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.

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 /18Z TAFS THROUGH 00Z SUNDAY/

Issued at 1213 PM CDT Fri May 1 2026

Key Messages:

- Easterly wind shift at the main Chicago terminals ongoing with lake breeze boundary.

- A few isolated to widely scattered showers and locally lower CIGs possible (20-30% chance) this afternoon.

- Northerly winds turn northeasterly again across northeastern IL and northwest IN Saturday afternoon.

Similar to yesterday at this time, a lake breeze boundary is again in the process of moving right across ORD and MDW. Winds at the main Chicago terminals will thus settle into an easterly direction for the remainder of the day. Winds will then ease this evening and may go variable for a period overnight. Wind trends on Saturday then look to mimic those of today, with directions settling in a north-northwesterly direction after daybreak Saturday morning, then shifting easterly with another lake breeze for the afternoon.

Otherwise, expect lower VFR CIGs this afternoon, along with some isolated to widely scattered showers. The coverage of showers this afternoon should be lower, and of shorter duration, than those observed yesterday afternoon and evening. The best chances (20-30%) for these showers will be in the vicinity of the lake breeze boundary across northeastern IL and northwestern IN. To account for this, we have opted to add a PROB30 mention for some showers. The threat for these will wane prior to sunset this evening.

KJB

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.