994
FXUS63 KLOT 300703
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
203 AM CDT Thu Jul 30 2026

.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 /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1245 AM CDT Thu Jul 30 2026

Main Item of Note:

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

Calm winds past sunrise will become light south-southwest by the
mid to late morning. A lake breeze will be able to slowly push
inland this afternoon, probably through GYY in the early
afternoon, and then possibly MDW in the mid to late afternoon.
Given the weaker lake breezes this time of year, ORD likely has
the lowest chance to observe an easterly wind shift. Will re-
assess for possible mention of an easterly shift at MDW in the
12z TAF. Primarily VFR conditions can be expected, aside from
temporary BR outside of Chicago early this morning.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

Visit us at weather.gov/chicago