455
FXUS63 KLOT 311110
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
610 AM CDT Mon Aug 31 2026

.KEY MESSAGES...

- Periodic shower and storms persist this week, though there
  will ample dry periods.

- Hotter and more humid conditions build midweek.

&&

.DISCUSSION...
Issued at 200 AM CDT Mon Aug 31 2026

Thunderstorms have fizzled out, but isolated showers fester over
the forecast area early this morning. Similar to last night, a
southwest to northeast low level jet (LLJ) is expected to
amplify prior to daybreak and help induce additional shower and
storm coverage. There is lower confidence on the exact
coverage; but considering how impressive last night/yesterday
morning`s coverage ended up being, widely scattered showers and
storms can be expected through daybreak before gradually
diminishing (as the LLJ weakens) and moves east. Forcing becomes
fairly nebulous behind the LLJ. Most areas should receive a
period of dry conditions around midday, but maintained slight
chance PoPs for a stray diurnal shower that might pop-up this
afternoon, mainly north of Interstate 88.

Stepping back and zooming out for a second, GOES IR and water
vapor satellite imagery currently shows broad cloud cover
sweeping across the CONUS from the Four Corners, to the
Minnesota arrowhead, down to our nation`s capital. Their
movement is riding over an upper level ridge that has dominated
the forecast for the last several days. IR is showing showers
and storms over eastern Colorado/northwestern Kansas that is
collocated with an upper level short wave embedded in the
ridge. This wave is expected to quickly move across the Great
Plains and head toward southwestern Wisconsin today. Model
soundings are showing a fairly strong cap and a robust dry air
over much of the area this afternoon and early evening with
DCAPE values around 1000 J/kg in northwest Illinois. The cap
might be enough to inhibit shower and storm development, but
lapse rates are expected to steepen along with modest deep layer
shear with the arrival of the short wave. Any elevated storm
that develops upstream strengthen and possibly produce some
hail up to the size of a quarter or potentially localized
damaging wind gusts, mainly for areas north of Interstate 88.
The main bit of uncertainty is the exact track of the wave. If
it follows the ridge, it could move right over northern
Illinois, with favored areas closer to the Wisconsin stateline.
But some models have a slightly more northern track into
Wisconsin which would limit shower/storm coverage.

Even as the wave passes to the northeast, broad warm air
advection behind the wave could allow for festering showers once
again overnight into early Tuesday morning. For now, kept
"silent" PoPs (10 to 14 percent chance) with no formal forecast
mention, but trends will be monitored. At the risk of sounding
like a broken record, but models are suggesting yet another
wave embedded within the ridge is expected to pass nearby on
Tuesday morning/early afternoon. However, models are showing a
lot of disagreement with this wave. The NAM currently has the
northernmost solution that may keep most of the forecast area
dry, whereas the latest 00Z guidance is starting to bring that
wave closer down into northern Illinois. With increasing
instability ahead of the wave, it provides another chance where
storms could become strong to severe. This also just highlights
the pattern that the region currently finds itself in with
persistent waves within the ridge pattern that keeps periodic
shower/storm chances in the forecast. However, shower/storm
chances on Tuesday could impact the temperature forecast.

As the upper level ridge slowly ambles east through midweek,
southwest winds are expected to advect in a warmer air mass and
higher dewpoints into the region. High temperatures are expected
to climb into the low 90s with dew point temperatures into the
low to possibly mid 70s. Models have been fairly consistent in
suggesting Wednesday could be the hottest day of the week where
heat index values could reach the mid to upper 90s for much of
the forecast area, and better chances that they could meet or
exceed 100 south of Interstate 80. Confidence lowers in exact
heat values from south to north across the forecast area with
greatest uncertainty closer to the top of the ridge (north of
Interstate 80) given the better chances for showers and storms
there.

Models are suggesting that the ridge could break down by the
end of the work week. That will help the hottest of temperatures
cool off a bit, but it will return the upper level flow pattern
to the northwest and thus bring with it additional chances for
showers/storms toward and over the weekend.

DK

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 609 AM CDT Mon Aug 31 2026

Per regional WSR-88D derived VWPs, a 40 to 45 kt low-level jet
is oriented from the middle Mississippi River Valley toward the
Lower Great Lakes. Showers and thunderstorms are percolating on
the nose of the low-level jet across Wisconsin and far northern
Illinois. In the past few hours, the orientation of the low-
level jet has subtly turned from southwesterly to westerly
directing the most focused zone of low-level isentropic ascent toward
northern Illinois. As a result, the expectation is for
additional showers and thunderstorms to develop over
RFD/DPA/ORD/MDW/GYY in the next few hours. Will keep the PROB30
groups going through about 15Z for this reason.

By mid-morning, the low-level jet will begin to weaken leading
to a corresponding decrease in the coverage of showers and
thunderstorms. Upward mixing into the decaying low-level jet
should still afford southwesterly gusts of 20-25 kt through
early afternoon.

Attention then turns toward an upper-level vorticity maxima
currently supporting an area of showers along the Nebraska and
Kansas borders. Broad west-southwesterly upper-level flow should
direct the wave toward the Lower Great Lakes this afternoon,
similar to the overall regime yesterday. Accordingly, do expect
isolated to scattered shower and storm development this
afternoon focused near the Wisconsin state line. The inherited
PROB30 groups focused on the afternoon/evening period handle
this nicely including implied uncertainties in coverage with
southward extent.

In the wake of the wave this evening, broad, largely unfocused,
and generally weak (20-25kt) southwesterly low-level flow within
the humid regime may encourage a random shower or two to
develop at any terminal through the overnight hours. Winds will
be light (5 kt our less) out of the west to southwest.

There is a low (20% chance or so) for a complex of storms to
develop and roll across northern Illinois after daybreak
Tuesday. The forecast outcome in which storms occur depends
entirely on the coverage and intensity of storms that develop
tonight (if at all) in the central Plains. At this point, there
is not nearly enough confidence to justify a mention a third
round of thunder in the last 6 hours of the ORD/MDW TAF, but
will brief the incoming shift on such a threat.

Borchardt

&&

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

&&

$$

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