The Chicago area remains at risk for severe weather Friday night-as we post this update at 3 pm Friday afternoon. NOAA's Storm Prediction Center (SPC) continues to outlook an area which includes sections of 7 states for possible severe weather--Chicago sits on the east side of that area (http://www.spc.noaa.gov/products/outlook/day1otlk.html). SPC has also just issued a tornado watch for eastern Iowa, northwest Illinois and southwest Wisconsin (http://www.spc.noaa.gov/products/watch/ww0617.html) until 5pm. Highly divergent winds over that area at jet stream level, daytime heating and an influx of energy-rich humid air form the basis of a significant severe weather risk even beyond 5pm in THAT region of the Midwest. But, Chicago area residents must remain vigilant as well--especially Friday night. The latest run of our in-house 4km RPM (Rapid Precision Mesoscale model) describes an atmosphere primed for potentially active thunderstorms across the Chicago area from roughly 8pm this evening far west and northwest sections to 2am in northwest Indiana. The city proper would seem at greatest potential risk somewhere in the 10pm to 1 am range. And, the model suggests at least some thunderstorms could continue develop across mainly southern sections beyond 2am. Of course, computer representations of the atmosphere and the way the atmosphere ACTUALLY develops don't always dovetail. But the fact that other models are and have been generating a similar set conditions in roughly the same time period makes the severe weather threat one which deserves to be watched.
Our RPM models---both 4km and 12 km versions--are run every 3 hours. The latest 4 km run available for this 3pm Friday posting is easily the most aggressive among the recent series of model runs in destabilizing an atmosphere Friday night predicted to be quite energetic. CAPE levels--an index of atmospheric energy levels--are predicted to surge during the 8pm to 2 am period to a worrisome 2500 to 3000 joules/kg in the most recent RPM run---1000 joules/kg is often cited as the level at which the severe weather risk becomes a concern. Several Weather Service models have been putting these values at or just above the 2000 joules/kg threshold in recent runs. At the same time, the RPM models suggest an even more unstable atmosphere than had been earlier predicted may develop with surface based lifted indicies--a reflection of how fast temperatures are to decline with altitude--- dropping to -6 to -8 during the 8pm to 2am. Not all of you work with these indicies and may not know what to make of these numbers. But a -6 to -8 lifted index value is worrisome to meteorologists because it suggests a sharp drop in temperatures with height. This means the increasingly humid air predicted to be entering the area overnight may well become quite buoyant because of faster than usual temperature declines with height and encouraged to ascend. These indicators plus the presence of 1.50" of evaporated water and shifting wind directions with height as well as the approach of a cold front along which low-level winds are to converge, all contribute to the view that Friday night's severe weather risk is elevated here and should be monitored.
There are a couple of important wildcards in the Friday night weather---there almost always are in severe weather situations. Thunderstorms, currently making their southeastward trek from Minnesota, northeast Iowa and Wisconsin toward Illinois, are producing so-called "debris" cloudiness which moves with upper winds out ahead of the storms themselve. This debris cloudiness can affect the degree of warming. These storms are also producing outflows of cool air dragged to the surface by their rainfall. These factors can both act to stabilize the atmosphere and cut into its ability to support powerful thunderstorms. Our models don't always do a terrific job of handling these effects from thunderstorms--especially thunderstorms already underway as the forecast cycle begins. That's why these potentially mitigating factors are being monitored. But, The a consensus of model forecasts suggests these potentially stabilizing factors may well be overcome by the meteorological set-up expected to be in place Friday night. Thus, a threat of severe weather can't be discounted over at least sections of the Chicago area--a threat even more acute in western Illinois.
If severe weather develops, we'll activate our wgntv.com "Severe Weather Blog"
http://weblogs.wgntv.com/chicago-weather/severe-weather/
to keep you posted on storm warnings and updates. And we'll have more on our WGN News programs at 5:30pm and 9 pm tonight---as well as here on our weather blog.
Tom Skilling
