548
FXUS63 KLOT 302009
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
309 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 /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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