


Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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999 FXUS63 KLOT 092350 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 650 PM CDT Wed Jul 9 2025 .KEY MESSAGES... - Dangerous swimming conditions are expected at Lake Michigan beaches through early Thursday - Scattered showers and isolated thunderstorms possible through this evening - Multiple potential opportunities for showers/storms/localized flooding Thursday afternoon through Saturday evening. Saturday PM currently appears to have the highest chance for a threat for more than isolated strong to severe thunderstorms. && .DISCUSSION... Issued at 322 PM CDT Wed Jul 9 2025 Through Thursday Morning: A lake enhanced cold front has worked its way down the Illinois lakeshore and continues to sag southward. Due to sustained onshore flow, the High Swim risk for waves up to 5 feet for both the Illinois and Indiana nearshore and Beach Hazard Statement will be maintained through early Thursday morning. This cold front has also kicked off showers in Cook County and southeastern Wisconsin. There have been isolated lightning strikes in a few of the tallest cells. Additionally, better coverage of scattered showers have cropped up along a ribbon of better moisture southeast of Interstate 57. There is plenty of uncapped MLCAPE to allow for thunderstorms to bubble up. However, due to a growing subsidence inversion with drier mid level air, cells will likely struggle to get the proper charge separation need for lightning. Have maintained shower chances (20% to 30%) through sunset, however, the chances for thunder were capped at 20% to message the isolated nature of it. Lastly, with over 1.5 inches of precipitable water and "very" slow storm motions, the main risk with these showers will be the threat of localized downpours. Flash flooding is not expected given the lower coverage, but having rain rates around and inch per hour or more is certainly possible and could lead to localized ponding on roadways. Shower chances quickly diminish after sunset with the forecast remaining dry through midday tomorrow. The only other concern that will be monitored through the overnight will be fog trends. Compared to this morning, the set-up for widespread dense fog looks more murky. Model soundings are showing drier air near the surface with only mid level condensation that would favor more stratus than fog. The forecast was capped at "patchy" mention, and it would mainly be for areas west of the the Fox Valley and south of Interstate 80. DK Thursday Afternoon through Wednesday: Convection emerging out of Iowa and southern Minnesota late tonight into Thursday morning may have yet another lower predictability MCV associated with it. In a change from most previous guidance, several recent CAM solutions simulated a convective footprint in advance of the MCV slowly spreading east-southeastward Thursday afternoon and evening, possibly lingering into the overnight. Forecast soundings valid early Thursday afternoon even near/west of I-39 feature notable dry air at the mid-levels as well as a subsidence inversion around 600 mb. If (likely decaying) convection advances eastward across the MS River Thursday afternoon, it will plausibly outpace the rather sharp west to east instability gradient across the area due to aforementioned antecedent mid-level dry air and capping issues. If this occurs, dissipating showers and isolated storms crossing the I-39 corridor may serve to stabilize the environment for any attempts at robust redevelopment and/or intensification late Thursday into Thursday evening. With this being said, a tongue of more favorable 800-600 mb RH is forecast to spread steadily eastward Thursday afternoon and evening in advance of the MCV. Even with sub-marginal deep layer shear, as is common this time of year, large DCAPE/steep low-level lapse rates and precip loading may present a threat for localized strong to severe downburst winds into Thursday evening (level 1 of 5 severe threat). Given an already low-predictability setup and the change from previous forecast cycles, we took an initial measured step of introducing chance PoPs near/west of the I-39 corridor Thursday afternoon (3pm onward) and then brought these east-southeastward through Thursday night. Seasonably high PWATs in the 1.5" to 1.8" range will also spread eastward Thursday night, in tandem with a modest low-level jet. As we saw with Tuesday evening`s MCV associated surprise flash flooding in Chicago, inherent slow storm motions (and any training convection) spurred by these features could yield a localized non-trivial flash flooding setup. WPC`s day 2 level 1 of 4 (marginal ERO) flash flood threat near/west of the Fox River valley appears reasonable at this time. On Friday, depending on how things evolve Thursday night, lingering showers and isolated embedded storms may serve to slow the diurnal warming some. Assuming any morning convection fizzles out, Friday afternoon-evening looks like a classic diurnal pulse convection setup amidst weak forcing, very warm and humid conditions, and little/no capping of a moderate to strongly unstable profile. Isolated to widely gusty scattered storms (20-35% PoPs/coverage) appear probable, with an IL shore hugging lake breeze potentially serving as a foci. A bonafide advancing mid-level trough Friday night into early Saturday could aid in scattered convection lingering deeper into the overnight, though as usual uncertainty abounds. Any slow moving storms Friday PM through Friday night will likely produce torrential downpours capable of causing at least localized flash flooding. Saturday`s afternoon-evening thunderstorm forecast will in all likelihood be modulated by the extent of convection lingering into Saturday morning. Synoptically speaking and mesoscale unknowns aside, though, Saturday has more apparent ingredients that could yield a scattered (level 2 of 5 type) severe wind threat. Mid-level short-waves embedded in a trough axis extending back to northwest Ontario will be accompanied at the surface by a weak surface low and cold front progressing across the region through Saturday evening. The front will encounter a very warm and moist (exactly how unstable TBD) air mass with dew points well into the 70s. 25-35 kt of deep layer bulk shear will support a bit more storm organization and longevity, especially if linear/upscale growth occurs just ahead of the cold front. For the reasons above, we`re advertising scattered thunderstorms Saturday afternoon (60-70% convective coverage) with this afternoon`s forecast package. Depending on the exact timing of convective initiation and cold frontal approach and passage late day Saturday, some storms should be in progress early Saturday evening, especially I-55 and southeast. Thunderstorms (and any wind and flooding threats) should then quickly end by the late evening as the cold front clears the area. The front should push far enough southeast into Sunday, along with very dry mid-level air and neutral to positive mid-level height tendencies, to result in a mostly (if not entirely) dry period at least through Tuesday morning. Thunderstorm chances may then again uptick midweek and beyond. Temps will continue to average solidly above normal with moderately humid to muggy conditions common to mid July. Castro && .AVIATION /00Z TAFS THROUGH 06Z THURSDAY/... Issued at 650 PM CDT Wed Jul 9 2025 Key messages for the 00Z TAFs: - Scattered SHRA/isolated TS for RFD early this evening along cold front. - IFR ceilings persist off the lake at GYY this evening before improving to VFR. - Low chance for patchy/shallow fog late tonight, mainly at RFD. - Low chance for TSRA to affect RFD Thursday afternoon, with chances decreasing toward the Chicago area. Additional TS potential into the evening, though timing/coverage of low confidence. Combined cold front/lake breeze boundary has pushed slowly inland to about a RFD-JOT-IKK line as of 23Z. Convergence along this boundary has initiated isolated SHRA/TSRA, with main area of TSRA threat now well south of the terminals in northwest IN. SHRA continue to develop along the boundary over RFD however, and expect a period of SHRA there for the first couple of hours of the TAF. Can`t completely rule out TS with this, though eventual loss of diurnal heating combined with fairly warm mid- level temps/poor lapse rates should limit that threat. Have indicated period of SHRA at RFD, though will monitor for any indication of increasing TS potential early this evening. Overnight, light northeast winds are expected to persist, with otherwise quiet aviation weather conditions. There is some potential for shallow/patchy fog to develop after midnight mainly southwest of the frontal boundary location, though can`t rule out some fog at RFD. Surface winds expected to be east to east-northeast across the metro Thursday, while shifting south- southeast and eventually south-southwest toward RFD during the afternoon. Scattered thunderstorm potential increases upstream of the area across MN/IA by Thursday afternoon, with CAM guidance suggesting current convective activity over the Dakotas will develop toward the region along with a mid-level disturbance. Confidence lowers with eastward extent across northern IL however, with stronger capping aloft farther to the east. Have included a PROB30 for TSRA at RFD in the afternoon, but maintained dry forecasts for the Chicago metro sites for now where confidence decreases. Additional scattered TS threat will exist into Thursday night, though again low-confidence in coverage/timing for now. Ratzer && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Beach Hazards Statement through Thursday morning for ILZ006- ILZ103-ILZ104. IN...Beach Hazards Statement through Thursday morning for INZ001- INZ002. LM...None. && $$ Visit us at weather.gov/chicago