STORM UPDATE
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