848 FXUS63 KLOT 110540 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1240 AM CDT Mon Aug 11 2025 .KEY MESSAGES... - Periodic bouts of scattered showers and isolated to scattered thunderstorms can be expected through the first half of the upcoming work week, probably followed by a few quieter days. - Any stronger thunderstorms the next few days will be capable of producing torrential downpours and localized flooding. && .DISCUSSION... Issued at 301 PM CDT Sun Aug 10 2025 Through Monday: In the wake of the earlier showers and storms, convective coverage has greatly diminished as of this writing, owing to likely subsidence amidst marginal mid-level lapse rates. Isolated to widely scattered showers and perhaps a few isolated thunderstorms should be the theme for the rest of the afternoon hours. Can`t completely rule out an isolated thunderstorm as far southeast as the Kankakee River Valley of northwest Indiana over the next few hours. Looking off to the west of the MS River, a slow moving MCV over eastern Iowa has recently resulted in the initiation of scattered convection. Given the worked over and more stable conditions over much of northern Illinois, along with this wave approaching after dark, how much bonafide deep convection is able to develop in our CWA is a big question mark. The next shift will be able to better refine things per observational trends this evening. Given the uncertainty, weak lapse rates, unfavorable timing, and only modest forcing, kept thunder chances in the slight chance range through tonight and into Monday morning. Our highest forecast PoPs (30-50%) for shower activity regardless of thunder chances/coverage are once again focused near/northwest of I-55. The slow moving cold front up across the upper MS Valley will gradually progress southeast through Monday. Specifics regarding shower and thunderstorm trends in the afternoon are unclear. However, outflow could push the effective boundary farther south resulting in better chances for scattered showers and thunderstorms farther south than previous forecast thinking. Exactly how things play out overnight into the morning hours will help determine how the rest of the day goes. All in all, weak forcing, possible cloud cover effects, and increased uncertainty necessitated lowering PoPs down into the 30-40% range through the day on Monday. If and where thunderstorms develop, coverage now appears more likely to be widely scattered in nature. Marginal mid-level lapse rates and sub marginal deep layer shear should further limit the threat for any damaging downburst winds on Monday. Slow storm motions and high PWATs will keep the threat for localized torrential downpours and flooding going, however. Aside from the uncertain convective details on Monday, it will be another warm and humid day, especially near and southeast of I-55 (upper 80s to around 90F). Castro Monday Night through Sunday: The potential for continued off-and-on rounds of showers and thunderstorms will persist through Tuesday night/Wednesday as an axis of high theta-e air with PWATs near 2 inches remains nearly quasistationary overhead. Several perturbations will slingshot their way through fairly active southwest flow during this time frame, as a robust shortwave drops out of southern Canada and sweep through the northern Plains. Timing each of these individual perturbations remains challenging, as, to some degree, convective augmentation will take place each day with successive rounds of thunderstorm development. That said, there is a decent signal in latest model guidance that a synoptic wave (positively tilted in most guidance) may impinge on the region Monday evening and overnight, yielding an uptick in broad warm and moist advection and an associated increase in shower and some thunderstorm activity. The ensemble signal remains fairly strong that the greatest potential coverage during this period overnight Monday night into Tuesday morning will remain near and northwest of I-55, within the core of the modeled deepest tropospheric moisture. While 925-850 mb southwesterly flow will be seasonably strong (near 35 kts at times), not currently seeing signs of particularly robust convergence axes which would spell more of a concerning heavy rain and flood scenario. Some training of individual convective clusters will be possible with cloud-bearing flow generally southwest to northeast, aligned parallel with the main moist axis, but nothing currently stands out as overly concerning from a widespread flash flood perspective into Tuesday. Guidance remains in decent agreement that a cold front will press south through the region during the Tuesday night/Wednesday timeframe. This should shift the shower and thunderstorm chances southward Tuesday afternoon and eventually bring an end to the daily convective potential. If the front gets hung up in our far south on Wednesday, isolated afternoon showers and thunderstorms will be possible. Moisture and instability looks like it`ll end up sloshing back across the region towards next weekend. Current extended guidance suggests that our region may remain on the northern periphery of lingering subsidence with a belt of stronger cyclonic mid and upper level flow displaced across northern Wisconsin and the UP of Michigan. This may end up keeping the more appreciable storm chances to our north, although there are some signals that the mid-level ridge could break down towards the Saturday night - Monday time frame ahead of another front which could open us up to the next round of MCS chances. Carlaw/Castro && .AVIATION /06Z TAFS THROUGH 12Z TUESDAY/... Issued at 1240 AM CDT Mon Aug 11 2025 The main concerns for the 06Z TAF period include: * MVFR-IFR cigs/vsbys possible overnight into early Monday, highest confidence at RFD * Low confidence on scattered shower chances throughout Monday Low ceilings and fog are beginning to fill in across the western half of ZAU airspace with a number sites around E IA and S WI reporting low MVFR or IFR conditions as of 0530Z. RFD is expected to go at least MVFR closer to 08Z with IFR or even LIFR conditions possible, especially after 10Z. Return to VFR looks like 13-14Z, but may hold off until the end of the morning. Confidence is low in how the worsening conditions will spread into Chicagoland, but category changes will be possible across all sites tonight. Uncertainty kept VFR conditions prevailing around Chicagoland for this TAF cycle. Scattered showers are expected around N IL during much of the day on Monday, although there is low confidence in exact coverage. Highest coverage appears to be well south of the terminals, so all sites were kept dry through the daytime, but the occasional shower can`t be ruled out throughout the day. Confidence increases for late Monday evening and night as a complex of more widespread showers is anticipated to move across. Doom && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago