262 FXUS63 KLOT 281149 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 649 AM CDT Mon Jul 28 2025 .KEY MESSAGES... - Dangerous heat and humidity today with a heat advisory in effect. - Periods of thunderstorms expected 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, possibly into Friday night. && .DISCUSSION... Issued at 324 AM CDT Mon Jul 28 2025 Through Tuesday: Primary forecast concerns continue to focus on thunderstorm trends this morning, and impacts of outflow and residual cloud cover on temperatures and heat indices into this afternoon. Similar concerns extend into Tuesday, with another potential for an MCS to reach the area in the morning. Regional radar depicts a large MCS developing south-southeast across southern MN/IA early this morning, with an apparent MCV over northeast IA. The strongest convection and severe wind threat was generally propagating southeast into central/eastern IA, where SPC/RAP mesoanalysis depicts a 4000 J/kg MLCAPE axis. An area of isolated to scattered thunderstorms were also noted extending from near the MCV into western IL, within modest west- southwest 850 mb flow/WAA wing along the eastern gradient in MUCAPE and an 850 mb moisture gradient. While the primary push of the MCS is expected to push across eastern IA into western IL this morning, the expectation per CAM trends is that scattered thunderstorms will continue this to spread east into areas generally along/west of the Fox River Valley and I-55 corridor regions through sunrise this morning, before weakening/dissipating during the morning. Precipitable water values around 1.50" will support torrential downpours with these storms, as well as localized wind gusts of 40-45 mph across our far western cwa early this morning. CAMs are largely dry across our forecast area this afternoon, though can`t rule out isolated shower/thunderstorm development along lingering outflow boundaries or the lake breeze. There appears to be little in the way of upper level support however, and have maintained a dry forecast at this time with weak subsidence/rising mid-level heights depicted by guidance in the wake of the morning MCV. The residual outflow and cloud cover footprint from morning storms does lower confidence a bit in temperature and heat index trends across the area into this afternoon. Far north/northwest cwa (especially northwest of I-88) appears to have the greatest potential for outflow to lower surface dew points, along with the immediate Lake Michigan shore areas where a weak lake breeze is expected to develop later this afternoon. High-res guidance suggests that the area should be generally convection-free from midday through this afternoon however, which should allow temperatures to warm into the low-mid 90s by peak heating. Current forecast afternoon heat indices (away from immediate lake shore) range from 100-105 north of I-88, to 105+ to the south, and thus am planning no changes to current Heat Advisory which runs from noon to 8 PM CDT. Attention then turns to another short wave trough which is forecast to propagate into the Northern Plains, along the northern periphery of the upper level ridge. Initial thunderstorm development over that area is expected to eventually evolve into a forward-propagating MCS this afternoon, tracking east-southeast across the Dakotas/Minnesota and northwest/northern IA into this evening. There remains some spread in CAM solutions with this, however there is general consensus in recent 4km NAM, High-res ARW, RAP and HRRR runs indicating this linear MCS may approach the forecast area toward sunrise Tuesday morning, though likely in a gradually weakening mode. SPC has included our northwestern cwa in the new Day 1 marginal (1 of 5) severe risk for wind, which appears very reasonable at this time. Thunderstorm trends beyond Tuesday morning then decrease in confidence, again due to potential for residual outflow and cloud cover which will likely impact the potential for redevelopment later in the day. These same impacts would likely affect temperatures/heat indices as well, and for that reason will continue to address any heat headlines on a day by day basis. If outflow and clouds are less impactful (or don`t appreciably make it into our area), the potential remains for us to heat into the low- mid 90s with peak heat indices around/above 105. Ratzer Tuesday Night through Sunday: Our brief heat wave will be in the process of coming to a close Tuesday evening as a cold front will be beginning to enter northern IL. While there could be some ongoing showers and thunderstorms with the front as it arrives, guidance continues to vary as to the coverage likely owing to differences in the amount of capping in place ahead of the front. Regardless, shower and thunderstorm coverage is forecast to increase late Tuesday night into Wednesday as a shortwave trough pivots into northern IL and provides more robust forcing. As per typical with convection, there will likely be a break between the showers/storms Tuesday night/Wednesday morning and the next round that develops along the front Wednesday afternoon/evening. In terms of severity, modest 500 mb flow associated with the shortwave is expected to generate around 30-35 kts of deep layer shear across northern IL and northwest IN which will be sufficient to support more robust updrafts and a severe weather threat mainly in the form of damaging winds. Additionally, the humid air mass ahead of the front (PWATs around 1.75-2.0 inches) will allow any showers/storms to be efficient rainfall producers which may result in instances of flooding especially if storms train over the same area. The showers and thunderstorms will gradually taper from north to south Thursday morning as the front exits into southern IL, but some lingering rain chance will be possible south of I-80 Thursday afternoon. Otherwise, expect notably cooler, less humid, and drier conditions to move into the Great Lakes behind the front as we become seated under the western periphery of an upper trough forecast to be centered over eastern Ontario and the northeast CONUS and a Canadian surface high. Therefore, expect highs Thursday through the weekend to be in the upper 70s to around 80 with dew points in the mid to upper 50s. However, the cold advection will also result deeper mixing and as a result breezier winds especially over Lake Michigan. Couple this with the onshore wind directions and conditions are in place for high waves and dangerous rip currents at southern Lake Michigan beaches. While the peak of the strongest winds and associated highest waves are expected Wednesday through Thursday night, continued 10-15 kt onshore winds on Friday may slow the subsiding wave action and thus prolong the rip current threat through Friday as well. So if you are planning a trip to the beaches this weekend be sure to check the forecast before heading out. Yack && .AVIATION /12Z TAFS THROUGH 18Z TUESDAY/... Issued at 649 AM CDT Mon Jul 28 2025 Forecast concerns for the terminals... - Scattered showers and isolated storms tapering through mid- morning. - 15-20% chance for additional isolated showers and thunderstorms along the lake breeze this afternoon. - Another period of thunderstorms early Tuesday morning, a few of which could be strong to severe with gusty winds. A decaying cluster of scattered showers and isolated thunderstorms will continue to drift across northeast IL and northwest IN through mid-morning before tapering. Once showers/storms end, skies are expected to gradually scatter out with winds turning northwesterly towards midday. However, there remains a 15-20% chance for an isolated shower or thunderstorm to redevelop along the lake breeze this afternoon. While the instability and moisture looks sufficient for showers and storms, the weak forcing with the lake breeze makes this forecast low confidence and thus the reason for leaving the TAFs dry for now. That said, the lake breeze does look to reach ORD and MDW between 23-00z this evening which will result in an easterly wind shift but speeds will be light (only around 5-6 kts). Another thunderstorm complex is forecast to develop across MN this evening and track into southern WI overnight. With the steering flow aloft aimed towards northern IL, confidence has increased that this complex could reach the terminals around daybreak Tuesday morning. While there is a chance the storms could weaken (like they did today), felt confidence was there to warrant the introduction of PROB30s at ORD, MDW, and RFD. Note that the complex may also reach GYY and DPA, but since the arrival time is after their respective TAF periods they were left dry for this issuance. If the storms do hold together there is the chance that a few storms could be strong to severe with gusty winds and torrential rain as the main threats. Yack && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Heat Advisory from noon today to 8 PM CDT this evening for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012- ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039- ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108. IN...Heat Advisory from noon CDT /1 PM EDT/ today to 8 PM CDT /9 PM EDT/ this evening for INZ001-INZ002-INZ010-INZ011-INZ019. LM...None. && $$ Visit us at weather.gov/chicago