773
FXUS63 KLOT 312014
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
314 PM CDT Fri Jul 31 2026

.KEY MESSAGES...

- Periods of rain are expected through Saturday night with a
  chance of thunderstorms, highest storm chances overnight and
  again Saturday afternoon.

- The potential exists for locally heavy rainfall with localized
  pockets or corridors of 2"+ rainfall totals.

- High waves and dangerous swimming conditions at Lake Michigan
  beaches late tonight through Monday morning.

&&

.DISCUSSION...
Issued at 314 PM CDT Fri Jul 31 2026

The first round of showers has moved into the area this
afternoon but have remained mainly light to moderate as far as
rainfall rates go (under 0.5"/hr). The rain and widespread cloud
cover have also helped cool temperatures back into the 70s.

This initial band of showers may decrease in coverage for a few
hours here late this afternoon before coverage increases again
heading into the evening as the center of the low gets closer
to the area (currently centered over Iowa). Instability is slow
to increase with this system which suggests that the lightning
coverage will remain low through this evening though a rogue
lightning strike or two can`t be ruled out prior to 9 PM. The
potential for isolated embedded thunderstorms then increases
from west to east through the late evening and continues
overnight (30-40% chance). Based on the favored track of the low
and a collective assessment of the available model guidance, the
threat for localized flooding this evening through the night is
greatest north of I-80 and toward northwest Illinois and the
WI/IL state line.

There may be a brief lull in coverage in the morning on
Saturday (though that is not consistent among all the guidance),
potentially mixed with reduced visibility in drizzle and/or
fog. Confidence is higher that shower coverage increases
through midday near and south of a Waukegan to Rochelle line,
especially across much of the Chicago metro area. This is the
period we are most concerned about a renewed threat of lightning
as well as heavy rainfall (rates in excess of 1-2"/hr) which
may lead to localized flooding. Concern is greatest in urban
areas and in any areas that experienced flooding from storms
earlier in the week (if showers/storms happen to stall or
redevelop over the same areas). We will also have to monitor
for potential funnel clouds south of the Illinois/Kankakee River
Valley where there is an area of higher low-level instability
closer to the center of the surface low. Heading into the
evening hours on Saturday the northern deformation axis of the
low (and associated area of greater shower coverage and higher
rainfall rates) will likely focus south and east of I-55 into
northwest Indiana and east central Illinois.

Confidence in if and where localized pockets or corridors of
rainfall amounts in excess of 2-3 inches occurs in the local
area remained too low to issue a Flood Watch with the afternoon
forecast update. Have instead issued a Hydrologic Outlook (ESF)
add a bit more visibility to the flooding potential in addition
to our graphical messaging. Will continue to monitor trends
closely in case a Flood Watch is needed with later updates.

While some lingering showers can`t be ruled out into portions
of northwest Indiana into Sunday, cooler and dry 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 though
confidence in the details remains low in that timeframe.

Petr

&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 120 PM CDT Fri Jul 31 2026

A low-pressure system with features more characteristic of fall
will drift over the area throughout the TAF period. Expect
several periods of SHRA mixed with summertime convective
elements.

An initial axis of RA/SHRA has already spread across the
terminals early this afternoon. At the Chicago terminals,
visibility and ceilings are expected to remain VFR. At RFD,
periodic MVFR visibility/ceilings are anticipated, with IFR
conditions also possible over the next several hours.

An arc of convection expected to soon develop from southern IA
to central MO will pivot across northern IL tonight, affecting
RFD through much of the night and the Chicago terminals late
evening and overnight. Periodic SHRA with embedded convection is
possible, with the current PROB30 for TSRA likely requiring one
or two targeted windows with TEMPO TSRA. For ORD/MDW, TSRA is
possible anytime from roughly 04-13Z, but will not occur every
hour during this time.

SHRA coverage and intensity will diminish mid to late morning
Saturday. IFR ceilings expected south and west of the terminals
will expand farther north during the morning, particularly with
added low-level moisture off Lake Michigan. While
climatologically uncommon in early August, prevailing IFR
ceilings may be required in the TAF through the morning hours.
Additionally, BR and spotty SHRA/DZ will lower visibility into
MVFR levels.

Diurnal influences combined with marginal low/mid-level rates
should result in a secondary pivoting arc of low-centoid SHRA
and possible TS over or just south of the Chicago terminals
Saturday afternoon. Have included PROB30 TS as the exact
location of this arc of precip remains unclear. However, any
SHRA/TS will produce torrential rainfall rates with low-end IFR
to LIFR visibility. The main precip arc is expected to begin
shifting south of the terminals very late in the TAF period
(early evening).

S winds 10 to 15 knots at TAF issuance will vary between 150
and 200 degrees throughout the afternoon. Winds will then
gradually back and increase through the rest of the period,
shifting E around 15 knots by midnight, and NE 15 to 20 knots
with gusts to around 25 knots Saturday afternoon.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from late tonight through Monday
     morning for ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement from Saturday morning through Monday
     morning for INZ001-INZ002.

LM...Small Craft Advisory from 3 AM Saturday to 3 AM CDT Monday for
     the IL nearshore waters.

     Small Craft Advisory from 9 AM Saturday to 3 AM CDT Monday for
     the IN nearshore waters.

&&

$$

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