133
FXUS63 KLOT 210745
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
245 AM CDT Fri Aug 21 2026

.KEY MESSAGES...

- Scattered showers and thunderstorms are expected today,
  mainly during the afternoon and evening hours. A few storms
  could become severe with damaging winds and hail the main
  threats, but an isolated tornado cannot be ruled out.

- Dangerous swim conditions are expected at Porter County Lake
  Michigan beaches Saturday, then all Illinois and Indiana Lake
  Michigan beaches late Saturday night and Sunday.

&&

.DISCUSSION...
Issued at 245 AM CDT Fri Aug 21 2026

A shortwave trough is pivoting across the northern Great Lakes
early this morning and has been leading to the development of
some isolated to widely scattered showers and thunderstorms in
WI, southeastern MN, and eastern IA. This festering of showers
is expected to continue through daybreak with a few of these
showers possibly making it into portions of far northern and
northwest IL. However, with the dry air mass across northern IL
and northwest IN and cloud bases with these showers generally
around 10000 ft any rain should struggle to reach the ground
locally aside from perhaps a stray sprinkle. Therefore, expect
dry conditions to prevail through the rest of the night with
just some passing clouds from time to time. Though, some shallow
ground fog may continue to develop in north-central IL and
northwest IN where temperature-dew point depressions are low
(around 1-2 degrees) and result in some locally reduced
visibilities around 3-5 miles through daybreak.

The aforementioned shortwave will continue to pivot eastward
across northern MI this morning which will allow the trailing
shortwave in northeast ND to pivot into the Upper Midwest and
then eventually northern IL later today. Ahead of this secondary
wave is a cold front that will also be drifting towards our
area. As these features approach, winds will become more south-
southwesterly and begin to advect in the more humid (higher dew
point) air in MO into northern IL and northwest IN. Couple this
warm-moist advection with daytime heating and cooling mid-levels
with the approaching shortwave, conditions are expected to
become unstable and favorable for showers and thunderstorms by
midday. Therefore, expect scattered showers and thunderstorms to
begin to take shape around 12-1 PM CDT in northwest IL and
gradually drift south and east across the rest of the area
through mid to late evening.

In addition to the favorable thermodynamics, a jet streak on
the southern periphery of the shortwave looks to generate around
30-35 kts of effective shear overhead which should allow for
some storms to become better organized and become capable of
severe weather. While damaging winds (up to 60-65 mph) and hail
(up to half dollar size) look to be the main threats, stout
westerly 850-700 mb flow will allow for some low-level turning
and may result in a threat for an isolated tornado or two. With
the better overlap of the low-level shear and surface
instability forecast to be across north and northwestern IL
(namely areas west of the Fox Valley and north of I-80), the
Storm Prediction Center has upgraded this part of our area to a
slight risk (level 2 of 5) for severe weather today. Though, all
of our area will have the potential to see some severe weather
today and thus remains under at least a marginal (level 1 of 5)
risk. Furthermore, locally heavy downpours will also be possible
with the showers and storms today but with their progress
nature and lower PWATs the threat for flooding is low at this
time.

Heading into tonight, the shortwave and cold front should push
through northern IL and northwest IN around midnight and should
bring an end to the shower and thunderstorm activity. Though, a
few lingering showers (15-20% chance) cannot be ruled out in
northwest IN tonight. Otherwise, expect dry conditions tonight
with overnight lows dipping into the low to mid-60s.

While mostly dry conditions should prevail through the day on
Saturday, broad upper troughing is forecast to still be over the
Great Lakes region with a couple of embedded disturbances
expected to be pivoting through northern IL and northwest IN
which may result in a few showers or storms Saturday afternoon.
Forecast soundings on Saturday do show that the mid-levels
should be drying out as the aforementioned disturbances pivot
overhead which should limit the coverage of any showers or
storms, but have opted to maintain a slight chance (15-20%)
mention in the forecast just in case any isolated activity is
able to materialize. The bigger weather concern on Saturday
however, will be the increasing northwest winds (gusting around
20-30 mph) which will result in building waves and an increasing
threat for rip currents at area beaches. Initially, the highest
waves will be in Porter County, IN on Saturday but with the
breezy winds expected to linger into Sunday that the larger
waves and dangerous currents will spread into the rest of
northwest IN and northeast IL by Saturday night. A Beach Hazard
Statement will likely be needed for this time period so make
sure to keep to check the forecast before heading to the beach
this weekend and remember to stay dry when waves are high!

Aside from the breezy winds, dry conditions are expected for
Sunday and the early part of next week as high pressure builds
into the region. However, despite the sunshine and easing winds
temperatures Sunday through Tuesday look to be on the seasonably
cool side with daily highs in the 70s to around 80 and
overnight lows in the 50s to lower 60s. The middle and later
portions of next week do look to turn active once again as
another shortwave trough and associated cold front are forecast
to traverse through the region and bring more chances for
showers and thunderstorms. While there is decent agreement
within the latest guidance suite, there is a lot of differences
in timing and coverage for our area at this range. So be sure to
check back as we get closer.

Yack

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1228 AM CDT Fri Aug 21 2026

Key Aviation Messages for the current TAF period:

- Chance for scattered showers and thunderstorms tomorrow,
  mainly in the afternoon/early evening

VFR conditions are expected through mid morning. Winds will be
light and variable overnight before becoming southwest around
daybreak and increasing to around 10 knots. There are light
returns on radar near KRFD at the time this discussion was
sent. While these are showers aloft, there is a layer of dry
air that would prevent much of the precip reaching the surface,
except for a sprinkle or two.

The next wave moves in tomorrow bringing with it chances for
showers and thunderstorms. The main piece of uncertainty remains
with timing, but models are consistently showing two rounds of
precipitation. A late morning round that has weak instability
and dry air near the surface limiting chances, and a second
round in the afternoon/evening. The second round is expected to
feature better instability and wind shear that could produce
some strong to locally severe storms in the area. Given the
uncertainty in timing a PROB30 was kept in for the Chicago
terminals with only adjusted start and end times, with a TEMPO
remaining at KRFD.

There is a chance for lingering clouds behind the rain to have
bases around MVFR levels, but for now kept the end of the TAF
VFR. Winds will slowly turn from southwest to northwest
overnight toward daybreak on Saturday.

DK

&&

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

&&

$$

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