103 FXUS63 KLOT 231130 AFDLOTArea Forecast Discussion National Weather Service Chicago/Romeoville, IL 630 AM CDT Tue Sep 23 2025
.KEY MESSAGES...
- Scattered showers and some storms possible this afternoon. Additional areas of showers and embedded storms tonight, but the greatest potential currently looks to focus mainly south of the forecast area.
- Intermittent showers may linger across the region Wednesday and overnight, but a trend towards drier conditions is expected through the end of the week and upcoming weekend.
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.DISCUSSION... Issued at 249 AM CDT Tue Sep 23 2025
Through early this morning: ongoing disorganized convective clusters currently across McHenry/Lake counties in northeast Illinois, and any additional spotty convection that develops along incoming outflows should gradually weaken and diminish through daybreak. In an unperturbed environment, fog development appears likely given weak near-surface flow and low/minimal dewpoint depressions. However, the presence of the aforementioned outflows/gravity waves and debris cloud cover may end up shunting the greatest fog threat south and east of the region. At the current time, the main fog corridor looks to be east of about a Gibson City to Valparaiso line. If the cirrus canopy diminishes across northern Illinois, the fog threat would increase commensurately there as well.
Things looks generally quiet this morning. The one exception might be along and lingering outflow which may continue pushing east towards the I-57/65 corridors. Additionally, this may serve as a mesoscale focus for isolated/widely scattered midday and afternoon convection.
This afternoon and evening, a cold front will shift south out of Wisconsin and down the lake. Dewpoints ahead of this boundary are forecast to remain in the low to mid 60s, with temperatures slated to rise through the upper 70s/near 80 degrees (assuming ongoing convective debris cloud cover thins through the morning). Forecast soundings reveal minimal inhibition to surface-based parcels with a fairly deep (~200 mb) thick low- level moist layer in place. Interestingly, most model guidance fails to convect along this convergent axis, potentially owing to lingering subsidence in the wake of the ongoing convection and generally limited large scale forcing mechanisms upstairs. Still, given the progged instability, elected to maintain 30-40 percent chances for showers/storms along the southward-advancing boundary this afternoon (the ECMWF in particular depicts quite a bit more coverage of afternoon convection). Finally, while 0-6 km shear is forecast to remain under 20-25 kts (owing to weak 500 mb flow), winds will increase towards 400 mb as a jet streak intensifies overhead. Based on forecast parcel profiles and ELs generally under 10 km, this flow should remain immaterial today, but if warmer temperatures/more instability is realized, we`d need to keep an eye on the threat for some stronger and more organized cores.
This evening and overnight, forcing will increase atop the near-surface frontal zone. This should lead to a general increase in the coverage of showers and embedded thunderstorms. As has been the case in this stretch, continue to see significant run-to-run changes in the handling of the pertinent larger scale features. A recent signal additionally shows a compact and potentially fairly deep surface low developing tonight into early Wednesday morning across downstate Illinois into southern Indiana. Latest ensemble output indicates this potential low track remaining south of the region, which would tend to focus the greatest precipitation chances across central Illinois and Indiana. Based on all of this, ended up confining the highest PoPs (low-end likelies) mainly well south of the Kankakee River. If a solution like the current 00z NAM, UKMET, ECMWF materializes, a locally heavy rain threat could be realized immediately to the north of the surface low track into early Wednesday morning as a coupled upper jet interacts with a compact zone of low-mid level convergence/f-gen. Presently, the most concerning signal for this remains just south of the forecast area.
Carlaw
Our region will be solidly post-frontal on Wednesday, with breezy north to northeast winds in place. Aloft, several shortwaves will transit the region, embedded within broad cyclonic flow in place across the Great Lakes. The broad forcing generated by these features in combination with ample moisture will allow scattered showers and perhaps a few thunderstorms to persist through the day on Wednesday with the highest coverage (30-40%) focused generally of I-80/east of I-55. While the highest rain chances will gradually shift southward with the cold front Wednesday night, the cooler air associated with the upper-low over the Great Lakes does look to develop some modest lake induced instability. Thus allowing some lake effect showers (and perhaps a storm or two) to linger into Thursday along the northeast IL and northwest IN shores before the low pivots east of the area Thursday night.
Temperatures on Wednesday and Thursday are forecast to be the coolest of the week due to the expected cloud cover and lingering rain chances. Therefore, expect highs during this period to top out in the low to mid-70s areawide with lows in the lower to mid-50s.
Heading into Friday, upper ridging is forecast to develop back into the Great Lakes region which will set up another period of dry and above average temperatures to close out September. While another cold front is forecast to dive through our area on Saturday/Sunday, the cold air reservoir behind the front does not appear to be sufficient enough to alter temperatures significantly, except perhaps near the lake. Thus highs should remain in the upper 70s to lower 80s through the weekend with overnight lows in the mid-50s. However, conditions will become more favorable for daily lake breezes towards the later half of the weekend which should keep temperatures cooler (low to mid-70s) at the lakeshore.
Yack/Carlaw
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.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/... Issued at 630 AM CDT Tue Sep 23 2025
Key Aviation Messages for the current TAF period:
- Chances for showers and storms along a lake breeze this afternoon
- Another round of showers possible overnight though better coverage expected to the south in central Illinois
- MVFR, and potentially even IFR, cigs expected tonight
Winds are light and out out of the west. Slowly they will become more northwesterly later this morning before turning to the northeast as a cold front moves south out of Wisconsin and is reinforced by a lake breeze. There is a chance for occasional gusts this afternoon and evening between 15 to 25 knots, but being more isolated in nature it was currently left out of the TAF. There is a chance for showers and storms to develop along this lake breeze this afternoon, which prompted an inclusion of a PROB30 for the Chicago terminals. Stratus will move southward behind the front, increasing the chance for MVFR cigs into Tuesday evening. Guidance is starting to lean toward IFR cigs being possible, but confidence was too low to add it in the TAF. MVFR cigs and northeast winds are expected to remain into Wednesday morning.
As the front moves southward tonight, models are suggesting that there will be renewed chances of showers, and maybe isolated storms, along a convergent axis that develops. The better chances remain in central Illinois. However, there was enough model agreement to keep a PROB30 in for KGYY and VCSH for KMDW.
DK
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.LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...Dense Fog Advisory until 10 AM EDT this morning for INZ019.
LM...None.
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