EXPLAINER: June 2008 Archives

July, historically the city's warmest month, has arrived and as if on cue, temperatures
are warming. After surging into the mid-80s Tuesday, temperature readings
Wednesday, propelled by southwest winds and the warmth-inducing subsidence of air
which takes place beneath the "nose" of a powerful jet stream, appear poised to
generate Chicago's second 90-degree temperature of the year. More ominously, the
ingredients for severe thunderstorms come together over the area making Wednesday
afternoon and evening a period which will have to be monitored.

The books closed on one of Chicago's warmer and wetter Junes overnight. The month's
70.8-degree average temperature was 2.4-degrees above the 137-year average here --
warm enough to rank among the area's warmest 28 percent of Junes. June's rain tally
of 4.18 inches ranked 48th of the 137 Junes for which there are records here.

UPPER WINDS BRING SMOKE FROM CANADA WILDFIRES TO MIDWEST

Smoke from wildfires burning in Manitoba and Saskatchewan is riding northwest upper
steering winds over sections of the Midwest. It may lend a yellow or orange cast to the
sky over parts of the region Wednesday.

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

For the second straight afternoon showers fell in the Chicago area, but unlike
Saturday's blasts of hail and damaging winds, Sunday's storms did little more than turn
a pleasant summer day into a chilly evening. Temperatures dropped 10 degrees to the
mid-60s with the onset of the showers, and by evening readings had fallen into the
lower 60s under leaden showery skies.
Sunshine should return Monday and Tuesday as temperatures climb back toward
seasonable normals. A burst of warmth should send the mercury into the upper 80s by
Wednesday, but another round of thunderstorms along a cold front should bring in
cooler weather for the 4th of July.
SEVERE WEATHER BLASTS EASTERN U.S.
High winds and hail raked the East Sunday with nearly 150 reports of severe weather
through mid-evening. Two-inch-diameter hail battered Jacksonville in southeast South
Carolina, while high winds damaged trees and power lines from New England to
Alabama. Severe thunderstorms blew over a row of tents at a Huntsville, Ala., air show,
killing one and injuring a dozen other people.
This same storm system was responsible Friday night for winds estimated as high as
115 m.p.h. per hour just west of Omaha.
--By Steve Kahn, WGN Weather Center Meteorologist

A delightfully sunny and warm Saturday afternoon turned violent as fast-moving severe
thunderstorms swept across the metropolitan area between 2 p.m. and 6 p.m. The storms,
packing high winds and hail, left a litter of limbs, branches and power lines throughout
the region. Wind gusts reached 74 m.p.h. at hard-hit DeKalb and 65 m.p.h. in Chicago’s
Rogers Park. Injuries were reported when the storms hit an Evanston art fair, knocking
down several tents while pelting the area with 1-inch hail. Lake Michigan boaters were
also affected by winds in excess of 60 m.p.h. when the storms reached the lakefront.
More showers are expected Sunday but aren’t expected to be severe.

TROPICAL CYCLONE UPDATE: EASTERN PACIFIC HEATING UP; ATLANTIC QUIET

Two tropical storms, Boris and Cristina, have developed in the Eastern Pacific, southwest
of Baja California. The storms are moving west and currently pose no threat to land. In
contrast, the Atlantic Basin remains free from tropical cyclones.

--By Steve Kahn, WGN Weather Center Meteorologist

Though a few showers and thunderstorms dampened parts of the Chicago area Friday,
severe weather and heavy rains plagued areas to the west, north and south of the city.
Nearly an inch and a half of rain inundated Burlington in southeast Wisconsin Friday
afternoon, most of it falling in just 25 minutes. Baseball-size hail pummeled Earlham,
Iowa, southwest of Des Moines, and in downstate Illinois, winds gusted to 80 m.p.h. at
Xenia, east of Salem.
It has been a rough week of severe weather across the nation: There have been more than
2,000 U.S. severe weather reports in 44 states since last Saturday.
All of the storms have a played a major role in Chicago's lack of hot weather so far this
summer. Only eight summers since 1928 have had fewer 90s at Midway Airport by this
date.
GREAT LAKES LEVELS ON THE RISE
Thanks to recent rainy weather across the Midwest, water levels in all of the Great Lakes
are higher than a year ago. Lake Michigan is up 6 inches in the last month.
--By Steve Kahn, WGN Weather Center Meteorologist

The heaviest thunderstorms have bypassed Chicago in recent days -- a trend which is
to end Friday. Cumulonimbus clouds -- the tall, cottony clouds from which
thunderstorms emanate -- towered over the city's skyline late Thursday. Radar scans
indicated several reached 42,000 feet in height and produced isolated downpours
responsible for 0.60 of an inch in Wilmette, 0.25 of an inch in Northfield and 0.21 of an
inch in Northbrook. But those storms paled in comparison to powerful clusters that
drenched sections of Iowa with as much as 5 inches of rain and produced deadly wind
gusts clocked at 68 m.p.h. in Indianapolis. Early reports indicated the winds may have
collapsed a building under construction resulting in three deaths.

Blistering heat in the Plains -- including a 103-degree high at Dodge City, Kansas --
spawned the storms that led to more than 300 reports of severe weather Thursday. A jet
stream settling south toward Chicago threatens to strengthen t-storms here later on
Friday and Friday night.

LAKEFRONT FLIRTED WITH 90 DEGREES THURSDAY

An 89-degree high was recorded at Northerly Island on Chicago's lakefront, where
nearby water temperatures reached a season high of 68 degrees.

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

Thunderstorms lambasted northern Missouri and sections of central Illinois producing
stunning 6 to 8" rains in a region still reeling from flooding and wet ground. The
stunning rains were part of a complex of thunderstorms -- what meteorologists refer
to as an "MCS or Mesoscale Convective Complex. These are the bright clusters of
thunderstorms visible on satellite animations which seem to spring from thin air --
then linger for hours. A powerful and persistent low level wind extending from the Gulf
of Mexico north to the Plains fueled early Wednesday's storms, supplying them a rich,
nearly unlimited supply of moisture of hot humid air which has dominated the Plains.
Thunderstorms with MCS's are prolific lightning producers and can linger for 6 to 8 or
more hours. Linneus, Missouri was hardest hit with 8.61" while nearby Ethel reported
7.88" . New storms, towering to heights of 55,000 ft., flared late Wednesday in Iowa
and threatened a new round of heavy. Golf ball size hail and 60 m.p.h. accompanied the
storms in Nevada, Iowa -- just east of Ames.

KANSAS BROILS NEAR 100∞ WEDNESDAY

Temperatures reached the century mark in Kansas. The hot dome of air will continue
supporting t-storm clusters along its northern periphery.
--Tom Skilling, Chief Meteorologist, WGN-TV/Chicago Tribune

Surging temperatures and humidities in coming days will lend an ominously explosive
quality to the atmosphere. Among the risks this poses to the Midwest is the potential for
thundery interludes in which downpours generate large rainfalls—bad news for a region
still reeling from near record flooding. Just where the heaviest storms erupt is always a
dicey call for forecasters. While rising temperatures and some of the summer season’s
most humid air provide the building blocks for thunderstorms, the rain cooled air that
gushes from such storms plays a big role in determining the areas exposed to the most
prolonged bouts of rain. With the atmospheric water content headed for nearly 2 inches,
there will be plenty of moisture for any storms to tap.

The delightful low humidities of recent days are a thing of the past for the remainder of
the work week. Dew points, a measure of atmospheric moisture, are headed for the 70s
later Wednesday for only the fifth time this year. A shift in wind direction from the ground
to higher altitudes is just the sort of environment in which thunderstorms rotate and
generate gusty winds. Hail and lightning are obvious risks in this situation.

Chicago's second 90-degree day of the year is just days away. Just one 90-degree high
temperature is on the book this year -- rare for so late into the season. Only 13 of the
80 years since 1928, at Midway Airport, have had as few or fewer 90s by June 24. There
had been five by this time a year ago, which equals the long-term average. City
residents had experienced 18 by this date in the Dust Bowl year of 1934. The 90-degree
drought this year is unusual, considering the opening 23 days of June have averaged
70.4 degrees -- one of the 10 warmest such periods of the past half century, and 35th
warmest such period since 1871.

HUMID AIR'S ADVANCE TO IGNITE TUESDAY NIGHT THUNDERSTORMS

Clouds increase Tuesday ahead of waves of thunderstorms in coming days which
threaten to deposit 1 to 3 inches of rain. The jet stream is shifting well north of the
area in typical summer fashion. It's a set-up in which light upper air winds here allow
downpour-generating thunderstorms to move slowly, a formula for generating large
rain totals.
--Tom Skilling, Chief Meteorologist, WGN-TV/Chicago Tribune

Scattered showers and thunderstorms popped up in an irregular pattern Sunday over the
Chicago area and northwest Indiana. There were many reports of pea- to quarter-size
hail. Half-inch hail covered the ground at 5:30 p.m. just south of Evanston. This
unstable air, associated with low pressure centered well to the north and east of Lake
Huron in Canada's Quebec province, moved to the east overnight.
Light northerly winds on the front side of cool high pressure over the western Great
Lakes will prevail over Chicago today with inland heating conducive to the setting up of
a lake breeze, resulting in cooler shoreline temperatures this afternoon.
DOWNPOURS COULD BRING MORE WORRY
A change in the upper-level jet stream pattern will open the gates for warm, humid air
to flow into the Midwest starting Wednesday. Weak warm and cold fronts will not
influence temperatures greatly, but will add instability to an already unstable
atmosphere. Frequent periods of showers and thunderstorms are likely, beginning later
Wednesday and continuing through Friday. Heavy rain with some of these storms could
bring more worry to residents of Iowa, Missouri, Wisconsin, Illinois and Indiana, who
are weary of battling flood-swollen rivers and streams.
--By Paul Dailey, WGN Weather Center Meteorologist

Showers and thunderstorms are expected this afternoon under the influence of a large
low pressure area east of Lake Huron in Canada's Ontario province. Steered by a strong
northerly jet stream flow aloft, cool high pressure will drift south and east out of
central Canada and hold over the Midwest through Tuesday. This will give northern
Illinois temperatures about 5 degrees below normal for the next few days. After today's
rains, a brief dry period will hopefully allow most of the area's swollen rivers and
streams to slowly fall.

WARMER, MORE HUMID AIR BY MIDWEEK

By Wednesday, the upper air pattern will change significantly with a west-to-east jet
stream flow establishing along the United States-Canada border. This will block the
southward movement of cooler Canadian air masses, and allow a gradual intrusion of
warmer, more humid air from the central and southern Plains into the Midwest. During
the last half of the week, Chicagoans should see readings average about 5 degrees
above normal along with much more humid conditions and frequent showers and
thunderstorms. Rainfall totals the latter half of the week could easily exceed an inch.

--By Paul Dailey, WGN Weather Center Meteorologist

Towering thunderstorms swept roughly half of the metro area in two waves Friday --
the first delaying the opening game of the Crosstown Classic at Wrigley Field with
downpours and peppering some northwest suburbs with hail as large as quarters.
The second wave of storms erupted mid-afternoon in 80-degree warmth and drenched
north sections of Geneva in the Fox Valley with an inch of rain in just 35 minutes. The
storms also toppled a tree across a house just southeast of Warrenville in DuPage
County.
The storms were the first to sweep the area since last Sunday. While hail and lightning
were the biggest threat posed by most storms, several generated wind gusts clocked at
45 m.p.h. in Kane County's Burlington. The largely storm-free south suburban area
hosted upper 80-degree temperatures -- including 88 degrees at Plainfield, Kankakee
and Orland Park.
SOUTHWEST WILTING IN 7TH DAY OF TRIPLE-DIGIT HEAT
Temperatures surged past 110 degrees for a seventh day Friday in parts of the
Southwest. Highs reached 117 degrees at Palm Springs and 110 degrees at Santa Maria,
both in California -- readings up to 38 degrees above normal.
--Tom Skilling, Chief Meteorologist, WGN-TV/Chicago Tribune

Temperatures surged above 80 degrees for the 14th time Thursday in the midst of one
of the driest June air masses here in three years. Slowly retreating flood waters still
cover areas adjacent to the Fox and Rock Rivers into next week. But that had little
impact on relative humidities here which by 3 p.m. Thursday had dropped to 23
percent. There hasn't been a lower relative humidity in June since the 18 percent
reading recorded June 22, 2005. High temperatures return to the 80s Friday and
Saturday--but the 2008 tally pales in comparison to the 30 days of 80-degree or
higher readings a year ago. The city had seen only one 90-degree-plus high to date --
less than the five considered normal.

HEAT'S RETURN NEXT WEEK COULD PROVOKE NEW T-STORMS

Summer-level warmth surges back into the area as June enters its final days the middle
and end of next week. It's a development that could produce regular clusters of
thunerstorms. It was another active night across the western Plains on Thursday. More
that 235 reports of severe weather were logged by the Storm Prediction Center in
Norman, Okla., including 185 reports of hail.

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

The clock may be ticking toward summer—the season gets under way at 6:59 p.m.
Friday—but you wouldn’t have known it late Wednesday. Temperatures across a
westward moving lake-breeze front varied so widely the front was discernable to area
weather radar despite the lack of precipitation with it. While most cool fronts powered
by breezes off Lake Michigan make it only 20 or 25 miles inland in mid June,
Wednesday’s traveled more than 50 miles in. By evening, temperatures near the lake
had tumbled into the 50s even as 80-degree readings lingered to the west. The
strength of this front was a product of Lake Michigan water temperatures 12-degrees
cooler than a year ago—a leftover from Chicago’s chilly May.

HAIL THE SIZE OF SOFTBALLS BOMBARDS THE PLAINS WEDNESDAY
More than 5 dozen reports of hail accompanied thunderstorms across eight Plains
states. Hardest hit was an area southeast of Westerville, Neb., and just north of
Center, N.D., where hail the size of softballs (4.25 inches in diameter) fell. Record
heat continued in the Southwest with Palm Springs, Calif., setting a temperature
record of 115 degrees.

The weather's turned beautiful in recent days and still water is on display everywhere in
the Midwest. Forecasts suggest the Fox River, which began falling Tuesday in
Wisconsin, is to crest later this week and only slowly retreat this weekend and early
next week. Water levels on parts of the flooding Rock River are at their second highest
levels on record.

Even Lake Michigan is responding to the copious rains of recent weeks. The U.S. Army
Corps of Engineers reports water levels are 3 inches higher than a year ago. That
doesn't sound like much. But, each inch the Lake Michigan rises represents an increase
of 390-billion gallons of water. Precipitation across the 45,000 square mile drainage
basin of Lake Michigan has averaged 181% normal since June began and the lake's
volume has risen 1.17-trillions gallons over the past year.

BLISTERING HEAT BLAZES ACROSS THE SOUTH

The hottest weather this season is baking the Southwest. Phoenix topped out at
113-degrees -- one degree shy of the record Tuesday while Tucson hit 109-degrees.
The warmth threatens to expand east next week and could signal a warm-up here
Wednesday and beyond.

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

It was only five days ago that area heat enthusiasts were relishing a temperature of 91
degrees. Dramatic changes in the upper atmosphere, including the reappearance of a
dome of warm air aloft to the west of Greenland over the Davis Strait -- a pattern
behind Chicago's persistent winter and spring chill -- have this week's weather on a
much different track. The pattern produces northwest steering winds aloft that tug cool
Canadian air south across the Midwest. Lacking the moisture delivered by south winds
from the Gulf of Mexico, this atmospheric set-up greatly limits thunderstorm
development -- a blessing for Chicago-area residents who have been forced to
contend with flooding in the past week. Only the Fox River in this area has yet to crest,
a benchmark to be reached Wednesday morning.

COOLEST WEATHER HERE IN TWO WEEKS

Monday's 72-degree high was Chicago's coolest since the 69-degree high June 3 and
the second chilliest daytime high to date this month. Temperatures over the coming
five days (through Saturday) will average 18 degrees cooler than the same period a week
ago. But warmth and humidity are to stage a comeback this weekend.

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

It's often the case that persistent and extreme weather in one region of the United
States is balanced by persistent and extreme, but opposite, weather in another portion
of the nation. While many residents of the Midwest struggle to cope with
unprecedented rainfall and record river flooding, residents in the western third of the
nation desperately wish the skies would open. They've been locked in a withering
drought that, in some locations, is now in its seventh year. As of June 13, 13 large
forest fires and literally hundreds of small fires were in progress from California to
Colorado. Flow rates in large western rivers (like the Colorado River) are nearing
historic lows, and, after a very dry winter and the driest May on record in California,
Gov. Arnold Schwarzenegger has declared an official statewide drought, the first such
designation since 1991.

'DRY THUNDERSTORMS' TORCH WESTERN FORESTS
Only very rarely are lightning ground strikes responsible for fires in the normally
well-watered forests of the eastern United States. It's a different story in the West.
There, thunderstorm rainfall, even in the best of times, is of limited extent, and
lightning often strikes outside a storm's precipitation area.
--By Richard Koeneman, WGN Weather Center Meteorologist

Impulses of energy zipping from northwest to southeast across the Midwest on Saturday
triggered scattered thunderstorms just where more rain wasn't needed: eastern Iowa and
southern Wisconsin. Many locations received no rain, but a few spots just west of Madison
and also near Iowa City picked up 1 to 2 inches of unwanted rain. A few of those
thunderstorms swept across Chicago's northern suburbs, though with much less rain.
That pattern of scattered but locally strong, hit-or-miss thunderstorms continues today
across the Chicago area as well as Illinois and surrounding states. Cooler, drier air arrives
late Sunday and the region dries out, at least temporarily.

LAST YEAR’S FATHER’S DAY WAS HOT

Today's warmth won't challenge the 91-degree Father’s Day reading registered here last
year, but thunderstorms might threaten his picnic. Storms will affect 50 percent of the
Chicago area, but there will be several rain-free hours even where storms do hit.

"Water flowing into the Chain is exceeding water flowing out," said Mike Warner,
Executive Director of the Lake County Stormwater Management Commission, speaking
of the increasingly serious flood situation on the Chain O' Lakes in northwest
metropolitan Chicago.
Even though heavy rains spared the immediate Chicago area Thursday and Friday, rain
totaling 3-5 inches swamped the headwaters of the Fox and Des Plaines rivers in
southern Wisconsin, bringing 10-day rain totals there to an overwhelming 10-16
inches. All that water is draining relentlessly south into Illinois. And so, rivers in
northern Illinois continue to rise today even as the sun shines.
In the East, a second day of modestly cooler temperatures further blunts a deadly early
summer heat wave (blamed for at least 30 deaths) that extended earlier this week from
Georgia to New England.
RENEWED RAIN THREAT ON SUNDAY
A new weather disturbance sweeps into the Midwest on Sunday, accompanied by
potentially vigorous thunderstorms. This system, unlike those that produced
unprecedented rains in the preceding two weeks, is moving rapidly and
rainfall-generating moisture available to it is limited.
--By Richard Koeneman, WGN Weather Center Meteorologist

The last thing Midwesterners want to hear is that more rain and more severe
thunderstorms are on the way. For them, the forecasts contain both good news and bad
news. A cold frontal boundary, the transition zone between cool air to the west and hot
air to the east -- and a fertile breeding ground for t-storms -- stretches from
Oklahoma to Wisconsin. It's been there for three days, and so have the storms. The
frontal zone is finally shifting southeast and its associated storms will also be shifting
southeast.

Severe storms continue to be a threat today, especially south and east of Chicago, but
excessive rainfall and unprecedented flooding are now affecting far more people. Rains
will be diminishing from west to east across Iowa, Wisconsin and northern Illinois
today (the good news), but Indiana, southern Illinois and Michigan are next in line for
the rains (the bad news). Those areas, too, desperately want dry weather.

CHICAGO'S 90-DEGREE SEASON BEGINS

It's two weeks late, but Chicago logged its first 90-degree day of the season on
Thursday: 91 degrees. On average, that first occurrence lands on May 29. For those who
are counting, the city's annual tally of 90-degree days is 24 at Midway Airport and 11
days in (lake-cooled) downtown Chicago.

--By Richard Koeneman, WGN Weather Center Meteorologist

Thunderstorms will be rolling into the Chicago area tonight, and they might be
severe. Heavy rains, too, seem likely. The same storm system battered the Plains with
three dozen tornadoes, one of which reportedly claimed four lives at a Boy Scout
camp near Little Sioux, Iowa.

When the eastern United States is warm, the West is often cool, which has been the
case in recent days. Cities like New York, Washington and Charlotte have set heat
records while the West has shivered.
Sporadic snows in excess of 15 inches have buried higher elevations of the Northern
Rockies and the Cascades.

UNPRECEDENTED FLOODING IN IOWA

Cedar Rapids, located on the Cedar River, is among Midwestern cities hardest hit by
flooding. Officials there have expanded mandatory evacuation in that city to areas
matching its 500-year flood-plain map. The Weather Service's River Forecast Center
in Chanhassen, Minn., predicts a flood crest of 24.5 feet on the Cedar River, 4.0 feet
above the previous record flood there in 1929.

Summer's start is not an astronomical event, but it's easy to think so. After all, the
summer solstice (an astronomic event that occurs this year on June 20) marks the official
start of summer. In the atmospheric sciences, however, summer is the three- month
period that began on June 1. But climatologically, the warmest quarter of the year at
Chicago is the 91-day period that begins today. With an average temperature of 73.5
degrees, June 11 through Sept. 9 is Chicago's summer -- its warmest 91 consecutive
days.

JUNE: WETTEST MONTH IN WETTEST SEASON

Summer is also Chicago's wettest season and, at Midway Airport, June is summer's (and
the year's) wettest month. Normal June rainfall, 4.16 inches, accounts for 11 percent of
the city's annual total. With 4.00 inches as of June 10, nearly all of the full month' s
normal allotment has already come down.

--By Richard Koeneman, WGN Weather Center Meteorologist

As if Saturday's severe thunderstorms and tornadoes weren't enough, the Chicago area
was expected to take another pounding Sunday night with waves of powerful storms
sweeping through. And it's not over yet -- but the end is in sight. The atmosphere
remains primed for yet another round of severe weather Monday, especially in the
afternoon, but cooler, less humid and more stable conditions finally promise quiet
weather by Tuesday. Until then, the possibility of flooding will add to our weather woes.
Recent heavy rains have left area rivers bankfull (or overflowing), and additional rain will
exacerbate the high-water situation. The Chicago area remains under a flash flood watch.
SATURDAY'S LOCAL TORNADOES
The National Weather Service confirmed the occurrence of six tornadoes in southwestern
metropolitan Chicago between 4 and 7:30 p.m. Saturday: two in northeast Livingston
County, three in southwest and central Will County, and the large, long-track tornado that
cut a 60-mile path from Odell (Livingston County) to Lansing (southern Cook County).
Further information can be found at the National Weather Service-Chicago Web site
at www.crh.noaa.gov/lot.
--By Richard Koeneman, WGN Weather Center Meteorologist

After tornadoes hit Saturday afternoon and evening, another round of thunderstorms,
some possibly severe, appears on tap today. Strong southerly winds wrapping around the
backside of high pressure centered over the southeastern U.S. continues to pump very
warm, humid air north into the Midwest and Great Lakes. Thunderstorms could pop up
almost anytime, but the peak heating afternoon and early evening hours appear most
likely. The National Weather Service Storm Prediction Center has projected a wide area
from the southern Plains to the western Great Lakes, including northeast Illinois and
northwest Indiana, for a chance of severe storms today.

RELIEF IN HEAT AND HUMIDITY IN SIGHT

A cold front is expected to move through the metro area Monday, preceded by showers
and thunderstorms, followed by a cooling less humid northwest flow. Hot, humid air is
forecast to briefly return midweek, peaking Thursday before the arrival of another cooling
cold front Friday. A cooler, less humid air mass should then hold through next weekend.

--By Paul Dailey, WGN Weather Center Meteorologist

Many areas of northeast Illinois were storm-free Friday. Where the storms did hit,
significant wind damage occurred. Thunderstorms moved at 55 m.p.h., and this
movement, combined with wind gusts out of the storms, resulted in localized 75 to 80
m.p.h. winds. Power lines and trees were downed, and a grain bin tipped over in
Lindenwood at Davis Junction in Ogle County. Extensive wind damage also occurred in the
Marengo area in McHenry County. Tornadoes were reported around 2:30 p.m. in Poplar
Grove and Capron in Boone County.
SOUTHERLY FLOW CONTINUES
Warm southerly flow and high humidity will continue over northern Illinois this weekend.
With temperatures forecast to approach or even exceed 90 degrees both Saturday and
Sunday, this weekend could be the city's warmest since last July 7-8.
--By Paul Dailey, WGN Weather Center Meteorologist

The Midwest becomes the focus today for all the atmospheric ingredients necessary to
produce a major outbreak of severe weather: warm air and moisture streaming into the
region from the Gulf of Mexico, ample sunshine providing additional heating, a cold front
approaching from the west and a powerful jet stream surging overhead. The Storm
Prediction Center places Chicago in a 45-percent risk area today(45 percent chance of
severe weather in any given circle 50 miles in diameter.)

CHICAGO'S HOT WEATHER SEASON

A temperature of 90 degrees is the benchmark definition of a hot day. By that standard,
Chicago's "hot season" extends from April 10 (the date of the city's earliest-occurring
90-degree day) to Oct. 6 (the latest occur- rence. The city logs an annual average of 24
days at or above 90 degrees, and the average first occur- rence is on May 29.

--By Richard Koeneman, WGN Weather Center Meteorologist

On Wednesday, Lake Michigan demonstrated yet again its ability to alter Chicago's
weather. While temperatures climbed into the 80s in the southern suburbs, lakeside
residents in northern sections donned jackets and sweaters to ward off the fog and
chill of afternoon readings only in the 50s. But that's a fading memory 24 hours later.
Borne in on brisk southerly winds, heat and humidity spread to all sections of the
Chicago area today. Such sharp temperature contrasts, aided by the energy from
strengthening winds aloft, fueled thunderstorms —many of them severe—Wednesday
afternoon and evening. A tornado touched down seven miles south of Flanagan (91
miles south of Chicago) near Pontiac in Livingston County at about 8 p.m. Wednesday
and another occurred near Rensselaer, Ind., at 7:30 p.m. Severe storms were ongoing as
of 9 p.m.

GULF-COAST LEVEL DEW POINTS ARRIVE

Humid air of Gulf origin arrives in the city Thursday in the wake of morning
thunderstorms. Dew-point temperatures, a preferred measure of atmospheric moisture,
become uncomfortable for most people when they surpass 65 degrees. Computer
forecasts indicate the level will be attained this afternoon.

--By Richard Koeneman, WGN Weather Center Meteorologist

Following a spring characterized by persistently cloudy skies and sub-normal
temperatures, Chicago's weather regime is embarking upon a dramatic change. Warm
—even hot—air that has maintained a multi-day presence across the southern U.S. is
expanding north. In response, Chicago's temperatures climb toward the upper 70s
Wednesday, then jump toward 90 degrees Thursday. The transition will be turbulent,
with a parade of vigorous disturbances across the area in upcoming days, each
accompaned by strong thunderstorms.

CENTRAL ILLINOIS WALLOPED BY HEAVY RAINS TUESDAY

Waves of t-storms roared across central Illinois on Tuesday, bringing additional rain to
an area already drenched from a week of excessive rainfall. Doppler precipitation
estimates indicated localized 8-inch amounts even before Tuesday's onslaught. And
it’s not over yet—periods of thunderstorms continue Wednesday.

--By Richard Koeneman, WGN Weather Center Meteorologist

Monday's 86-degree reading (the city's highest temperature thus far this season and
the warmest in nearly eight months) yields to cooler air today, but heat returns
Thursday -- and persists. Chicago's temperatures have climbed to 80 degrees or higher
on only four days this year (17 days by this time last year), but forecasts on five of the
next seven days contain readings above 80 degrees. Higher humidities, too, will
accompany the arrival of this season's first multi-day stretch of summer warmth. It's
also likely to be a turbulent period. Computer models indicate a powerful 70-90 m.p.h.
jet stream shoots from Missouri into central Illinois today, and that will enhance
thunderstorm development. Other disturbances sweep across the area Thursday and
Friday and again on Monday, and thunderstorms will accompany each of these events.

SEVERE THUNDERSTORM POTENTIAL

Chicago sits within a 30-percent-chance area Tuesday. That means there is a 30
percent chance that a severe storm will occur within 25 miles of any given point.

--By Richard Koeneman, WGN Weather Center Meteorologist

Originating in the central Plains, storms will track through the Midwest this week. After
a sunny start to the workweek Monday, warm, moist southerly flow riding over a warm
front to the south will spread showers and thunderstorms into northern Illinois on
Monday night. The storms will continue Tuesday with occasional downpours. By the
time the rain diminishes, totals well in excess of an inch may be widespread over the
Chicago area. Northeast Illinois should experience a brief break in the clouds early
Wednesday, only to have another warm front produce showers and thunderstorms later
in the afternoon and at night.

JULY-LEVEL HEAT LATE THIS WEEK
As the warm front moves well north of the Illinois-Wisconsin line, strong southerly
winds are expected to usher in midsummer-like temperatures and humidity Thursday
and Friday. Depending on cloud cover, some high temperatures -- especially to the
south -- could approach or even touch 90 degrees.
Another band of showers and thunderstorms could hit Friday night into Saturday.
Rainfall totals for the week ahead could easily reach 2 to 3 inches at many locations
across northeast Illinois, southern Wisconsin and northwest Indiana.
--By Paul Dailey, WGN Weather Center Meteorologist