865
FXUS63 KLOT 211832
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
132 PM 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 in the evening with damaging winds and hail the
  main threats, but an isolated tornado cannot be ruled out.

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

&&

.DISCUSSION...
Issued at 132 PM CDT Fri Aug 21 2026

Early this afternoon, a decaying MCS is pushing south across
far northern IL along an north-south oriented instability
gradient draped very roughly along the I-39 corridor.
Additionally, scattered showers are moving east near and north
of the Kankakee River valley as a subtle leading shortwave works
across. This MCS has been on a very gradual weakening trend,
but it managed to hold together much better than any recent
guidance had thought. It`s still generating a fair amount of
lightning and pockets of heavier rainfall near its leading edge
which will likely persist as it continues to drop south across
the Chicago metro through the afternoon. No strong or severe
storms are expected through mid-afternoon with this system, but
embedded downpours will likely remain.

Farther upstream, a more potent shortwave trough axis is
swinging into the upper Midwest coincident with a surface cold
front stretching from western WI southwest into northeast and
central IA. It`s along and ahead of the front this evening where
we expect a chance for severe convection. The first showers
have just started to go up along the front near La Crosse.
Thunderstorms are expected to blossom along the boundary as it
continues southeastward and eventually crosses into northern IL
near the end of the afternoon. SPC mesoanalysis suggests
instability is building quickly west of I- 39 in a region of
efficient theta-e advection ahead of the front. CAM guidance has
been rather consistent on a tongue of 2,500 to 3,000 Joules of
MLCAPE building into our western and northwestern CWA by late
afternoon. It`s not clear the quality of the thermo environment
farther east this evening between this MCS, added daytime cloud
cover, and eventually diurnal cooling into the evening. All
along, guidance had been resolving notably less instability with
eastward extent largely owing to a drier low level airmass and
diurnal cooling through the evening. However, the added rain and
cloud cover could be enough to greatly inhibit much of a severe
threat east of the Fox Valley and especially into northwest
Indiana, but there`s not great confidence there.

Effective shear this evening is marginal to fair at about 25 to
35 kt, highest in our west. However, models do resolve a
veering low level shear profile which would support a low-end
tornado threat where the low levels can destabilize
sufficiently. Even when discounting this ongoing MCS, CAMs had
been consistently hanging onto some low level capping roughly
east of the Fox Valley through the afternoon, and would imagine
that`s even more likely now, before being further reinforced by
diurnal cooling. For that reason, this evening`s marginal
tornado threat will be focused west of the Fox Valley and north
of I-80 where the more supportive low level thermo environment
is anticipated. This is also where the severe wind and hail
threat will be maximized given the greater instability and
generally more support expected from the left exit region of a
700 mb jet max out to our west. Severe hail and damaging winds
are on the table area-wide but it`s not clear the
eastward/southeastward extent of deep convection this evening
and it`s very possible much of the area is spared from severe
weather.

Areas of very heavy rainfall are expected as well, in addition
to the ongoing pockets of heavy rain within the remnant MCS.
The storms look progressive enough in nature to avoid many
widespread flooding concerns, but scattered pockets of 1-2" with
the evening coverage alone is a strongly favored signal among
guidance which would likely lead to localized flooding impacts,
mainly in low-lying and other flood prone areas. The severe
threat looks to exit the area to the southeast shortly after
midnight.

The upper trough will continue digging into the Great Lakes
region through tomorrow and drive a vort max across WI and Lake
Michigan during the day tomorrow. Isolated to widely scattered
showers and storms will attempt to develop on the back side of
the system and fall on parts of the area tomorrow afternoon.
With the cooler mid level air spilling over, as many as 1,500 to
2,000 Joules of MUCAPE is favored to develop over NE IL and NW
IN for the afternoon. Nothing too strong or severe is
anticipated, but if one or two more robust updrafts can develop
and survive through significant dry air aloft, it may be enough
to spit out some locally heavy rain and gusty thunderstorm
winds. Most of the area will be staying dry tomorrow with highs
forecast in the upper 70s to around 80.

Strong northerly winds will develop on Lake Michigan tomorrow
and generate high waves and dangerous currents at northwest
Indiana beaches leading to hazardous swimming conditions. Such
conditions will work west to include the Illinois lakeshore for
Sunday. High pressure will dominate the Midwest and Great Lakes
regions early next week keeping conditions largely quiet. Daily
highs are forecast to range from the middle 70s to lower 80s
with rain chances looking to return closer to midweek.

Doom

&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 114 PM CDT Fri Aug 21 2026

Key Messages:

- Thunderstorms may occur at/near the terminals during the
  early-mid afternoon today.

- Another round of thunderstorms is expected late this afternoon
  into this evening. The highest thunderstorm chances during
  this time period are at RFD.


The thunderstorm complex that tracked southeastward across
central and southern Wisconsin earlier this morning has decayed
to a considerable degree as it has entered northern Illinois
early this afternoon. The outflow boundary from this complex is
still pressing southward at the time of this writing, and new
cells may continue to develop along it as it tracks across the
terminals, so there is still some chance for lightning to occur
at/near the terminals through the mid-afternoon. Regardless of
whether storms affect the terminals during this time frame, the
southward-propagating outflow boundary will likely result in a
temporary northerly wind shift at the terminals over the next
few hours.

Another wave of storms is expected to develop along a cold front
this afternoon and track southeastward into northern Illinois
during the late afternoon/early evening. The degree to which
these storms may affect the Chicago metro terminals is not yet
certain due to the earlier showers, storms, and associated cloud
cover hindering destabilization there this afternoon. RFD --
being farther to the west and closer to the better instability --
stands a better chance to be affected by these storms later
today, and have maintained a TEMPO group for TSRA there. This
second round of storms is likely to be stronger than the first
and may feature strong to severe wind gusts and hail.

Winds will shift to a northwesterly direction later tonight
behind the aforementioned cold front. They will strengthen after
sunrise tomorrow, and gusts to around 20 kts may be observed
during the late morning and afternoon. Additionally, there is a
slight (~15-25%) chance for showers and maybe a few storms to
affect the terminals tomorrow. That said, it`s very much
possible that the post-frontal air mass will be too dry to
support much in the way of shower and storm activity near the
terminals tomorrow afternoon, so do not have any formal
precipitation mentions in the TAFs for tomorrow at this time.

Ogorek

&&

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

&&

$$

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