703
FXUS63 KLOT 182346
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
646 PM CDT Tue Aug 18 2026

.KEY MESSAGES...

- Scattered thunderstorms are possible this evening (50%
  chance), some of which may be capable of damaging wind gusts
  and heavy downpours which may lead to localized flooding.

&&

.DISCUSSION...
Issued at 130 PM CDT Tue Aug 18 2026

An area of showers, with a few embedded storms, have been
developing across parts of northwestern IL and far southern WI
over the past couple hours. It appears this activity is tied to
a corridor of warm air advection just in advance of a remnant
MCV shifting into northeastern IA. We anticipate this isolated
to widely scattered activity to shift eastward across far
northern IL (mainly north of I-80) this afternoon well in
advance of an approaching cold front over the Upper Midwest.
Lightning activity is expected to remain isolated, and likely
focused closer to the WI state line the next few hours as the
instability gradient begins to slide southeast. We do not
anticipate much of a severe threat with these initial showers
and storms this afternoon.

The cold front currently extends from southeastern MN east-
northeastward into northeastern WI. Additional strong to severe
thunderstorms are expected to develop near the front late this
afternoon as the prefrontal airmass moistens/destabilizes.
Deep-layer shear will support organized severe weather, driven
by 45-50 kt mid-level westerly flow overspreading the warm
sector at the base of a digging mid-level trough. Initial
supercells along the front are expected to upscale into a
southeastward-propagating clusters into southern/southeastern
WI late this afternoon, potentially reaching northern IL this
evening. The primary local threats include torrential rainfall,
frequent lightning, and damaging wind gusts exceeding 60 mph.
While the highest threat is currently near the WI state line,
localized damaging winds may persist across the Chicago metro
area this evening, roughly between 7 and 11 PM.

On Wednesday, the primary threat for convection shifts toward
central IL and IN, well south of I-80. However, lingering low-
level moisture in northeastern IL may support isolated afternoon
showers and storms, particularly along the lake breeze
boundary. No severe weather is expected Wednesday afternoon, and
most areas should remain dry. Given recent trends, a 20% chance
of afternoon storms has been included for northeastern IL. High
temperatures will reach the low 80s inland and mid-70s along
the lakeshore.

Surface high pressure will move overhead Thursday, bringing
light winds, mostly sunny skies, and lower humidity.
Temperatures in the upper 70s to near 80 will make for a
pleasant day. Forecast confidence decreases late in the week as
additional waves move through the northwest flow pattern,
bringing a return of shower and storm chances on Friday. While
some storm threat could linger into Saturday, Sunday currently
looks dry with pleasant temperatures in the 70s.

KJB

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 646 PM CDT Tue Aug 18 2026

Forecast concerns include...

Scattered thunderstorms this evening.
Possible mvfr/ifr cigs and fog overnight/Wednesday morning.
Wind shift to northeast Wednesday morning.

A line of thunderstorms across southern WI is expected to move
southeast across the terminals during the mid/late evening
hours. Some weakening is possible, but close enough now to
include tempo thunder mention for a few hours. A wind shift to
the northwest is expected with these storms along with brief
heavy rain that may reduce visibilities to 1sm. Some lingering
showers are possible after the storms move through.

Models continue to show the potential for fog overnight into
Wednesday morning, especially across northwest IL and at RFD.
Guidance is also showing a better coverage for low mvfr and ifr
cigs overnight into Wednesday morning. There is still quite a
bit of uncertainty and have maintained tempo fog mention at
RFD/DPA but have only included scattered mvfr/ifr cloud level
mention for now. There is also a chance for localized dense fog
across northwest IL around daybreak Wednesday morning.

Southwest winds around 10kt will slowly diminish this evening,
with a few higher gusts possible through sunset. Behind the
thunderstorms, winds will likely become light and variable and
then favor a light westerly direction overnight. Winds will
turn northerly after sunrise and then north/northeast Wednesday
morning. A lake breeze is expected Wednesday afternoon, which
may turn winds more easterly and also increase speeds into the
10kt range.

There is a low chance for showers, possibly an isolated
thunderstorm Wednesday afternoon. Confidence is quite low for
this potential and the better chances, while still low, look to
favor areas south of the terminals. Have maintained a dry
forecast during this time period for now. cms

&&

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

&&

$$

Visit us at weather.gov/chicago