Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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839 FXUS63 KLOT 200833 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 333 AM CDT Fri Sep 20 2024 .KEY MESSAGES... - Scattered showers and isolated thunderstorms ending early this morning - Widely scattered thunderstorms expected to redevelop this afternoon, mainly near/east of I-57 - Unseasonable warmth will continue through Saturday, with one last 90 degree day of 2024 possible - Periods of showers and some thunderstorms late Saturday night possibly continuing into Monday && .DISCUSSION... Issued at 333 AM CDT Fri Sep 20 2024 Water vapor imagery early this morning shows a vigorous shortwave trough moving eastward across the upper Great Lakes. Some showers and scattered thunderstorms across northwest and far northern IL appear to be on the southern flank of the stronger ascent associated with this shortwave trough. The convection across northern IL is generally expected to be on a weakening trend over the next few hours as it moves east of the more slowly eastward advecting instability axis and the 30-35kt low level jet weakens/veers. By sunrise, expecting just some widely scattered showers and possibly an isolated thunderstorm, with activity expected to dissipate by completely by 13/14z. Weak cold front extending from NW WI south across eastern IA and into northern MO at 08z with move eastward across the CWA today. Low level moisture pooling along and just ahead of the cold front (sfc dewpoints mid-upper 60s) will support an axis of moderate instability (MLCAPE 1000-1500 J/kg) by early afternoon across eastern portions of the CWA. Mid-upper level trough associated with the initial shortwave over the upper Great Lakes should be east of the area this afternoon. However, another, more subtle shortwave is evident on water vapor imagery moving into western MN early this morning. This wave should remain to our north, but perhaps close enough to provide just enough synoptic ascent to allow some widely scattered convection to develop along the approaching weak cold front. Can`t rule out development midday over far eastern IL near/east of I-57, but the slightly better chances should be over northwest IN this afternoon. Any afternoon convection should be out of our CWA by late afternoon. Can`t completely discount a marginally severe storm, but there are quite a few negatives working against a meaningful severe threat, including weak synoptic forcing and weak frontal convergence, substantially dry mid-upper level air mass, and poor mid-level lapse rates. There`ll be a break in the precip threat tonight into Saturday with Saturday looking to feature a good deal of sunshine. Guidance is in pretty good agreement with afternoon 925mb temps 22-25C, which would support high temps near or just above 90F. Lower dewpoints in the wake of today`s front should keep heat indices generally lower than ambient temps, but it will still be quite warm for this time of year. In fact, highs could end up just a couple/few degrees shy of the record high of 94F (both Rockford in 1920 and Chicago in 2017). Saturday night into Sunday, closed mid-upper level low over the desert southwest will move eastward and emerge over the central High Plains. Lead shortwave emanating from this upper low, possibly with some assistance from strong upper low tracking east across the U.S. Canadian border, should lead to showers and thunderstorms developing across the Cornbelt and northern Missouri later Saturday into Saturday night. This activity is expected to move/develop eastward into our CWA, possibly as early as later Saturday night, but more likely Sunday. Moisture pooling near a stalled boundary certainly would support some potentially beneficial rainfall, though there are some lingering questions about the magnitude of forcing this far east of the main southern stream upper low and south of the northern stream shortwave. Assuming the boundary stays parked nearby, the threat of showers and thunderstorms could continue Sunday night and even into Monday as the weakening upper low over central Plains moves east into the mid Mississippi Valley Monday. In the wake of that system, long wave troughing should become established next week over the Great Lakes which should result in more seasonable temperatures and dry conditions as Canadian high pressure ushers in a dry and very pleasant air mass. - Izzi && .AVIATION /06Z TAFS THROUGH 12Z SATURDAY/... Issued at 1240 AM CDT Fri Sep 20 2024 SHRA/TSRA extending from southern WI southwest into IA will continue to move gradually eastward into northern IL during the predawn hours. This activity is likely to weaken as it progresses eastward across northern IL. Greatest TS chances remain at RFD (60% chance) with chances dropping off considerably over the Chicago terminals (less than 30%). In fact, it is possible that this activity could dissipate altogether before reaching Chicago toward dawn, but maintained TEMPO for now and will monitor trends overnight. Cold front will move across the terminals today with southwest winds this morning expected to veer to west and then northwest by later this afternoon. Lake breeze will develop this afternoon and result in wind shift to northeast at GYY and could threaten MDW with a wind shift to northeast for a time early this evening. Confidence wasn`t high enough to include wind shift in MDW TAF, but this will need to be monitored for later TAF issuances. - Izzi && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago