612
FXUS63 KLOT 302348
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
648 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 309 PM CDT Thu Jul 30 2026

Mostly sunny skies earlier in the day allowed temperatures to
warm well into the 80s by early afternoon with increasing high
cloud coverage helping to stall further warming, with highs in
the mid to upper 80s expected for inland areas. A weak lake
breeze has kept temperatures a bit cooler near the lake, in the
mid 70s to around 80, making for a pleasant day for late July.

We are monitoring a wave currently moving across the SD/NE
border this afternoon that is expected to gradually drift east
toward the Upper Mississippi Valley tonight into Friday,
eventually becoming a cut-off low. An associated deepening
surface low and area of showers and embedded thunderstorms will
move across Iowa into Illinois during the day on Friday. The
expectation here locally is for (mainly light) rain to move in
from west to east through the afternoon, with the potential for
pockets of higher rainfall rates favored during the evening
hours through Saturday morning. While there remains a fair
amount of variability in placement of the highest rainfall
amounts (upwards of 2-3+ inches) amongst the various suite of
available guidance as well as from run to run, the signal for
heavy rainfall in the local area has remained fairly persistent.

The slow moving nature of this system paired with anomalously
high PWATs would be supportive of torrential rainfall rates. Any
areas where repeated rainfall occurs over multiple hours Friday
evening through early Saturday could lead to flooding,
especially in urban and poor drainage areas. Accordingly, have
coordinated with the Weather Prediction Center (WPC) to expand
the Level 2 of 4 threat of flooding to include most of the area,
including the Chicago metro.

Confidence in how quickly showers end Saturday remains on the
lower side with the potential that showers persist south of
I-80 through the evening/overnight hours before ending toward
early Sunday morning. Mainly dry conditions conditions are then
favored during the first part of next week with gradually
increasing temperatures back into the 80s by TUE-WED. An upper
level wave and attendant surface front may swing through the
area toward midweek bringing renewed shower/storm chances.

Petr

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 648 PM CDT Thu Jul 30 2026

Aviation forecast concerns for the 00Z TAFs:

- Low confidence in lake breeze wind shift at MDW next 1-2
  hours.

- Showers/rain develops Friday afternoon and persists through
  the remainder of the TAF. Thunder potential fairly low,
  greater later Friday evening south/southwest of the
  terminals. Moderate SHRA may produce at lest brief MVFR
  conditions Friday evening.

- SSE winds though much of the period, gradually increasing
  later Friday/Friday evening and becoming more ESE.

Center of surface high pressure has drifted southeast of the
terminals over IN/OH this afternoon. Light southwest surface
winds were not strong enough to hold off the lake breeze moving
through ORD, though radar currently indicates the boundary has
stalled at the northeast edge of the field at MDW. Can`t rule
out a wind shift there in the next 1-2 hours before the breeze
fades, though confidence is low. Otherwise, winds should settle
into a very light (<5 kt) SSE from lat)<5er this evening into
Friday morning before slowly increasing.

A disturbance currently tracking east from the northern
Rockies and northern High Plains will approach the area Friday,
gradually amplifying and slowing as it does. Guidance is in good
agreement in showers developing into the terminals during the
early (RFD) to mid (Chicago) afternoon, with gradually lowering
VFR ceilings. Increasingly deep moisture should produce some
decent showers/rain especially by Friday evening, with periods
of MVFR visibilities (and perhaps eventually patchy MVFR
Ceilings) in the heavier showers. Thunder potential appears
fairly low (<20%) through early Friday evening, then increases
slightly late - though mainly south/southwest of the terminals.

As the surface low pressure center tracks toward west-central IL
late Friday/Friday night, winds should gradually shift more
east-southeast and increase. Gusts into the 15-20 kt range are
possible, though with flow off the lake likely limiting mixing
somewhat have held off mentioning gusts just yet in the ORD/MDW
forecasts.

Ratzer

&&

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

&&

$$

Visit us at weather.gov/chicago