textproduct: Chicago

KEY MESSAGES

- Rain expected to develop during the day Friday and continue into Friday night before ending sometime Saturday.

- The potential exists for locally heavy rainfall late Friday through early Saturday, with areas of 2"+ rainfall totals.

DISCUSSION

Issued at 203 AM CDT Thu Jul 30 2026

Water vapor imagery early this morning shows a shortwave trough over the central Rockies. This feature is progged to crest the upper ridge over the central/southern High Plains today before turning southeast and amplifying as it moves into the Midwest and mid-Mississippi Valley Friday into Friday night. The 00z runs of the GFS and ECMWF both close off a 500mb low Friday night into Saturday morning, supporting an unusually deep surface low for this time of year.

Even in the farther north ECMWF solution, the warm sector and stronger instability remain mostly south of our CWA. Impressive upper level divergence with some coupling of upper level jet streaks will enhance ascent in a developing deformation zone on the north side of this cyclone. PWATs at or above 150% of normal, coupled with weak static stability, and strong ascent point to the potential for heavy rainfall within the deformation band on the north side of this cyclone. Can't rule out a few embedded thunderstorms given the weak elevated instability, particularly with parcels being lifted isentropically, but even without thunder, it still looks favorable for potentially convective rainfall rates at times Friday night into Saturday morning.

There's still some spread where exactly the deformation band and heaviest rainfall totals will set up, but potential exists for widespread rainfall totals over an inch with localized swaths of 2"+ totals. Flash flood guidance is pretty high over most of the CWA, though lower in the Chicago metro area where Monday storms have left soils more saturated and streams more elevated. Certainly possible that any transient f-gen driven mesoscale bands of heavier rain/isolated thunderstorms could result in a localized flash flood threat, particularly if it were to occur in the Chicago metropolitan area where antecedent conditions are wetter. Later shifts may need to consider an ESF or possibly even a flood watch, but enough spread in guidance at this point to justify holding off.

There's also spread in guidance with how quickly this system and its associated rain move out of the area Saturday. NBM pops look pretty reasonable with highest pops in the morning trending down in the afternoon. This upper low is progged to be shoved south of the area as upper ridge flops eastward across the Great Lakes Sunday into the start of next week. That should result in a return to dry conditions and seasonably warm temperatures.

- Izzi

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 608 AM CDT Thu Jul 30 2026

Main Item of Note:

- Potential for a lake breeze wind shift at the near lake terminals this afternoon

Another quiet VFR period is in store through Friday morning. Light south-southwest winds will be conducive to a lake breeze pushing slowly inland this afternoon, probably through GYY in the early afternoon. It appears that the boundary may be able to make it past MDW for 2-4 hours later today into this evening, bringing east-southeast winds, followed by a return to light south-southwest winds into early Friday.

Winds will increase to near or over 10 kt from the south to south-southeast by mid to late Friday morning as surface low pressure approaches from the west. Showers will gradually develop Friday afternoon, followed by rain, heavy at times, Friday evening, which will be addressed in subsequent TAF cycles.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.