Chicago, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

748
FXUS63 KLOT 252335
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
635 PM CDT Tue Aug 25 2026

.KEY MESSAGES...

- Isolated thunderstorms are possible across northern Illinois
  late tonight through daybreak. The strongest storms closer to
  the Wisconsin state line may contain small hail.

- Potential for strong to severe thunderstorms Wednesday
  afternoon and early evening along a cold front, with greatest
  severe threat (level 2 of 5) around and east of a Valparaiso
  to Paxton line.

&&

.DISCUSSION...
Issued at 206 PM CDT Tue Aug 25 2026

An upper-level low centered over the southern Manitoba and
Ontario border will track to east of Lake Superior by Wednesday
evening. Weak mid-level waves rotating around the upper-level
low will cross Wisconsin and clip northern Illinois and
northwest Indiana overnight through Wednesday evening.
Meanwhile, a cold front will cross the forecast area late
Wednesday morning through the afternoon.

The first mid-level impulse will brush the area to the north
overnight. A reservoir of higher 900 to 750 hPa moisture
advecting in from the Missouri River Valley should arrive in
time to interact with a N/S axis of low-level isentropic ascent
and steeper mid-level lapse rates. Expectations are for a broken
to scattered line of showers with embedded thunderstorms to
cross northern Illinois overnight through daybreak Wednesday.
Elevated effective shear may be marginally sufficient for a
couple stronger storms with small hail.

A lull in precip is expected mid to late morning Wednesday.
Guidance has come into better agreement with an incoming cold
front, with latest trends indicating a much faster frontal
passage than what was noted just 24 hours ago. The cold front
likely will have cleared much of the northwest CWA by late
morning, most of the Chicago metro early in the afternoon, and
northwest Indiana mid-afternoon. With deep convection hinging on
destabilization via both daytime heating and advection of
higher mix-layer dew points ahead of the front, trends suggest
that convective initiation may not occur until near or east of a
Valparaiso to Paxton line mid to late afternoon. Conditional on
these storm developing before the front clears most of the
forecast area, deep-layer shear will be favorable for a mix of
supercells and merging multicell clusters. Large hail will be a
threat with discrete cells while damaging wind is of most
concern with any storm.

With the main mid-level wave trailing the cold front by a few
hours, have maintained low-end chance PoPs (under 30%) over
northeast Illinois mid to late afternoon as the post-frontal
environment may not fully stabilize and/or mix-out. Isolated to
widely scattered showers and perhaps a couple narrow but deeper
cores with lightning are possible during this window.

Mid-level ridging over the Canadian Prairies will deamplify
while shifting eastward late this week into the weekend.
Associated surface high pressure will track east while becoming
elongated across the central Great Lakes Thursday and Friday.
Cannot fully rule out sprinkles or weak showers on the lake
breeze boundary across northeast Illinois late Thursday
afternoon, but the subsidence inversion should be strong enough
to stunt growing diurnal cumulus.

A loosely zonal pattern across North America this weekend into
early next week favors a more active storm track north of the
forecast area, but close enough to warrant periods of low-end
precip chances for showers and some thunderstorms as early as
Saturday night (and especially toward Monday and Tuesday).
Summer warmth will also return Sunday and possibly Monday with
increasing humidity and daytime highs pushing well into the 80s
(assuming active weather remains bottled up to the north).

Kluber

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 635 PM CDT Tue Aug 25 2026

Key Aviation Messages for the current TAF period:

- Slight chance for showers, and possibly isolated storms, to
  develop around or just before daybreak

- Another chance for showers and storms to develop along a front
  by midday and the afternoon, mainly around and south of KGYY

A stalled lake breeze is slowly retreating to the lake.
Southwest winds are less than 10 knots and will gradually
diminish toward midnight. Winds will increase tomorrow morning
ahead of a front with gusts to around 20 knots. After the
frontal passage midday/afternoon, winds are expected to turn
westerly (if not occasionally northwesterly).

The previously mentioned front is still currently in central
Minnesota, but will descend southeastward through Wisconsin
overnight. A weak boundary out ahead of it may produce some
showers after midnight and closer to daybreak. The main
uncertainty with the showers is coverage so maintained a PROB30
in the TAFs. Weak instability will be present during this
window, so thunderstorms are expected to be more isolated (if
at all). So, thunder was kept out of the TAF presently.

There is lower confidence on the exact timing of the front`s
arrival, but better instability and forcing along the front
increases the chances for thunder development. While the
mid/late afternoon still looks like the prime window for
strongest storm development well southeast of the Chicago
terminals, there was enough model agreement to suggest a small
window for thunder in the late morning/early afternoon near
MDW/GYY. With the better chances around and south of KGYY, no
-TSRA was included in the ORD TAF. However, if and unexpected
slower front transpires, it may need to be reconsidered.

DK

&&

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

&&

$$

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