970
FXUS63 KLOT 152337
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
637 PM CDT Sat Aug 15 2026

.KEY MESSAGES...

- Another round of heavy rain producing showers and
  thunderstorms late this evening into Sunday morning.

- A brief break in the stormy pattern is expected Monday into
  early Tuesday before storm chances return ahead of the next
  cold front late Tuesday into Tuesday night.

&&

.DISCUSSION...
Issued at 236 PM CDT Sat Aug 15 2026

The remnant MCV responsible for this morning`s heavy rain
across central Illinois is shifting into northern Indiana this
afternoon. In its wake, widely scattered showers and drizzle
persist within a broad warm air advection pattern. While locally
heavy downpours remain possible, limited coverage should
mitigate renewed flooding concerns through late afternoon.
Expect muggy conditions with temperatures near 80 degrees under
mostly cloudy skies.

Another round of showers and thunderstorms is likely (60%+)
tonight ahead of an approaching cold front and mid-level
impulse. Timing favors an arrival into western Illinois around
and after 10 pm, reaching eastern Illinois and northwest Indiana
after midnight. Poor lapse rates and marginal deep-layer shear
should limit the severe threat, but very heavy rainfall and
localized flash flooding remain concerns given precipitable
water values exceeding 2 inches. While most activity appears
progressive, we are monitoring the potential for backbuilding or
training storms, particularly along the southern periphery of
the MCS as the low-level jet veers overnight. Central Illinois
and Indiana are most at risk, encompassing areas saturated by
this morning`s rainfall. A Flash Flood Watch was considered, but
we opted to hold off due to low confidence in the exact
placement of training cells. The evening shift will continue to
monitor trends to identify high-risk areas.

A cold front will move southeast across the region Sunday,
maintaining a 20-30% chance for scattered showers and
thunderstorms, primarily southeast of I-55. Humidity will be
slow to recover following the frontal passage; expect warm,
humid conditions with highs in the low to mid-80s.

Mainly dry weather is expected Monday and Tuesday as surface
high pressure settles over the Mid-Mississippi Valley. Lower
humidity and slightly cooler temperatures will provide a brief
reprieve before another round of convection arrives late
Tuesday. This next system, driven by a cold front and shortwave
digging into the Great Lakes, brings a renewed threat of strong
storms and heavy rain Tuesday night. Quieter, cooler weather
returns Wednesday and Thursday.

KJB

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 637 PM CDT Sat Aug 15 2026

Forecast concerns include...

Chance of thunderstorms late this evening and overnight.
Ifr/lifr cigs/vis overnight/Sunday morning.
West/northwest winds gusting to 20kt Sunday.

A few showers will be possible this evening but the focus for
new shower and thunderstorm development is across southeast IA
and western IL. With at least scattered coverage expected to
develop late this evening and continue into the overnight hours.
Current tempo mention has this potential handled well and made
no changes for now, but some timing tweaks can be expected once
trends emerge. A few lingering showers may persist through
daybreak.

While cigs have lifted to low vfr in many areas south of the
terminals early this evening, mvfr cigs continue to redevelop
across northern IL and confidence is low for cigs trends this
evening and opted to continue prevailing mvfr cigs which are
expected to persist into the overnight hours. Cigs are then
expected to lower into ifr and likely lifr for at least parts of
the area in the predawn hours and continue through mid morning.
Cigs will slowly lift back through mvfr by late morning with
vfr expected by early Sunday afternoon. Visibilities may drop as
low as 1sm across parts of northern IL by daybreak but overall
confidence is low for how low visibilities may drop.

Light southerly winds currently may become light southeast over
the next few hours and then light southwest overnight. A cold
front will move across the area Sunday morning, shifting winds
to the west/northwest with speeds increasing into the 10-12kt
range and gusts in the 15-20kt range Sunday afternoon, which
will diminish with sunset Sunday evening. Winds may turn more to
the north/northwest Sunday evening. cms

&&

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

&&

$$

Visit us at weather.gov/chicago