972
FXUS63 KLOT 202326
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
626 PM CDT Thu Aug 20 2026

.KEY MESSAGES...

- Showers and thunderstorms chances return Friday afternoon and
  evening, some may be severe, especially west of the Fox Valley
  in Illinois.

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

&&

.DISCUSSION...
Issued at 211 PM CDT Thu Aug 20 2026

Some much deserved sunshine and gorgeous summer weather today
with comfortable humidity and mild temperatures. High pressure
that`s responsible for today`s weather will move east into New
England tonight and make way for the next system that will bring
the a threat of showers and storms to the area Friday afternoon
and evening.

Fairly potent looking shortwave trough and closed mid-upper
level low is evident on water vapor imagery this afternoon
straddling the international border with Minnesota. This trough
is progged to continue moving rather sluggishly southeastward
into northern WI by Friday morning. The associated surface low
will move into the upper Great Lakes by Friday afternoon with a
cold front trailing southwest from the low into northwest IL by
early Friday evening.

Low level moisture in advance of the this front will be somewhat
limited, consisting largely a recycled moist continental air
mass and lacking a direct Gulf connection. Despite the absence
of a feed of moisture from the Gulf, evapotranspiration and
pooling of moisture ahead of the front may allow for a reservoir
near 70 degree dewpoints ahead of the front Friday afternoon.
Heating of that boundary layer air mass juxtaposed beneath some
modestly cooling mid level temps is likely to result in an axis
of 2000+ J/kg of MLCAPE along and ahead of the cold front.

A northwesterly mid-upper level jet extending from the northern
Plains into the mid-Mississippi Valley will result in a corridor
of seasonably strong deep layer shear. The strongest shear looks
to remain southwest of, and mostly divorced from the stronger
forcing over the Great Lakes. There is spread in guidance
regarding strength of the shear across our CWA, with some
models bringing the more favorably long hodographs into our
western CWA Friday afternoon, while other models keep favorable
shear farther west and southwest.

If the the stronger shear does materialize into our area, then
a corridor of conditions quite favorable for northwest flow
supercells would likely materialize over eastern Iowa into
western portions of northern IL. Weak low level flow should
greatly limit the tornado threat, but large hail and damaging
winds would be in play with this more strongly sheared set-up.
If more favorable shear remains farther southwest, then the
severe threat would be more marginal, limited to mainly a
potential for localized microbursts.

Instability is expected to wane after sunset and that should
result in a weakening trend in the convection as it spreads east
across our CWA Friday evening. How quickly that diminishing
trend takes place and how far south and east a severe threat may
materialize is something that we hope to have a better handle
on by tomorrow morning. Showers and storms should end from the
northwest Friday night with northwest winds ushering in a less
humid and slightly cooler air mass for Saturday.

The upcoming weekend largely looks dry, though a secondary push
of cooler air Saturday night could result in just enough lake
induced instability to support a slight chance of some lake
effect showers into Porter County late Saturday night into
early Sunday morning.

The only other concern weather wise this weekend will be with
the likelihood of hazardous swim conditions developing. Behind
the initial cold front, northwest winds are likely to increase
Saturday morning and could lead to hazardous swim conditions
for mainly Porter County beaches and points east Saturday.
Saturday afternoon, land-lake breeze circulation should result
in a diurnal relaxing of the winds over southern Lake Michigan
in the afternoon, which will allow waves to ease temporarily
late Saturday afternoon into Saturday evening. A secondary cold
front will down the lake Saturday night, with that push of
cooler air steepening up the marine layer lapse rates and
turning winds more briskly from the north down the full fetch of
the lake. This will likely result in hazardous swim conditions
at all IL and IN Lake Michigan beaches later Saturday night
through Sunday.

Otherwise, look for dry and pleasant weather to continue into
the start of next week.

- Izzi

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 626 PM CDT Thu Aug 20 2026

Key Aviation Messages for the current TAF period:

- Chance for scattered showers and thunderstorms tomorrow, with
  best chances near KRFD

VFR conditions prevail through the overnight. Any shower
activity in southeastern Minnesota this evening is expected to
erode entering western Wisconsin as it moves away from the
better instability. Winds have already started to diminish and
will become light and variable overnight.

The next wave moves in tomorrow bringing with it chances for
showers and thunderstorms. There is a chance for showers to move
in mid to late morning out ahead of the main wave. Confidence in
it holding together is low, so a PROB30 was only added to the
KRFD TAF at this time. It may need to be included in the Chicago
terminals, but held off for now. Regardless, the instability
looks weak, so thunder is not expected during the morning round.

Instability really ramps up in the late afternoon and evening
as the wave descends over northern Illinois. Widely scattered
showers and thunderstorms are expected, particularly for areas
closer to KRFD. A TEMPO was included in KRFD given the higher
confidence. For now, the Chicago terminals were kept at a PROB30
given the model uncertainty with the timing, though once
confidence increases enough it will likely be converted to a
TEMPO.

Southwest winds will prevail out of the southwest through the
day around 10 knots, diminishing behind the system. 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.

DK

&&

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

&&

$$

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