885
FXUS63 KLOT 300722
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
222 AM CDT Sun Aug 30 2026

.KEY MESSAGES...

- Periodic shower and storms through the early part of the week,
  though there will ample dry periods.

- Hotter and more humid conditions build through midweek.

&&

.DISCUSSION...
Issued at 222 AM CDT Sun Aug 30 2026

As a low level jet (LLJ) increases this morning, as sampled
around 40 knots just 2000 ft AGL on the KLOT VAD, it is no
surprise that shower coverage is gradually expanding over
northern Illinois. PoPs were increased to around 30 to 40
percent chance to match the coverage being observed. This
southwest to northeast oriented LLJ is expected to pivot and
become slightly more easterly oriented through the morning and
be the driving force behind an increase in shower coverage.
There is modest effective shear present, but CAPE values and
lapse rates are not providing confidence in overall coverage of
thunder. That being said, there have already been small clusters
of embedded thunderstorms develop early this morning. Most
thunder is expected to be isolated, but there could be an uptick
in thunder coverage right around and after daybreak. Current
guidance is suggesting the LLJ will gradually diminish around
midday allowing the showers to eventually erode and sink
southward as it falls apart.

An upper level ridge, the center of which is currently analyzed
over Oklahoma and Texas, keeps northern Illinois and northwest
Indiana along its northern periphery through midweek. There
will be plenty of dry times, but as embedded waves moves through
the pattern, there will be additional periods of showers and
storms. For example, RAP analysis is currently showing an upper
level short wave from Rapid City, SD to around Dodge City, KS
just behind ongoing storms in the Great Plains. There is an
additional short wave in western Wyoming and Colorado. Models
are suggesting that that both these waves are expected to ride
along the top of the ridge and provide additional chances for
precipitation overnight tonight (the Plains wave) and again
overnight into Tuesday morning. If the ridge were to strengthen,
it could force these waves (and subsequent shower/storm
chances) farther north. Whether or not they maintain their
current trajectory or strength remains to be seen. For now,
chances for showers and storms remain in the forecast through
the middle of the week.

As the ridge remains over the CONUS through midweek, so does
southwesterly flow at the mid-levels. This wind direction will
aid in advecting in a warmer air mass to the western Great Lakes
with projected 850 mb temperatures over 20C, potentially pushing
25C. Winds at the surface will also remain out of the south,
gradually increasing dew points through the week back into the
70s. Hotter temperatures in the upper 80s and lower 90s are
expected. This combination of hot temperatures and humid
conditions are expected to increase heat indices into the mid to
upper 90s by midweek, with the southwestern portion of the
forecast area with a chance of exceeding 100. Chances for
shower/storms through the weak lowers confidence in specific
temperatures values; therefore, there was no consideration at
this time for any heat hazard products (advisory/watch).

Models are suggesting that the ridge could break down by the end
of the work week. That will help the hottest of temperatures to
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 over the weekend.

DK

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1228 AM CDT Sun Aug 30 2026

Spotty showers are ongoing across the central ZAU airspace at
press time associated with isentropic ascent being forced by a
southwesterly 35-40 kt low-level jet oriented toward the Great
Lakes. As the orientation of the low-level jet turns more
westerly toward daybreak, the expectation is for coverage of
showers to increase and peak generally in the 11 to 15Z
timeframe, give or take an hour. With mid-level lapse rates
gradually steepening through the night, opted to convert the
inherited PROB30 groups to TEMPOs for -TSRA this morning (will
note there has already been lightning in activity across
southern Wisconsin early this morning). Winds overnight through
mid Sunday morning should remain generally where they are right
now out of the southwest at 10 to 15 kt (southeast at rural
terminals).

As the low-level jet starts to weaken by mid-morning, coverage
of showers should correspondingly decrease. Upward mixing into
the base of the decaying low-level jet should still promote
southwesterly gusts increasing to 25 kt or so from mid-morning
to early afternoon. A shower or two may remain in the area
through early afternoon.

Toward mid-afternoon a remnant MCV originating out of ongoing
convection in southern Nebraska will move into the Lower Great
Lakes. Suspect there will be another period of showers across
the region as the MCV moves through, though am not confident the
highest coverage will be over the terminals or over southern
Wisconsin. For now, will simply cover the threat with a
prolonged VCSH through early evening.

Tonight, currently expect dry conditions with a southwesterly
breeze around 10 kt (southeasterly directly at rural terminals).
A few showers may develop toward daybreak Monday at the nose of
another low-level jet. Will tackle that window in a later TAF
package.

Borchardt

&&

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

&&

$$

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