316 FXUS63 KLOT 262355 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 655 PM CDT Sat Jul 26 2025 .KEY MESSAGES... - Scattered thunderstorms are likely this evening mainly south of a Pontiac to Valparaiso line. Torrential rain with these storms could lead to localized flooding. - Dangerous heat and humidity may return on Monday. - Periods of thunderstorms return Monday through Wednesday, some possibly strong to severe with torrential rain which could lead to flash flooding. - Dangerous swimming conditions at Lake Michigan beaches Wednesday into Thursday. && .DISCUSSION... Issued at 235 PM CDT Sat Jul 26 2025 Through Sunday Night: The primary forecast concern is thunderstorm coverage/location and the impact to the current flood watch. The bulk of additional thunderstorms this afternoon into this evening are expected to remain south of Interstate 80 and may end up mainly south of the Kankakee River. Confidence is increasing that any heavy rain and localized flash flooding, if any occurs, will be south of the current flood watch area and have cancelled the watch. Its possible a new flood watch may be needed for the far southern cwa this evening, but confidence is too low for a new watch at this time. Scattered thunderstorms continue to develop across northern MO into west central IL and this activity will continue to move east/northeast this afternoon into this evening. If this activity were to train over the same areas, then flooding would be the concern. Current trends suggest that if this does materialize, it would be across the far southern cwa, across central IL and central IN. Main severe threat (level 1 out of 5) with any storms through this evening will be locally damaging wind but all of these storms are rotating and with the very moist low levels, there remains a low potential for a possible tornado. Again this would be south of I-80 and more likely across the southern third or so of the cwa. Depending on where these thunderstorms track later this evening, they may continue into the overnight hours across the far southern cwa, but the overall trend should be for these to gradually shift southeast and eventually out of the local area. Confidence is also increasing that Sunday will now be mainly dry for most of the area with little support for thunderstorm development. If anything develops, the best chance would be across the far southern cwa and maintained slight chance pops across this area but later forecasts may be able to go dry for the entire area. Much of Sunday night is expected to be dry. However, there is a consensus among guidance for a possible thunderstorm complex moving southeast from MN/WI into northeast IA/northwest IL early Monday morning, perhaps around daybreak. Low confidence for this possibility, but opted to include low chance pops across the far northwest cwa for now and trends will need to be monitored with later forecasts. With the expectation that Sunday will be dry with at least partly cloudy skies by afternoon, high temps in the upper 80s to around 90 look on track. Dewpoints generally in the mid 70s, perhaps a little higher in the rural areas, maybe slightly lower in the urban areas. This would yield peak heat indices generally in the 95 to 100 range and not planning any heat headlines for Sunday. Finally, with the light winds, very moist low levels and a weak frontal boundary sagging into the northern cwa, fog will be a concern overnight into early Sunday morning and have added fog to the forecast for the north and northwest cwa. Some dense fog is possible but confidence is too low to mention dense fog for now. cms Monday through Saturday: Early Monday, a low amplitude upper wave will track nearby around the northern periphery of the upper high centered over the Gulf coast. At the surface, a counterpart cold front is expected to get hung up to our north while the rest of the wave comes and goes through the morning. The departing upper wave will steal a lot of layer moisture away from the region following the morning. Some convection may linger from Sunday night while these features remain more congealed. For more on the precip chances through early Monday, see the short term discussion above. Plentiful moisture will remain at the surface leading to a highly unstable and largely uncapped midday airmass. Conceptually, this could lead to instances of free convection during the day, although few pieces of guidance resolve any precip on Monday beyond the morning likely due to a lack of forcing mechanisms. So the forecast was kept dry Monday afternoon/evening for the time being. Following what will likely be an uncomfortably warm Sunday, even more sweltering heat is expected on Monday, although there are uncertainties. The footprint of the upper high will expand across much of the Plains and Midwest late Sunday into Monday keeping the hot airmass pumping into the region. Highs Monday afternoon are expected to be in the lower to middle 90s, including along the lakeshore. When coupled with dewpoints forecast in the upper 70s to near 80 degrees, we could be looking at afternoon heat indices between 105 and 110, possibly even nearing 115 in spots. It`s not well understood yet how morning convection and lingering cloud cover will influence daytime heating, and any activity that brews later in the day would also play a role. But for now, it looks as though we`re headed for heat headlines to kick off the work week. Even more uncertainties loom for Tuesday as that cold front is expected to move across the CWA sometime Tuesday-Tuesday night. Model camps have differing opinions on the timing of FroPa and the magnitude of cooling in its wake. Generally, Tuesday should be another cooker around the area with afternoon heat indices again nearing or exceeding 100 degrees. A lake breeze looks to move onshore which would keep communities near and downwind of the lake noticeably cooler, but a couple pieces of guidance do suggest offshore flow throughout the day until that front drops through which would offer little to no relief near the lake. The front meandering about the area will provide low chances for scattered showers and storms on Tuesday. Tuesday night into Wednesday, the upper high will break and give way to a large upper trough swinging across the Great Lakes and another upper high forming over the High Plains. This will allow for more comfortable temperatures on Wednesday with highs forecast in the lower and middle 80s. With the relatively moist zonal flow and ambient forcing around the nearby jet, there is a signal for a wave or two of precip to move through the region Tuesday night through Wednesday. Surface high pressure situating over the Great Lakes will keep the more temperate conditions around during the back half of the week with the high looking to steer most precip systems away from the local area. Doom Increasing north/northeast winds Wednesday afternoon into Wednesday night, possibly gusting to 30 mph will lead to high waves and dangerous swimming conditions at Lake Michigan beaches. These northerly winds may continue through Friday leading to uncertainty for when the dangerous swimming conditions may end. cms && .AVIATION /00Z TAFS THROUGH 06Z MONDAY/... Issued at 655 PM CDT Sat Jul 26 2025 Key Messages: - Spotty showers through this evening into the overnight hours. - Period of IFR to MVFR VSBYs at RFD/DPA overnight - East wind shift behind a lake breeze late Sunday morning. The threat for thunderstorms is expected to remain well south of the terminals through tonight. A few spotty showers can`t be ruled out any time this evening into the early overnight hours and have opted to include VCSH mention through 9Z to account for this. If confidence in a narrow window of greater shower coverage increases a targeted TEMPO can`t be ruled out later tonight. Overall the lightning threat continues to look low (less than 15%). Light winds and ample low-level moisture support reduced visibilities to IFR later tonight across interior northern Illinois, particularly out toward RFD and potentially as far east as DPA. Confidence at GYY is low, but wouldn`t fully rule out visibility reductions there as well. Some MVFR to near IFR cigs may also try to lift in as far north as ORD/MDW but confidence remains too low to include with this update. Light southwest winds this evening will turn light and variable overnight at all sites, a late morning lake breeze is expected to move inland turning winds northeast (GYY) and east (ORD/MDW) during the 15-17Z timeframe. Petr && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago