FXUS63 KILX 311044 AFDILX Area Forecast Discussion...Updated Aviation National Weather Service Lincoln IL 544 AM CDT Sat May 31 2025 .KEY MESSAGES... - There is a 15-20% chance of a stray shower this evening. The risk for severe weather is low (less than 5%). - Wildfire smoke has degraded air quality into the moderate category. The Illinois Environmental Protection Agency (EPA) is forecasting this to continue through at least tomorrow. - Seasonably warm conditions return early next work week, especially with increasing humidity on Tuesday. There is a 20-30% chance heat indices reach 95 degrees on Tuesday, increasing the risk of heat illness. && .DISCUSSION... Issued at 204 AM CDT Sat May 31 2025 Water vapor satellite shows northerly flow across the Prairie State on the western periphery of a trough extending across Ontario/Quebec and the Great Lakes Region. Deterministic guidance agrees a backdoor cold front, invigorated by a lake breeze of off Lake Michigan, will drop southwest through central and southeast IL this evening through tonight, potentially (15-20% chance) sparking a shower or storm. In the wake of that boundary, temperatures will run a couple degrees less warm tomorrow, but heat will build as deep southwest flow overspreads the region ahead of another trough early next work week. Near the middle of the week, the forecast features a 50-70% chance for showers and storms as a cold front sweeps through the district. ***** STRAY SHOWER/STORM THIS EVENING? ***** The CAMs remain in discord over depth of afternoon mixing, low level moisture/instability, and hence whether a couple showers or storms fire along a backdoor cold front slipping south through the area this evening. The HRRR and RAP are sticking to their story of a dry frontal passage while the NAMNest, ARW, and especially the FV3 suggest a couple showers and/or even a storm pop along the boundary. Given the discrepancies, we'll continue to message NBM and HREF's probabilities (15-20%) for precip this evening. Otherwise, expect a mostly sunny day with seasonable warmth, i.e., highs in the low to mid 80s. ***** SMOKEY CONDITIONS CONTINUE ***** Geocolor satellite revealed significant smoke plumes generated by fires in Manitoba and Saskatchewan driven south into the Midwest yesterday. PM2.5 particulates have consequently brought air quality into the moderate category across central and southeast IL; this suggests those who are unusually sensitive to particle pollution should consider reducing their activity levels and shortening the amount of time they're active outdoors. While uncertainty is substantial regarding how much of the smoke gets diurnally mixed into/out of the boundary layer, it seems plausible that there will be periods of degraded air quality through tomorrow and possibly even into Monday as northerly flow keeps a steady stream of smoke coming into the region; this is also forecast by the RAP and HRRR, so we maintain that in the forecast, consistent with the Illinois Environmental Protection Agency's (EPA's) forecast of moderate air quality through tomorrow. ***** SUMMER WARMTH AND STORM CHANCES RETURN NEXT WEEK ***** Temperatures are forecast to run a few degrees cooler (highs around 80 degF) and humidity values lower (afternoon RH < 30%) tomorrow in the wake of the backdoor cold front that will sweep the area tonight. This changes Monday as shortwave ridging builds into the Midwest ahead of the next trough. The dry low levels and full sunshine under the ridge axis Monday should foster highs well into the 80s area-wide, making a run for 90 near/west of the IL River (NBM gives a 30-50% chance). Meanwhile, the ridge will shift east permitting deep southwest flow to overspread the Midwest/Ohio Valley Regions Monday night into Tuesday, resulting in increasing humidity which will make Tuesday feel warmer despite similar temps. While NBM's mean features dewpoints in the low 60s, we have some suspicion those could wind up higher due to model differences in timing of the most pronounced moisture surge ahead of the incoming cold front; using a timing tool over the LREF, we get 30-45% chances dewpoints reach 70 degF sometime between Tuesday and Thursday morning. If that materializes during the afternoon when temperatures are nearing 90, heat indices will reach the mid 90s; at this point, we'd give that about a 20-30% chance of occuring. Given we haven't had a whole lot of heat yet this spring to acclimate folks, we have a key message highlighting the increased risk for heat-related illness, particularly for those sensitive to the heat like the elderly, those without air conditioning, and anyone participating in vigorous outdoor activities. It's unclear how long the heat will linger because it's uncertain when thunderstorms enter the picture. Once upon a time, the global models and their respective ensembles brought us storms on Tuesday, but things have slowed and now the highest precip chances (50-70%) are on Wednesday, with some hints that the front may not clear the area until Thursday. The strongest forcing looks to be displaced to our north, and given modest instability (LREF mean of 500-1000 J/kg SBCAPE) the severe threat doesn't look all that worrisome at first glance. However, with the front getting hung up in/near our neck of the woods there is some concern for repeated rounds of rainfall becoming problematic. The ECMWF has a low Extreme Forecast Index (EFI) but also a Shift of Tails (SOT), implying low confidence but a notable subset of members advertising a climatologically significant precip amount. 6 hour Flash Flood Guidance (FFG), or the amount of rain required to fall during a 6 hour period to result in flooding, is running 2.5-3.5 inches, so we'd really have to be running on the high end of the ensemble envelope (95+ percentile) for precip totals to have significant hydrological issues. Nonetheless, we'll keep an eye on model trends to continually reassess that potential. It'll be breezy, especially by June standards, ahead of this system. NBM gives the area a 60-80% chance (highest west) the strongest southerly gusts exceed 35 mph Tuesday and 40-70% chance (highest east) Wednesday, which seems reasonable from Bufkit soundings which depict top of channel winds around 35 kt. Chances are much lower for over 45 mph, so a Wind Advisory is looking unlikely (less than a 15% chance) but we'll continue to monitor. Bumgardner && .AVIATION... (For the 12z TAFs through 12z Sunday Morning) Issued at 542 AM CDT Sat May 31 2025 VFR conditions are forecast to continue throughout the period, with some (afternoon) diurnal cu around FL080 and haze from wildfire smoke. Winds will increase from the northwest during the late morning, ease during the evening, then briefly pick back up from the east-northeast behind a cold front sinking southwest through the area between 00z and 04z (7-11pm CDT). There is a 10-20% chance for a shower along and ahead of that front, which is too low to include a prob30 group; if confidence increases however, we'll add that to the TAFs. Bumgardner && .ILX WATCHES/WARNINGS/ADVISORIES... None. && $$