the
large low pressure trough that has guided a cool northerly flow aloft over the Great
Lakes and Midwest will modify as it slowly drifts east in the coming days. By Thursday
a strong low pressure center is forecast to develop over the Southwest and the upper
Great Lakes low will be centered over the northeast U.S. This should bring significant
cooling to the west coast and allow a warm-up in Chicago by the end of the
workweek.

A dome of incredibly hot early season air scalded California on Thursday. The heat,
which produced readings 25 to 30 degrees above normal at many locations, smashed
records from California to Oregon. Perennially mild Seattle is predicted to turn
downright steamy Friday with highs surging into record territory, topping out near 90
degrees. It's looking increasingly likely the air mass could bring Chicago and the
nation’s Heartland it's first surge of summer-level warmth later next week.
Temperatures Thursday soared to a record-breaking 99 degrees at Oakland and
Sacramento, 103 degrees at Redding and 104 degrees at Red Bluff. The heat,
expected to blaze into the weekend there, will send the risk of wildfires soaring.
In Chicago, lakeshore residents shivered in Thursday's east winds. Temperatures at
the University of Chicago only reached 49 degrees and O’Hare's 57 degrees was 13
degrees below normal and far below the 80 degrees on the date a year ago.

MORE HEAVY RAIN DOWNSTATE

Downstate farmers, anxious to get into their fields and far behind in planting this
year's crops, were dealt another blow by heavy rain Thursday which reached 1.75
inches at Christopher—just northeast of Carbondale.

Temperatures returned to the 70s Tuesday for the first time in a week. Readings of
74 degrees at Rockford, Wheeling and Chicago's Midway Airport topped the day's
high temperature list. It marked the 14th time in 2008 that the mercury has topped
70 degrees and set the stage for late afternoon thunderstorm development. By
evening, nickel- and quarter-size hail had been reported from the Mississippi River
east to Dixon. Hailstones 1 inch in diameter showered down on Ogle County's
Oregon around 5:25 p.m., prompting a severe thunderstorm warning. By that time,
lightning was flashing earthward from the base of 43,000-foot-tall cumulonimbus
clouds (thunderheads) within a 200 mile radius of Chicago at nearly 400 times a
minute.

Powerful winds gushed out of the leading edge of the eastbound storms, reaching 52
m.p.h. at suburban Long Grove, Mundelein and Carpentersville early Tuesday
evening.

ONLY FOUR MAYS IN THE LAST HALF CENTURY WERE WETTER HERE

May's rainfall at O'Hare International Airport reached 2.99 inches prior to Tuesday's
storms, making it the 17th wettest opening for the month here since 1871. Only four
Mays in the past half century have opened wetter.

It was nasty Sunday in Chicago as heavy rainfall lashed the city, blown about by
northeast winds gusting to nearly 50 m.p.h. The greatest rain totals occurred from
the city westward, with Midway Airport recording 1.93 inches and Aurora measuring
2.05 inches. The rain let up by early afternoon, but strong northeast winds kept an
unseasonable chill in the air throughout the day. The sky is expected to clear
Monday, and with the return of sunshine, temperatures should climb into the lower
60s inland—though lake-chilled winds will keep shore areas in the 50s. It should be
even warmer Tuesday as south winds send the mercury into the lower 70s for the
first time in nearly a week.

A STEAMY MOTHER'S DAY IN FLORIDA

In contrast to Chicago's rainy chill, record heat gripped the Sunshine State. Record
highs were established in many areas, ranging from 93 at Pensacola to 96 at Miami
and Ft. Lauderdale.

TWISTERS STRIKE EASTERN CAROLINAS

Severe weather continued Sunday evening with tornado damage and injuries reported
in eastern North Carolina near Elizabeth City and Morehead City and in South Carolina
near Charleston.

--By Steve Kahn, WGN Weather Center Meteorologist

Chilly, wind-driven rain is expected to pummel the Chicago area Sunday, forcing
Mother’s Day celebrations indoors.

More than an inch of rain is likely to fall in many locations in heavy downpours before
the rains diminish to light showers in the afternoon. High temperatures will struggle
to reach the lower 50s, and as strong northeast winds gust in excess of 40 m.p.h. in
the afternoon, readings should drop back into the 40s, making 2008 the coolest
Mother’s Day here since a 50-degree high back on May 12, 2002.

DEADLY TORNADOES SWARM SOUTHWEST MISSOURI

Numerous tornadoes struck southwest Missouri late Saturday. Early reports indicate
three were killed, with many injured and extensive damage. Towns hit included
Neosho, Newtonia and Seneca.

Saturday's sunny open isn't to last. Clouds marking the approach of a windy Mother's Day
storm expected to soak the area Sunday arrive as the day proceeds, but not before
Chicago temperatures reach the 60s away from the lakeshore. However, cooling east
winds are expected to restrict shoreline highs to the 50s.
LATE SHOWING OF BACK-TO-BACK 50s
The chill that has gripped the area since Thursday is more than a bit unusual by historic
standards. The 59- and 58-degree highs here Thursday and Friday are the first
back-to-back 50s to occur on a May 8 and 9 here in a quarter century. In addition, 80
years of weather records dating back to 1928 at Midway Airport indicate that fewer than a
third of mid-May highs fail to crack the 60-degree mark.
IT WAS WARM THIS TIME A YEAR AGO
By contrast, Chicago was in the midst of early-season warmth at this time last year.
Saturday's predicted 63 degrees falls 21 degrees short of the 84-degree high a year ago.
Interestingly, last year's first 90-degree day was to occur in only a week's time on May 14.
May 2007 ended up with 13 days of 80 degrees or higher.
--Tom Skilling, Chief Meteorologist, WGN-TV/Chicago Tribune

Temperatures on Chicago's North Shore failed to break out of the 40s Thursday. Such
readings are more typical of late March than May. Wilmette topped out at just 48
degrees while Highland Park struggled to 49 degrees and Glencoe only made it to 50
degrees. Chicago's official 59-degree high at O'Hare International Airport was 8
degrees below normal and a far cry from an 82-degree high only a year earlier. It
marked the chilliest May 8 high here since the 51-degree high for the date 24 years
ago in 1984.

The cool weather continues Friday beneath increasing clouds and ahead of an
approaching disturbance that could spark a shower in spots late Friday.

LATEST SOAKER HITS DOWNSTATE

Unwelcome rains drenched the southern Midwest on Thursday. As much as 1.59
inches fell at Freeburg while 1.54 inches was measured at Flora—both east of St.
Louis in Downstate Illinois. The same storm spawned half a dozen twisters across
Tennessee, Alabama and Mississippi. Meantime, still another storm entering the
western Plains produced 68,000-foot-tall thunderstorms responsible for 70 m.p.h.
gusts at Lewis in southwest Kansas.

Two separate waves of heavy rain drenched the Chicago area Wednesday—the latter
embedded within an eye-catching, southbound shelf cloud. The ominous roll-
shaped, wind-tossed bank of clouds developed as winds shifted northeast tapping
chilly air to the north and sending temperatures into a 20-degree dive in less than
three hours. This month’s opening seven days had produced 2.04 inches of rain even
before Wednesday’s downpours arrived—the 11th wettest May opening of the last
138 years.

Area rain totals by day’s end were impressive and included: 1.34 inches in Wilmette,
1.17 inches in Palatine, 1.08 inches at Chicago’s Whitney Young High School and 1.05
inches at Mt. Prospect. In the city, 0.76 of an inch fell O’Hare International Airport
and 0.57 of an inch at Midway Airport. The “normal” full May rainfall tally is 3.38
inches.

SECOND STORM TARGETS AREAS
DOWNSTATE WITH A THURSDAY DELUGE
An unusually active pattern is taking shape—one likely to send a series of storms
across the Midwest over the coming two weeks. A powerful system, responsible for
Thursday’s gusty, cool northeast winds in Chicago wallops areas downstate with
downpours.

May, already wetter than normal—1.28 inches versus 0.67 of an inch—delivers its
latest thundery soaking in waves Wednesday. Warm, more humid air arrives as south
winds increased overnight and 35+ m.p.h. gusts keep the moisture needed to fuel
some potentially energetic downpours coming. While rainfall totals of a half inch or
more will be widespread, locations visited by the heaviest showers and
thunderstorms could get an inch or more.

The system behind Chicago’s Wednesday storms walloped the Plains and western
Midwest with thunderstorms late Tuesday believed to have produced three tornado
touchdowns in Winona County in southeast Minnesota —west of LaCrosse, Wis. In
Texas, 4.5-inch diameter hail (softball size) pummeled Olton while 4.25-inch
diameter hail knocked out windows near Grand Falls. Upon arriving home, a fire
official in Rose, Neb., witnessed a barrage of tennis-ball size hail that covered the
ground to a depth of 8 to 10 inches.

A 30-DEGREE TWO-DAY TEMPERATURE DIVE AHEAD—THE EQUIVALENT OF A LATE
JUNE TO LATE MARCH TEMP SHIFT
Unseasonably chilly air follows the rains. A blocking pattern aloft in Canada means
Thursday’s near 30-degree temp pullback won’t be the last in the next two weeks.

Temperatures surge to within striking distance of 80 degrees the next two days—
potentially the warmest readings here since the 82-degree high on April 25. An 80-
degree high Tuesday would make it the second warmest day of 2008. Crisscrossing
winds through the atmosphere—southwest at ground level and west/northwest only
two miles aloft—combined with faster than normal temperature declines with height
signal the possibility of several thunderstorms erupting this afternoon, especially in
northern sections of the area and out over the lake. The cool winds flowing out of
these isolated storms could surge ashore late today and set up a situation where
storms would build south near the lake—but that’s not yet a certainty. What appears
more likely is the eruption of more numerous storms with the arrival of more humid
air overnight.

DESPITE PREDICTED TEMPERATURE CRASH, 72% OF MAYS THAT START OUT WARM
WIND UP ABOVE NORMAL

The temperatures in May’s first week can offer clues on how the remainder of the
month may fare. When the month’s opening seven days average at least a degree
above normal, May has finished above normal 72 percent of time.

--By Paul Dailey, WGN Weather Center Meteorologist

Saturday's weather in Chicago was more reminiscent of late March, with gusty northwest winds, cloudy skies, scattered showers and high temperatures barely making it into the 50s. Sunday marks the start of a rapid turnaround with sunny skies, southwest winds and temperatures some 10 to 15 degrees higher. Warming peaks in the mid-70s Tuesday. Showers and thunderstorms, featuring downpours and heavy rains, precede the arrival of a cold front later Tuesday night. This storm system signals a change in the upper air pattern featuring a strong west-to-east jet stream flow directly overhead.

STORMS TRACK THROUGH THE MIDWEST
The rain-free period forecast from Sunday into Tuesday may be the longest seen in northeast Illinois for some time. Beginning Wednesday, low pressure systems will move frequently out of the central Plains through the Midwest and Ohio Valley. Computer models indicate that for the next couple weeks in Chicago, periods of cloudiness and showers will linger longer than the infrequent intervals of clearing skies. Starting Wednesday, a cool easterly air flow off Lake Michigan will probably persist well into the weekend. Rains should hit Thursday and then again over the weekend.

Gusty thunderstorms raked the Chicago area in waves Friday, peppering some
locations with pea-size hail while unleashing downpours so heavy in other
areas that motorists were forced to the side of the road. The 1.28 inches measured
at O'Hare Airporat broke the record of 1.26 inches set in 1979 and included 0.43
inches that fell in just 7 minutes.
An evening cloudburst that hit west suburban Elgin just before 7 p.m. swamped the
city with 1.50 inches in only 15 minutes. Other heavy rain totals included
1.32 inches in Glenview, 1.20 inches in Lombard and 1.14 inches in Northbrook.
Powerful south winds topped 40 m.p.h. in and out of Friday's thunderstorms, and
evening storm gusts were estimated at 50 m.p.h. in sections of Rockford while
WeatherBug wind sensors clocked gusts of 59 m.p.h. at Marshall High School in
Chicago and 56 m.p.h. gusts at Lansing Municipal Airport in southern Cook
County. Storm winds toppled a 2-foot diameter tree in Kankakee.

RAINBOWS CLOSE A STORMY DAY
Friday's stormy weather closed with rainbows visible in many areas as the
setting sun interacted with thunderstorm rains.
--Tom Skilling, WGN-TV Chief Meteorologist

An immense spring storm 2,200 miles across—easily the season’s biggest to date—is
driving the weather literally from one corner of the U.S. to another. The potent
system threatens a second Friday with severe weather in the Chicago area—including
rotating supercell thunderstorms and possible tornadoes if warming isn’t terribly
inhibited by regular thundery downpours.

A 100-degree north/south U.S. temperatures differential is driving the latest
atmospheric behemoth, which by nightfall Thursday had unleashed crippling snows
on sections of the Plains and Rockies—areas in the 80s only days earlier--while
promoting explosive thunderstorm growth in the Plains. Among the 10 twisters
reported across the Plains by nightfall was a multiple vortex twister—one in which
mini-tornadoes rotate within a larger funnel similar to the one which devastated
Plainfield in August, 1990. That tornado was one of two on the ground
simultaneously near Ralston, Oklahoma. The storms sprung to life as incredibly dry
air featuring desert-like single digit dewpoints, roared at 50 m.p.h. out of the Texas
Panhandle into humid air over the Sooner State.