203
FXUS63 KLOT 301746
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1246 PM 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 /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1245 PM CDT Thu Jul 30 2026

Key aviation forecast messages:

- Potential for east wind shift at ORD/MDW behind lake breeze
  this afternoon.

- Showers begin to move into the area Friday afternoon

Winds are light out of the south to southwest around 5 kt early
this afternoon. A lake breeze is gradually drifting inland
toward ORD/MDW (already through GYY). Confidence whether it
reaches the terminal remains on the lower side but given overall
weaker surface flow and hi-res model support, opted to
introduce a wind shift around 21Z to account for slightly
farther inland progress. Will continue to monitor radar and
observational trends closely as it remains possible that it
stalls just east (or right over) the terminals. Winds behind the
lake breeze could increase into the 5-10 kt range before easing
with sunset this evening.

Light and variable winds overnight return to a prevailing
southerly direction after daybreak then gradually trend
southeast ahead of our next system. Shower coverage through the
afternoon increases from west to east across the area,
remaining mainly VFR through the daylight hours.

Petr

&&

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

&&

$$

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