624
FXUS63 KLOT 180830
AFDLOT

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

.KEY MESSAGES...

- Scattered thunderstorms are possible late this afternoon into
  the 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 330 AM CDT Tue Aug 18 2026

All is quiet weather-wise across northern Illinois and
northwest Indiana early this morning with temperatures in the
60s and increasing high cloud coverage. Attention is instead on
monitoring upstream features which will play a role in our
shower and storm chances later today. Overall confidence is on
the lower side in how things unfold today owing to weaker
forcing mechanisms for ascent initially in the afternoon and
less favorable diurnal timing of the cold frontal passage this
evening. This has led to continued variability from model run to
run in the location and coverage of storms later this afternoon
through the evening.

There are currently a few different thunderstorm complexes to
our west and northwest, including southeast Minnesota,
southeast South Dakota and eastern Nebraska tied to mid-level
impulses moving within the broader northwest flow aloft. The
Minnesota storms have been struggling to hold together for
several hours and are likely to continue decaying with
southeast extent across southern Wisconsin. The complex of
storms in Nebraska remains more organized early this morning but
is expected to eventually weaken with eastward extent into Iowa
and northern Missouri. However, it is possible that a remnant
MCV feature within the northern stratiform rain area persists as
it crosses Iowa this morning into the area by this afternoon.
This paired with a mid-level vort may lead to renewed shower
development this afternoon near and north of I-80. Modeled
soundings suggest that the layer of steeper lapse rates will be
fairly narrow through mid-afternoon suggesting that instability
will be limited and keep the lightning threat lower initially.

Farther to the northwest a cold front is currently diving
southeast across the SD/ND border and northwest MN. As this
moves into central Wisconsin later this afternoon it may lead
to increasing thunderstorm coverage as the mid-level flow
strengthens to 45-55 kt. This combined with increasing PWATs
into the 1.5-2.0" range supports an increasing damaging wind
threat from precip loaded downbursts with any storms that
develop. This activity may then drift southeast into northern
and northeast Illinois this evening (~5-10 PM timeframe). Given
the higher PWATs, locally heavy rainfall will likely accompany
any storms that develop and could result in a localized flash
flood threat, particularly if multiple cells move over the same
areas. The slow southward progress of the cold front may lead to
showers and thunderstorms lingering into the early overnight
hours south of I-80 before they fully exit the area toward
daybreak Wednesday.

Another break in this active pattern is in store midweek as a
cooler and less humid airmass settles into the area.
Temperatures will be slightly cooler for this time of year
(typically lower 80s in mid August) with highs in the mid to
upper 70s each day amidst cooler onshore flow. Confidence in the
details then decreases late week into the upcoming weekend as
additional waves move within the upper level northwest flow
pattern which will bring periodic shower and storm chances into
the weekend.

Petr

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 114 AM CDT Tue Aug 18 2026

Expect VFR conditions through the morning with plenty of cirrus
transitioning to increasing mid cloud cover. SW winds under 10
knots around 5 knots will increase to 10 to 15 knots by late
morning, then gust to around 20 knots through the afternoon.

Ongoing convection over Nebraska and less-so over southeast
Minnesota should steadily weaken with eastward extent this
morning. However, SHRA originating from a residual mid-level
cloud deck may survive across northern Illinois (particularly
RFD) this afternoon.

An elongated mid-level wave and trailing cold front will drag
across southern Wisconsin early this evening and northern
Illinois and northern Indiana late evening into early Wednesday
morning. There remains some question as to how far south
convection develops early to mid evening owing to increasing
capping with southward extent over northern Illinois. While
isolated to scattered SHRA are expected during this time (VCSH
in current TAF), coverage of TS remains low enough to maintain
PROB30 (roughly 23-03Z at ORD/MDW). Will need to continue
refining the time window while determining if confidence is high
enough to convert to a TEMPO TS, but a more concrete time
window may not be apparent until much later today.

The cold front will shift winds NW under 10 knots early
Wednesday morning amid VFR conditions.

Kluber

&&

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

&&

$$

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