textproduct: Chicago

KEY MESSAGES

- Frost development is expected tonight into early Saturday morning across many areas.

- There is a low (~15-25%) chance for thunderstorms late Sunday followed by a better (~30-40%) chance for thunderstorms on Monday.

- Outside of a brief warmup early next week, near to below normal temperatures are favored through next week with occasional chances for rain showers.

DISCUSSION

Issued at 329 AM CDT Fri May 1 2026

A few lingering showers are still ongoing early this morning as an upper-level shortwave trough continues to pivot through the area. Subsidence on the backside of this wave should bring an end to this precipitation by mid-morning, but steepening low- level lapse rates amidst filtered diurnal heating and modest cold air advection may pave the way for the redevelopment of isolated showers or sprinkles in some spots this afternoon as another shortwave approaches the area. It's possible that a slightly greater coverage of showers may be realized along the lake breeze, where surface convergence will provide a localized boost to cumulus growth, but with low-level moisture looking somewhat limited, didn't see a need to go with anything higher than slight chance PoPs in the forecast grids at this time. The combination of cloud cover and cold air advection will result in unseasonably cool temperatures for the start of May, with high temperatures today expected to range from only the mid 40s to the mid 50s (coolest near Lake Michigan).

Surface high pressure will build into the region tonight, causing winds to become light/calm. Skies should also start to clear out as the secondary shortwave departs, and frost development should ensue thereafter with efficient radiational cooling allowing for air temperatures to dip into the low-mid 30s across most of the area. Have hoisted a Frost Advisory with the expectation for widespread frost development across most of our CWA, though there are a couple of exceptions. One of these exceptions is our southern/southwestern counties, where uncertainty remains regarding whether clouds will clear out in time and allow for temperatures to fall enough to be conducive for frost development. It is also probable that the urban heat island effect may keep temperatures high enough to preclude frost development within most or all of the city of Chicago and its closest suburbs, so have kept central Cook County out of the advisory as well, at least for now.

Dry conditions and similar to perhaps slightly milder high temperatures are expected on Saturday before the next shortwave trough in this northwest flow pattern reaches our area on Sunday, bringing our next chances for precipitation. Existing uncertainties with respect to the trajectory of the shortwave and the timing of its arrival preclude having any PoPs higher than 30-40% in the forecast grids at this time, with several ensemble solutions suggesting that there are many scenarios where most or all of our forecast area could stay dry through Sunday night. That said, should the shortwave arrive at an optimal time and take a favorable track that would maximize synoptic-scale lift over northern Illinois and northwestern Indiana, then mid-level lapse rates would become steep enough to be supportive of a few thunderstorms in our area during the afternoon and/or evening.

Breezy southwesterly winds are also expected to develop on Sunday as a weakening surface low tracks into Ontario and will help push temperatures back into the 60s across the area. 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 into the northern Great Lakes. Continued warm air advection off of these winds will propel temperatures into the 70s south of an inbound cold front on Monday, while also inducing an influx of better low-level moisture that will bring an increased chance for showers and thunderstorms to our area along and ahead of the cold front as it scoots on through. Strengthening mid- to upper- level flow along the southern periphery of the aforementioned trough could support some organization to these storms, but only modest destabilization and subpar lapse rates will be limiting factors for how strong storms will be able to get across our forecast area.

Aggregate troughing and northwest flow over the Great Lakes and Midwest will then remain the theme for the synoptic upper-level pattern for the remainder of the week and into next weekend. This will favor the persistence of near to below normal temperatures after Monday's cold frontal passage and should allow for additional periodic opportunities for rain showers as more disturbances embedded within the mean longwave troughing and northwesterly flow aloft dive into the region.

Ogorek

AVIATION /12Z TAFS THROUGH 18Z SATURDAY/

Issued at 608 AM CDT Fri May 1 2026

Some patchy MVFR CIGS are possible early this morning, otherwise look for VFR conditions through the TAF cycle. Isolated light showers or sprinkles are possible near the lake breeze this afternoon, however given minimal impacts and low confidence and coverage, opted to keep the TAFs dry.

Light north-northwest winds this morning will increase a bit later this morning with a few gusts up to 20kt possible midday. A lake breeze is expected to push inland during the early-mid afternoon with a wind shift to northeast at the Chicago terminals this afternoon. Winds will diminish and become near calm tonight.

- Izzi

LOT WATCHES/WARNINGS/ADVISORIES

IL...Frost Advisory from 1 AM to 8 AM CDT Saturday for ILZ003- ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013- ILZ020-ILZ023-ILZ033-ILZ103-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Frost Advisory from 1 AM CDT /2 AM EDT/ to 8 AM CDT /9 AM EDT/ Saturday for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.