282
FXUS63 KLOT 171952
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
252 PM CDT Mon Aug 17 2026

.KEY MESSAGES...

- Threat for severe thunderstorms and flash flooding Tuesday
  afternoon/night.

&&

.DISCUSSION...
Issued at 252 PM CDT Mon Aug 17 2026

While the weather is pleasant and quiet for a change today,
attention quickly shifts to Tuesday afternoon and evening as a
mid/upper trough digs into the Great Lakes. As this occurs, a
belt of enhanced mid and upper level flow will sag southward
into the Midwest and steer a series of mid-level impulses right
across our area as a surface cold front drops southward into
southern WI into northern IL.

The kinematic environment is expected to be quite favorable for
organized severe thunderstorms, owing to the 45-55 kt of mid-
level westerly flow developing through the base of the Great
Lakes trough. The main uncertainty at this time regarding the
severe thunderstorm threat across our area is afternoon
destabilization. There is some potential for storm development
this evening across parts of the northern Plains to shift east-
southeastward across the Corn Belt tonight, and then across our
area in a weakening fashion late Tuesday morning/early afternoon.
If this were to pan out, afternoon destabilization could be
limited ahead of the front due to extensive cloud debris and
remnant showers, hence leading to a lower severe threat into the
evening. This potential thus keeps confidence in the severe
threat over our area low at this time. Nevertheless, if ample
destabilization occurs in the afternoon, conditions will be
favorable for the development of severe thunderstorms, including
supercells. The main hazard from these looks to be wind gusts
in excess of 60 mph. The threat of a tornado or two is low, but
non-zero. Locally heavy rainfall may also accompany these storms
into the evening, and could result in a localized flash flood
threat, particularly if multiple cells move over the same area.

The surface cold front will shift south into central IL and IN
into Wednesday. In its wake, a cooler and less humid airmass
will filter into the area on northerly winds. A period of
pleasant and dry weather is expected to persist for a couple of
days with this airmass as a surface high shifts over the Great
Lakes on Thursday. High temperatures are forecast to remain in
the mid to upper 70s both days under partly cloudy skies.
Confidence in the details then decreases late week into the
upcoming weekend as additional waves move within the upper-level
northwest flow pattern, with a potentially more robust and
slower moving wave bringing increasing shower/storm chances for
the weekend.

KJB

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1223 PM CDT Mon Aug 17 2026

VFR conditions expected through much of the period. A weak low level
boundary may be enough to generate isolated showers/sprinkles
through mid afternoon, though chances remain rather low (10-
20%). Light winds otherwise tonight into Tuesday morning before
southwesterly winds increase to near 12 knots in advance of a cold
front Tuesday afternoon. There could be just enough heating and
instability near the approaching boundary and/or pre-frontal feature
to produce a few showers and storms mid-late afternoon tomorrow.

Steinwedel

&&

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

&&

$$

Visit us at weather.gov/chicago