FXUS63 KLOT 070845 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 345 AM CDT Tue May 7 2024 .KEY MESSAGES... - Thunderstorms expected this morning, which may produce gusty winds and/or small hail. - A second round of scattered thunderstorms is likely this afternoon, with the highest confidence east and southeast of the I-55 corridor. A few of these afternoon storms could become severe with large hail and damaging winds as the primary hazards. - Another round of storms may develop late Wednesday, some of which could be strong to severe, mainly south of I-80 into central IL/IN. && .DISCUSSION... Issued at 344 AM CDT Tue May 7 2024 Through Wednesday Evening: Early morning Midwest radar mosaic depicts an extensive north- south linear MCS propagating east-northeast across eastern IA/MO. This MCS, associated with a potent negative-tilt mid- level short wave and surface cold front spreading east from the mid/upper Missouri Valley, will continue to push into northern IL and the WFO LOT forecast area after 5 am. The surface cold front was in the process catching up to a warm front stretching from IA into central IL, which should allow storms to become slightly elevated farther east into IL, which along with the gradual diurnal decrease in low-level instability should result in a slow weakening trend as it moves into our area. Despite these trends, localized sub-severe gusty surface winds up to 50 mph will likely still be possible across our western cwa as this line arrives. While a few isolated to widely scattered cells may develop ahead of the line within increasing warm advection flow, the main line of storms should reach the Chicago metro area around 7-8 am. The convective outflow/cold pool footprint from these morning storms will stabilize low levels through midday, with some question remaining regarding location/extent of diurnal destabilization across the forecast area this afternoon. Kinematics certainly favor the potential for strong to severe storms, with the mid-level short wave trough steepening mid- level lapse rates and a 75+ kt mid-level jet spreading across the area. Guidance continues to indicate the bulk of morning convection clearing the area around noon, with around 1000 J/kg of MLCAPE developing by early afternoon. CAMs depict at least widely scattered to scattered thunderstorm redevelopment early- mid afternoon, though location and evolution is of somewhat low confidence. Severe storms are possible area-wide this afternoon with all hazards possible, though areas east/southeast of the I-55 corridor look to have a higher overall and higher tornado threat better low-level instability and stronger shear as the mid-level jet spreads east. The window for greatest severe threat is roughly from 1-2 pm through about 5 pm, with storms becoming more isolated thereafter. Coverage may be a bit better across northwest IL and along the IL/WI border late in the afternoon. Generally quiet weather is expected tonight into Wednesday morning, as the mid-level short wave lifts northeast of the area and short wave mid-level ridging develops overhead. Breezy southwest winds later today will diminish overnight, as weak surface high pressure slides across the area. Another round of showers and thunderstorms is expected to develop Wednesday afternoon into Wednesday night, as the next short wave approaches the area. A surface low pressure wave is expected to develop along the cold/stationary front which will trail across the southern Plains in association with this wave, which will then propagate northeast into the mid-Mississippi Valley by Wednesday evening. Renewed warm advection return flow will help push the front north across central IL/IN as a warm front during the period, with showers and thunderstorms developing northeast into the forecast area during the afternoon. Details remain somewhat unclear with the track of the surface wave and frontal position by Wednesday evening, with various guidance showing quite a bit of spread. A more northerly low/warm front track into our southern cwa would likely present a greater severe weather threat. Ratzer Late Wednesday Night through Monday: A mid-level trough extending southwest across Quebec and southern Ontario will phase with a mid-level low over the Missouri Valley Wednesday night. As the phasing wave crosses the area on Thursday, a residual low to mid-level TROWAL combined with PWATs around 1" and a deep layer favoring warm rain processes supports the potential for a low-end localized band of heavy rain (1-2") across portions of northern Illinois into southern Wisconsin late Wednesday night into Thursday. Outside of this band, scattered diurnally enhanced showers and perhaps a few storms are expected. After a quick-moving ridge yields dry and seasonable temps on Friday, another upper-level low is progged to cross the western Great Lakes on Saturday. Anomalously cold mid-level temps shifting across the area through the day will yield scattered showers and storms, with some stronger cores likely producing locally strong wind gusts. Diverging solutions by early next week with regards to handling of the departure of Saturday's upper low and a building ridge to the west greatly diminishes the forecast by early next week. Kluber && .AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 104 AM CDT Tue May 7 2024 Key Aviation Messages: - Line of TSRA with potential gusts to 30 knots 13-15Z - SE winds shifting SW and gusting over 20 knots around 18Z - Scattered TSRA, possibly locally severe, mid-afternoon A line of TS currently across central Iowa will track eastward across northern Illinois after sunrise. TS onset timing for ORD/MDW is favored roughly around 14Z, with an expected duration less than one hour at any given location. While these storms will be weakening with time, locally higher wind gusts to 30 knots are possible. Behind the initial line of TS, MVFR ceilings are expected to develop as isolated -SHRA persist through the remainder of the morning. SE winds will then veer SW and gust to 25 knots beginning around 18Z. Another round of TS is expected this afternoon. While confidence in TS coverage this afternoon is lower than this morning, it is high enough to change the PROB30 to TEMPO with the most recent TAF issuance. TS may very well be in the vicinity of northeast Illinois for several hours early to mid afternoon, but the most likely window of direct impacts to the terminals is from 19-21Z. These storms may be strong to severe and capable of wind gusts over 30 knots and hail. Beyond the second round of TS, VFR conditions and SW winds are expected through tonight. Kluber && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago