SEVERE WEATHER UPDATES

Tracey Surface provides us this photo of Tuesday afternoon’s snow as it swept into Montrose Harbor earlier this afternoon. Many thanks Tracey! By late evening (5:30 p.m.), Chicago area snow tallies have reached the 2-5” range—including 2” at O’Hare, 3” at Downers Grove and our meteorological colleague Steve Kahn reports 4” is down at Arlington Heights. Chicago’s Midway Airport—where the mid-afternoon 2007-08 seasonal tally has risen to 49.7” (14.7” of it having falling in February alone)—is likely to have played host to a total of 50+” before the evening ends, says veteran Midway and Chicago weather observer Frank Wachowski. Frank also reports this snow, because of the cold temperatures in which it has formed and fallen, has a 22 to 1 snow to water ratio—indicating this snow has more than twice the volume of conventional snow here.
Radar (at 5:45 p.m.) is indicating a band of snow is approaching from the west—snow likely to be reinforced by lake moisture riding NE winds. So the snow’s not done as of this posting—but will end later tonight. It’s not the last snow likely to occur here in the coming week. Though glorious (and welcome) sunshine is due Wednesday—more snow is predicted Thursday (Valentine’s Day) night. The area’s lack of February sun is running at record levels. Only 12% of Chicago’s possible sun has occurred—February’s “normal” tally is 46%.
More on WGN-TV’s Nine O’Clock News Tuesday evening, on our wgntv.com “weather blog” and in Wednesday’s Chicago Tribune.
Tom Skilling
WGN-TV Chief Meteorologist

Photo courtesy: Tracey Surface, Chicago

We’re boosting our Chicago accumulation forecasts further, a foot or more of snow likely in parts of the city—much lighter amounts south
The early onset of snow suggests even higher storm snow totals are likely in the city. Expectations precipitation would start as a rain/sleet mixture in the northern suburbs due to a layer of warm air aloft have NOT materialized suggesting northeast winds have injected a layer of drier air in the lower atmosphere producing evaporative cooling there. That’s why snow has begun as soon as it has. This means assumptions that the first hours of this storm’s precipitation would come down as liquid or a mix must be updated and that moisture must be added to our predicted snow tally. As a result, it now appears likely 8-14” will fall from Chicago north and west. The presence of potential thunderstorms could lead to locally higher totals. Our latest in-house RPM (Rapid Precision Mesocale) model forecast now puts totals at 13” at O’Hare and 10” at Midway. The University of Wisconsin NMS model is even suggesting 12-18” totals in portions of northern Illinois.
Thus a major winter storm, by far this winter’s biggest to date, looks likely to produce serious travel problems well into Wednesday. We’ll update as new information becomes available.
Tom Skilling
Chief Meteorologist, WGN-TV/Chicago Tribune

The tornado watch for the Chicago metropolitan area has been canceled. Scattered showers will continue overnight with some heavier rain and possibly more thunderstorms developing late tonight and Tuesday morning.
Steve Kahn WGN-TV Weather Center Meteorologist

ENTIRE CHICAGO AREA UNDER A TORNADO WATCH UNTIL 9 PM THIS EVENING
Fast-moving tornadic thunderstorms swept across portions of extreme northern Illinois into southeast Wisconsin this afternoon from north of the Rockford area to near Kenosha. The storms left a trail of damage from the Machesney Park area north of Rockford east northeast through Poplar Grove passing north of Harvard before moving into southeast Wisconsin causing damage near New Munster in Kenosha County. Two twisters were also sighted near the Kenosha County Airport.
The twisters damaged numerous homes, downed trees, transformers and powerlines. High winds were also reported with gusts to 80 m.p.h. at Hebron in far northern McHenry County. A semi was overturned north of Harvard. Hail also accompanied the storms.
The storms erupted in an unseasonably warm record-breaking air mass that sent the mercury in Chicago to a balmy 65º shattering the day's previous record high of 59º set 101 years ago in 1907.
The entire Chicago Metropolitan area along with northwest Indiana remains under a tornado watch until 9 p.m.
Steve Kahn WGN-TV Weather Center Meteorologist

Severe thunderstorms developing rapidly, hail in the Loop
Marble size hail fell in the Loop on Michigan avenue and 60 m.p.h. wind gusts measured by the WeatherBug sensor atop the LaSalle Bank Building this evening just after 5:30 p.m. as severe thunderstorms rolled northeast into the city from the Joliet area. Shortly after
5:45 p.m. golf-ball size hail was reported in Joliet and winds gusts to 75 m.p.h. were clocked at Wilmington in Will County.
High winds blowing from the southwest, not associated with thunderstorms but directly related to the intensity of the storm system sweeping the Midwest have been raking the Chicago area throughout the afternoon and evening hours.
Some of the highest wind gusts are recorded on our WeatherBug network include
Tonti Elementary School Chicago 53 m.p.h.
Curie Metropolitan High School Chicago 52 m.p.h.
Simeon Academy Chicago 52 m.p.h
Bednarcik Junior High Aurora 51 m.p.h.
Gale Academy Chicago 50 m.p.h.
Steve Kahn WGN Weather Center Meteorologist

Powerful winds raking the area beneath jet stream, Chicago continues on west side of severe weather risk area into early tonight
A tornado watch has just been posted by NOAA’s Storm Prediction Center until 11 pm for Chicago and its south and southeast suburbs—including ALL of northwest Indiana. Counties in Illinois under the watch are Cook, Will, DuPage, Kankakee and Lake. Another tornado watch covers Wisconsin until 10 pm—from the Illinois/Wisconsin border northward. The Chicago area is being raked by powerful non-thunderstorm winds within the “dry slot” of a huge autumn storm. The dry slot is an area of large scale atmospheric subsidence, limited cloud cover and strong winds directly beneath the strongest winds of the jet stream. It is most often several hundred miles across and creates the indentation in huge storm’s cloud cover as viewed from space, lending storms the “comma-shaped” appearance we often refer to on our television weather programs when we display satellite movies. Powerful non-thunderstorm winds have been raking the Chicago area beneath this dry slot and have been clocked by WeatherBug sensors since 2 pm at 53 mph on Chicago’s West Side at Tonti Elementary School, 48 mph in west suburban Aurora and Lombard, 47 mph at Sugar Grove and 45 mph at Elburn.
The Chicago area continues under a severe weather risk, though the storms are likely to be selective, affecting only portions of the metro area. Storms have been actively developing across southern Wisconsin within the past hour (Note: This update is being filed at 3:30 pm Thursday afternoon). Thunderstorms rarely affect a region as large as the Chicago metropolitan area uniformly and the weather situation today will be no exception. What continues to concern us is the POTENTIAL for the development of fast-moving, north-northeastwardbound t-storms over at least SECTIONS of the metro area. Satellite imagery shows a field of enhanced cumulus clouds extending from central Illinois northward into the northeast quarter of the state and into Wisconsin. This corresponds with a region of varied wind speeds at jet stream level—what is known as a “shear-zone”. There, air parcels slow as they depart the region of strongest winds aloft— which encourages air to ascend, a development which enhances cloud development. Allowed to proceed, this cloud development can go on to produce showers and thunderstorms. Wisconsin and areas east may be particularly prone to t-storm development the remainder of the afternoon and early evening, given the presence of particularly “unstable” air there----air which cools faster than usual with height.
Several computer models area are hinting an upper disturbance is to swing northward into Chicago and/or its south and southeast (Indiana) suburbs later today and this evening---about the time these enhanced cumulus clouds reach the metro area. This may enhance cloud development and help initiate the formation of at least some scattered showers or t-storms, which would ascend into a band of powerful steering winds and therefore move at speeds of as much as 50 mph. Especially fast-moving storms, in combination with the normal flow of air out of thunderstorms often become capable of damaging wind production and of spawning microbursts. Not all parts of the Chicago area are likely to see severe weather and the most widespread severe weather is likely to be to our east in Indiana and Michigan. But, given the presence of such strong winds through the atmosphere today, any thunderstorm has a better than usual chance of producing strong gusts beyond the powerful non-thunderstorm wind gusts already occurring.
Ultimately, these storms—expected to selectively affect sections of the metro area—are likely to assemble into a more solid line/band of storms as they proceed eastward into Indiana and Michigan. The arrival of cooler air beyond 10 pm makes that the most likely time beyond which any threat of severe weather in the Chicago area ends.
Tom Skilling
WGN-TV Chief Meteorologist

As of 4:55 pm, severe thunderstorms have hit hard across the west and northwest suburbs. At west suburban Sugar Grove, winds were clocked at 60 mph along with torrential downpours. In McHenry county's Marengo, lightning struck a 4 foot diameter tree, knocking it onto the road at U.S.-20 and IL-23. More storms are possible as the most of the Chicagoland area is under a severe thunderstorm watch until 9pm. In addition, Dekalb county remains under a flash flood warning until 7:45 pm. due to 3-4" rains which fell yestersday along with new heavy rainfall today.
Bill Snyder
WGN-TV Weather Producer