textproduct: Chicago

KEY MESSAGES

- Gusty showers and storms will move across the area this afternoon and evening, favoring areas north of Interstate 80.

- Temperatures will trend cooler for the rest of the week, with a threat for patchy frost Wednesday night.

DISCUSSION

Issued at 217 PM CDT Mon May 4 2026

The center of a 982 mb surface low pressure system was located over central Ontario at press time, with a cold front extending southwestward through central Wisconsin and into central Iowa. Ahead of the cold front, strong southwest winds (locally gusting in excess of 40 mph south of Interstate 80), nearly full sunshine, and a relatively dry low-level moisture column (surface dew points in the upper 30s to mid 40s) have led to temperatures rocketing into the upper 70s to lower 80s.

While low-level southwesterly flow should allow for at least modest moisture recovery through the afternoon (dew points returning to the lower 50s from south to north), the expectation is for the boundary layer to remain effectively deep and well- mixed through this evening. With that said, uncapped thermal profiles by late afternoon should result in further deepening of the developing high-based cumulus field with eventual development into scattered showers and thunderstorms. Coverage may initially be highest across northwestern Illinois between 4 and 6 PM before spreading eastward through the remainder of the area this evening (ending by midnight or 1 AM). With the deep and well- mixed boundary layer, instability values will not be off the charts today (MLCAPE of 500 to perhaps locally 1000 J/kg, largely driven by cooling mid-level temperatures). With that said, we continue to suspect that that the tallest convection will be able to produce localized damaging winds via efficient evaporation in to the dry sub-cloud layer especially if any conglomeration of individual cells can occur to support a composite cold pool. Ironically, such a threat may end up where moisture quality is poorest today and largely along and north of Interstate 80 where dew points may remain below 50 degrees. Cannot sleep on the threat for severe hail today as well given the cool mid-level temperatures and marginal shear profile. With that said, the threat for tornadoes will be curtailed by large surface temperatures/dew point spreads and high cloud bases.

Toward and especially after midnight, any remaining thunderstorms should shift southeast of the area. Increasing upper-level jet forcing acting upon the broad baroclinic zone draped across the Lower Great Lakes will support occasional flares of frontogenesis, altogether leading to episodic showers after midnight and through the morning hours. Additional waves of showers and perhaps a storm or two will continue through the afternoon hours, though with highest coverage shifting southward through time (if not entirely out of our area by mid-afternoon) as the low-level baroclinic zone similarly shifts southward. Tomorrow otherwise looks much cooler than today with mostly (largely upper-level) cloudy skies and prevail cold air advection leading to highs in the upper 50s to around 60. Onshore flow near Lake Michigan will hold shoreline locations to the upper 40s. Continued mostly cloudy skies tomorrow night will hold overnight lows in the upper 30s (north) to lower 40s.

The greatest push of cool air will arrive on Wednesday as 850mb temperatures fall toward -2C. When paired with mostly cloudy skies, highs will struggle to get out of the 50s. There may be some attempt for upper-level clouds to scatter Wednesday night, setting the stage for overnight lows in the mid to upper 30s. Should clouds end up clearing completely in parts of the area, lows could make a run for the lower 30s with an associated threat for frost (especially across interior northern Illinois).

Looking toward the end of the week and beyond, ensemble guidance favors a gradual warming trend as upper-level cyclonic flow shifts eastward. With that said, will note that interquartile ranges of 850mb temperatures get quite large (over 10C spread) by early next week, suggesting uncertainty in the placement of the baroclinic zone and with overarching implications on temperatures. Regardless, there is a notable trend upward in temperatures by the middle to end of the month advertised by long-range ensemble guidance, in line with climatology.

Borchardt

AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 1236 PM CDT Mon May 4 2026

Concerns: -Scattered thunderstorms this evening, isolated this afternoon -A few wind gusts to 35-40 kt in stronger thunderstorms -Lake breeze timing on Tuesday

Challenges with the 18z set is the timing of scattered thunderstorms Confidence is high enough to shift from a Prob30 to TEMPO forecast for thunderstorms. Recent ensemble guidance would suggest the main window for scattered coverage will be along and ahead of the cold front, with more isolated to widely scattered coverage prior to it. We would anticipate the highest coverage initially in the late afternoon will be along and south/southeast of ORD/MDW where better instability will be, though it could be close enough to MDW to warrant carrying a VCTS a bit earlier.

Expect showers to continue along the front which should pass through the Chicago terminals around 5z. On the back side the front there is another wave that will lead to some lighter rain showers overnight into early Tuesday, though conditions should remain VFR.

Finally, there will be a lake breeze on Tuesday. TAFs reflect the best timing but with medium confidence at this distance.

KMD

LOT WATCHES/WARNINGS/ADVISORIES

IL...Blowing Dust Advisory until 6 PM CDT this evening for ILZ019- ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ107-ILZ108.

IN...Blowing Dust Advisory until 6 PM CDT /7 PM EDT/ this evening for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory until 7 PM CDT this evening for the IL and IN nearshore waters.