551
FXUS63 KLOT 211141
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
641 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 /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 641 AM CDT Fri Aug 21 2026

Forecast concerns for the terminals...

- Period of scattered showers and thunderstorms this afternoon
  and evening ahead of a cold front.

- A few storms could become severe with damaging winds and hail
  the main threats.


A shortwave trough continues to pivot over the northern Great
Lakes this morning which continues to develop some isolated
showers across eastern IA and southern WI. So far these showers
have been struggling to reach the ground given the high cloud
bases (around 7000-10000 ft) and the dry sub-cloud layer and
that trend should continue through mid-morning. However, there
is a non-zero (~10%) chance that an isolated sprinkle makes it
to the terminals, especially RFD. Otherwise, expect SCT to BKN
VFR skies this morning with southwest winds around 5-10 kts.

Heading into this afternoon, a secondary shortwave (currently
over eastern ND) and its associated cold front will begin to
pivot into the area. By this time, better moisture and
instability should be in place which is expected to result in
the development of scattered showers and thunderstorms through
the afternoon and evening hours. The generally favored time
window for storms at the terminals will be between 20z and 02z,
but with coverage of storms being more uncertain have opted to
maintain the PROB30 groups at the Chicago area terminals for
now. At RFD the TEMPO group timing was adjusted to reflect a
slightly early storm development, but given that storms may
fester longer than the four hour TEMPO allows have opted to
introduce a VCTS here through 03z for that potential. Regardless
of storm coverage, any storms today will have the potential to
become severe with damaging winds (up to 60 mph) and large hail
(up to half dollar size) the main threats. Though an isolated
tornado cannot be ruled out, particularly west of the Fox River
Valley.

Shower and storm chances should diminish after 03z this evening
as the cold front makes its way across the area. As a result
mostly dry conditions are expected overnight with VFR
conditions. While there is a signal that some lower ceilings
(around 2000-3000 ft) could develop after midnight, confidence
on their coverage remains low. Winds will also become
northwesterly behind the front with speeds gradually increasing
through the night into the 10-12 kt range by Saturday morning.

Yack

&&

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

&&

$$

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