textproduct: Chicago

KEY MESSAGES

- Widespread frost development is expected tonight into Saturday morning across portions of northern IL and northwest IN.

- There is a low (~15-20%) chance for showers and thunderstorms 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.

UPDATE

Issued at 916 PM CDT Fri May 1 2026

Have expanded frost advisory slightly into La Salle and Grundy counties for tonight, and lowered temps slightly into these areas with clear/partly cloudy skies and weak surface ridge axis resulting in light/calm winds.

Broad upper level trough remains in place across the Great lakes and upper Mississippi Valley region this evening, though the main trough axis will shift southeast of the area into early Saturday morning. Mid-level height rises already noted in the wake of a couple of embedded short waves which are moving east of the area this evening, with the associated subsidence and loss of low-level instability with sunset helping to scatter out the extensive stratocu deck from this afternoon. Weak surface high pressure ridge is expected to develop into the forecast area overnight in response, and the combination of mostly clear to partly cloudy skies, light winds and a cool air mass setting up conditions for widespread frost to develop by morning across much of the forecast area. Given current trends, have lowered temps a bit farther south into the I-80 corridor west of Chicago overnight, and have expanded the frost advisory slightly to include La Salle and Grundy counties.

Otherwise, quiet weather conditions are expected into Saturday, with no other significant changes made to the forecast at this time.

Ratzer

DISCUSSION

Issued at 205 PM CDT Fri May 1 2026

The broad upper low continues its vacation over the Great Lakes with a shortwave disturbance pivoting along the low's western periphery in IA, WI, and southern MN. This shortwave has lead to the development of partly to mostly cloudy skies across northern IL and northwest IN this afternoon and may even give way to a few isolated showers as well. While recent radar trends do show some light echoes developing in eastern IA, northwest IL, and southern WI; none of them are reaching the ground at this time. However, with dew points across the area mixing down into the upper 20s and lower 30s confidence on showers actually being able to reach the ground has diminished. Nevertheless, have opted to maintain the slight (15-20%) POPs in the forecast for now. Otherwise, expect temperatures to remain in the lower 50s for most with mid to upper 40s near the lake.

Any showers that develop this afternoon will taper shortly after sunset as the shortwave pivots east. Skies behind the wave should quickly clear as modest height rises and a weak surface high moves overhead. The combination of clearing skies, light winds, and cooling temperatures into the low to mid-30s will result in widespread frost development across much of the area tonight. Therefore, a Frost Advisory remains in effect for late tonight into Saturday morning for most of northern IL and northwest IN. The exception however is for Central Cook County (downtown Chicago) and our southwest CWA where milder temperatures and lingering cloud cover may limit frost coverage.

The modest ridging will persist through Saturday night which will keep dry conditions in play to start the weekend. However, the continued northwest winds will keep highs only in the mid to upper 50s inland and in the upper 40s to lower 50s near the lake. With the ridging overhead it seems skies should favor a mostly to partly sunny conditions which may once again lead to some patchy frost development Saturday night into Sunday especially in northwest IN.

The forecast for the Sunday through Tuesday timeframe remains generally unchanged but there is a lot of uncertainty in the exact timing of the key features. The upper low and associated troughing over the Great Lakes is forecast to shift east on Sunday which will allow a couple of shortwave troughs to dig out of Canada and into the region. At the same time, another trough/upper low is forecast to develop off the southern CA coast and should begin to move inland Monday into Tuesday. These features will help to generate stout southwesterly flow across northern IL and northwest IN on Sunday which will lead to a slight warming trend with highs in the mid to upper 60s forecast areawide Sunday afternoon. This warm advection in addition to the forcing from the leading shortwave looks to be sufficient to generate at least some isolated to widely scattered showers and perhaps a few thunderstorms on Sunday. While most guidance continues to favor a later afternoon and evening timeframe for the higher rain chances Sunday, recent guidance trends have started to hint at a faster shortwave arrival which could lead to some showers developing Sunday morning. Given the inconsistencies run to run in guidance have opted to just maintain the low chance (~20%) POPs for the entire day on Sunday for now but suspect some dry hours are possible. Outside of the rain, winds on Sunday will be breezy with gusts likely around 30-35 mph especially during the afternoon.

The better chances for rain are still expected to occur with the passing of the more amplified secondary shortwave and its associated cold front late Monday into Tuesday. With guidance trending later with the frontal passage, it appears likely that highs on Monday will top out in the 70s areawide courtesy of the continued breezy southwest winds. Despite there being a stronger signal for a later frontal arrival, there continues to be uncertainty as to the degree of moisture return ahead of the front which will dictate the coverage and intensity of thunderstorms. Regardless, a fair coverage of showers and thunderstorms is expected for Monday especially during the afternoon and evening hours. Given that moisture still looks to be a limiting factor the threat for any severe weather continues to look low, but if more moisture is able to advect in then that may change considering that 35-40 kts of deep shear are forecast to be present.

The cold front is progged to shift south of the area late Tuesday which is expected to return cooler temperatures to the region as broad troughing redevelops over the Great Lakes. Thus, highs in the 50s (locally cooler near Lake Michigan) can be expected for the middle and later half of next week. Additionally, periodic shortwave disturbances will continue to dive through the broader trough and result in occasional chances for showers especially towards the later half of next week.

Yack

AVIATION /00Z TAFS THROUGH 06Z SUNDAY/

Issued at 651 PM CDT Fri May 1 2026

An axis of light showers associated with a weak, elongated mid- level wave over or just north of the terminals early this evening should gradually dissipate through mid-evening with the loss of daytime heating. Additional axes of mid-level moisture/clouds upstream over Wisconsin should also diminish with time this evening and potentially result in SKC conditions overnight, though residual FEW/SCT clouds may continue to advect over the area. Outlying sites in northern Illinois (DPA/RFD) may experience BR/MIFG late tonight. Shallow VFR cumulus will then redevelop Saturday afternoon.

NE winds up to 10 knots at TAF issuance will subside through the evening while becoming VRB/calm overnight as a surface ridge extends eastward over the area. Winds will then settle NW up to 10 knots Saturday morning. While there is some uncertainty on if/when a lake breeze affects ORD/MDW during the afternoon, expectations are that winds at both sites should shift ENE during the mid-afternoon. Winds will then shift SW during the evening ahead of an approaching trough on Sunday.

Kluber

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- ILZ019-ILZ020-ILZ021-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.