textproduct: Chicago

KEY MESSAGES

- Rain showers are expected in the area on Sunday, followed by a 30-50% chance for thunderstorms Monday night. If storms occur Monday night, a few could become strong to severe (primarily near and south of I-80).

- Outside of a brief warmup Sunday into Monday, near to below normal temperatures are favored through next weekend with occasional chances for rain showers.

DISCUSSION

Issued at 256 AM CDT Sat May 2 2026

Aside from the frost and localized below freezing temperatures this morning, today is looking be a pretty uneventful day weather-wise with surface high pressure drifting through the region. Temperatures will climb from the 30s this morning into the 50s this afternoon, though a lake breeze may keep some of our lakeside locales just below the 50 degree mark. Increasing mid- and upper-level cloud cover tonight will prevent Sunday morning from featuring a repeat of this morning's frost, though still couldn't completely rule out some patchy frost development in our far southeastern CWA, which will have more time to spend underneath clear skies compared to areas farther to the north and west.

Sunday morning, the upper-level shortwave responsible for the increasing cloud cover will arrive in earnest. The ascent offered by the shortwave, augmented by low- to mid-level frontogenesis and isentropic lift, will drive some rain showers through the area during the morning and early afternoon without much additional fanfare. The descending hydrometeors will still have to work through a wedge of dry air below 700 mb in order to reach the ground, and it remains plausible that areas removed from the main mesoscale axis of frontogenetical forcing will see little to no precipitation as this activity moves through the area. Nevertheless, with deterministic and ensemble guidance converging on a common solution with measurable precipitation occurring in at least a portion of our forecast area, an increase in PoPs during the daytime hours on Sunday was warranted. The earlier arrival time of this shortwave and an associated weak cold front will likely relegate most of any threat for thunderstorms during the late afternoon and evening to our south and southwest, though it still couldn't be ruled out at this time that at least some attempts at convection occur closer to home while the front still hasn't fully cleared our forecast area.

The low-level pressure gradient will also compress on Sunday as the surface pressure trough associated with the cold front encounters the influence a ~1020 mb surface high centered over the southeastern CONUS. This will yield breezy southwesterly winds across our area that will likely gust in excess of 30 mph at their peak and push air temperatures into the 60s (and possibly even into the low 70s) after the earlier rain showers depart. These southwesterly breezes will continue into Monday as a deeper surface low associated with a more pronounced upper- level trough dives southeastward from the Canadian Prairies towards Lake Superior. Continued warm air advection off of these winds will propel temperatures into the 70s south of another inbound cold front on Monday, while also inducing an influx of better low-level moisture that will bring increasing chances for showers and thunderstorms to our area.

It appears that there will be two main areas/forcing mechanisms that we'll need to monitor for convection late Monday afternoon and evening. One area will be along the aforementioned cold front, which may not reach our CWA until after sunset based on most of the latest deterministic model runs. By this point, an already modest axis of MLCAPE along/ahead of the front will become further depleted, putting into question how long any frontal convection that developed during and just after peak heating in the afternoon can sustain itself into the evening. Additionally, a southwest-northeast oriented low-level jet centered just to our south/southeast and juxtaposed by better (but still not exactly stellar) low-level moisture and instability will likely try to force convection in or near our southern counties as it strengthens during the evening. Eventually, the cold front will encounter the slightly higher dew points across our southern CWA and cross paths with the low- level jet, but this may not happen until late in the evening or during the early overnight hours -- by which point, the available instability coincident with the better moisture will have likely dwindled quite a bit.

Thus, while thunderstorms remain possible on Monday, a lot of uncertainty remains with whether everything will come together just right for them to occur, and it's not a sure thing yet that they will occur in our area. That said, if storms do end up occurring here late Monday, then 30-40 kts of deep-layer shear would support the potential for some degree of storm organization. This would particularly be the case across southern portions of our forecast area, where the Storm Prediction Center has delineated a level 1/5 severe thunderstorm risk in their latest Day 3 Convective Outlook.

Shower and storm chances may continue into Tuesday amidst cooler post-frontal temperatures with plentiful large-scale ascent overspreading the frontal zone that should still be situated in close proximity to our forecast area. Aggregate troughing and northwest flow over the Great Lakes and Midwest will then be the theme for the synoptic upper-level pattern for the latter half of the week and into next weekend. This will favor the persistence of near to below normal temperatures and should allow for additional periodic opportunities for rain showers in the coming days as more disturbances embedded within the mean longwave troughing and northwesterly flow aloft dive into the region.

Ogorek

AVIATION /06Z TAFS THROUGH 12Z SUNDAY/

Issued at 1235 AM CDT Sat May 2 2026

No significant weather concerns for aviation through this evening. While there could be some patchy shallow light ground fog at DPA and RFD early Saturday morning, the current most likely worst case scenario is that VSBY could briefly get to MVFR around and just past sunrise.

Light winds will become northwest after sunrise. Guidance has been trending slightly strong with the westerly wind component this afternoon, which suggests the lake breeze may remain east of ORD/MDW/DPA.

- Izzi

LOT WATCHES/WARNINGS/ADVISORIES

IL...Frost Advisory until 8 AM CDT this morning for ILZ003-ILZ004- ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019- ILZ020-ILZ021-ILZ023-ILZ033-ILZ103-ILZ105-ILZ106-ILZ107- ILZ108.

IN...Frost Advisory until 8 AM CDT /9 AM EDT/ this morning for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.