637 FXUS63 KLOT 271130 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 630 AM CDT Sun Apr 27 2025 .KEY MESSAGES... - Turning very warm and windy Monday, with highs in the lower 80s and southerly wind gusts of 35 to 45 mph. - There is a chance for thunderstorms Monday night, some could be strong to severe. - Next chance for showers and storms will be Wednesday night through early Friday morning && .DISCUSSION... Issued at 338 AM CDT Sun Apr 27 2025 Through Monday Night: While some areas of higher level cloud cover are moving across the area early this morning, they have had little impact on temperatures thus far this morning. In fact, many areas outside of the Chicago urban heat island have already dropped in the mid to upper 30s as of this writing. With light winds expected under a surface high the remainder of the predawn hours, suspect that areas of at least some patchy frost will continue to be a threat through 7 to 8 am this morning, before temperatures begin to warm out of the 30s. As the surface high shifts into Lower Michigan this morning, surface winds will turn southeasterly across our area. This will foster warmer weather for the area this afternoon under mostly sunny skies. Expect the warmest locations today inland from the lake, where readings should top out well into the 60s to around 70 degrees. Conversely, persistent onshore east-southeasterly winds will promote notably cooler conditions (low to mid 50s) this afternoon along the northeastern IL Lake Michigan nearshore areas. Following our quiet and pleasant weather today, forecast attention quickly turns towards the next lee cyclone developing across the high Plains into this evening. Ultimately, this storm system will track northeastward into the Arrowhead region of MN by early Monday evening. As it does, a very warm, moist and strongly sheared warm sector airmass (70s to 80s temps and 60s dewpoints) will overspread much of the Upper Midwest into the western Great Lakes region. This is expected to set the stage for a notable severe weather event late Monday, particularly to our northwest across the Upper Midwest in closer proximately to the surface low track. Confidence does continue to increase with the severe weather threat northwest of our region this period. However, uncertainty continues with the extent of the severe weather threat into our area Monday night, owing largely to the unfavorable diurnal timing and lingering capping associated with a stout EML inversion. The current forecast continues to favor a mainly dry period for the area during most of the daylight hours of Monday as the region remains capped under the EML inversion. The only exception to this being the possibility for a short period of widely scattered elevated high based showers and storms with the warm front early in the morning (currently ~20% chance). Accordingly, increasingly gusty and mild southerly winds look to be the main story during the day as strong pressure falls overspread the Upper Midwest in advance of the surface low. Currently, it appears these strong southerly wind gusts will peak in the 35 to 45 mph range later in the afternoon and continue through the evening. These strong winds will drive much warmer conditions (upper 70s to around 80 degree temperatures) all the way to the lakeshore Monday afternoon and evening, so lake cooling is not expected along the Lake Michigan shore. As it stands now, the severe weather threat across northern IL looks to come after sunset Monday evening into Monday night as a cold front approaches the region. Again questions on the coverage of severe storms remain for much of the area. With that being said, any storms that are able to develop into the area Monday evening and night will do so in a strongly sheared environment that would be very supportive of organized severe convection with all hazards (damaging winds, large hail and tornadoes). The highest threat area across northern IL continues to be favored over northwestern parts of the state (highlighted by a level 3 of 5 threat by the SPC), including the Rockford vicinity. Farther east into the Chicago metro area, a level 2 of 5 threat exists, while a level 1 of 5 threat exists southeast of I-57. Stay tuned for later forecast updates as this is a period we will need to monitor closely for nighttime severe weather. KJB Tuesday through Saturday: The surface cold front will still be making its way east- southeastward on Tuesday morning. There is still some uncertainty on the exact placement and speed of the frontal passage which will be influenced by what happens Monday night. Nevertheless, lingering showers and thunderstorms can be expected during the morning commute, though there is lower confidence in coverage. With the exit region of a stout jet streak aloft over the region in the morning, there is still decent forcing to allow the potential for some of the storms to grow stronger. But, recent guidance has sped up the progression of the front allowing the better chances for any severe weather on Tuesday to be mainly south and east of the forecast area. The front is expected to move south into central Illinois through central Indiana and become quasi-stationary Tuesday night into Wednesday. While the chances for showers diminish for areas to the north of I-80 Tuesday afternoon, at least slight chances for showers will persist to the south. Winds will turn to the northwest Tuesday afternoon and gradually usher in a cooler airmass. Temperatures on Tuesday may still reach the 70s, but continued cool air advection will limit highs on Wednesday in the 60s. The next upper-level wave will eject out of the southern Rockies Tuesday night into Wednesday morning developing a new surface low Wednesday night into Thursday morning. A warm front extending east of the low will gradually lift into the area increasing shower and thunderstorm chances Wednesday afternoon (particularly south of I-80). The surface low is projected to continue a northeast track toward Chicagoland Thursday through Friday afternoon. While much of the forecast area will likely get rain and some thunder, the better forcing looks to be south and east of the low. And with some discrepancies in models on the exact track of the low (the GFS is slightly more north than the Euro), there is still some uncertainty with overall impacts as well coverage. Surface high pressure will gradually grow on Friday through the weekend as upper-level ridging moves east over the area. Drier conditions should be expected over the weekend with high temperatures in the 60s (if not some low 70s in certain parts). DK && .AVIATION /12Z TAFS THROUGH 18Z MONDAY/... Issued at 630 AM CDT Sun Apr 27 2025 Key Aviation Messages for the current TAF period: - Gusty southerly winds expected on Monday Winds will become southeasterly this morning. They are expected to mostly be around 10 knots. Isolated gusts around 15 knots are possible, but lower confidence around the Chicago terminals. There is a better chance for 10-15 kt around KRFD where the pressure gradient is slightly stronger. Any lake breeze push for KORD/KMDW should result in an ESE direction, not too different from the synoptic background flow for those locations. VFR conditions are expected throughout the period with only patchy high clouds expected. A weak wave is expected to pass over the area Monday morning around 12Z. Most models are dry, but a few high res models have better saturation with enough lift to suggest some isolated VFR showers and thunderstorms. Chances remain below 30 percent, so it was left out of the TAF presently. DK && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago