841
FXUS63 KLOT 122339
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
639 PM CDT Wed Aug 12 2026

.KEY MESSAGES...

- Fog possible tonight into early Thursday morning, with the
  next chance of showers and storms late Thursday morning into
  the afternoon.

- Our active, stormy weather pattern is expected to persist into
  the weekend with periodic chances of severe weather and heavy
  rainfall which could lead to flooding.

- A period of dry weather may finally arrive early next week.

&&

.DISCUSSION...
Issued at 311 PM CDT Wed Aug 12 2026

A much quieter afternoon and evening is anticipated across the
area following the stabilizing influences of this morning`s
round of showers and thunderstorms. Accordingly, we have
continued the trend of lowering POPs through tonight, with dry
conditions expected across most of the area. Our main concern
for tonight is the threat for fog development, as a very moist
low-level airmass nocturnally cools amidst light winds. Also
cannot rule out the possibility for some of the fog to become
locally dense into early Thursday morning, particularly in areas
that become mostly clear.

On Thursday, forecast attention will be focused on the
evolution of the next MCS expected to track eastward from the
Plains out across the Corn Belt tonight. Ultimately, this MCS is
expected to begin to cross the Mississippi River around, or a
bit after daybreak on Thursday, though likely in a weakening
state as the primary instability gradient over IL looks to
remain south of I-80 Thursday morning. This may thus act to
steer the stronger storm potential with the MCS farther south
across central parts of IL later in the morning. In spite of
this, high precipitable water values (~1.9") and deep warm cloud
depths will support locally heavy rainfall with this remnant
MCS.

Later Thursday, and especially Thursday night, a south-
southwesterly low-level jet is expected to ramp up across the
mid- Mississippi Valley and nose into IA and partitions of
northern IL. Interestingly, this could result in regenerating
showers and thunderstorms along a west-northwest to east-
southeast corridor in the wake of Thursday MCS. Where this
occurs, a flash flood threat would be a big concern Thursday
night, especially given the storm motion to favor training cells
over the same areas. At this time, there are signs that the
best potential for several hours of training storms may remain
southwest of the Chicago metro area, and potentially even
southwest of our area altogether, but this will need to be
monitored.

Unfortunately, medium-range guidance continues to depict our
active pattern remaining in place and supporting continued waves
of showers and thunderstorms across our region into the weekend.
Fortunately, there does appear to be some decent agreement in
the global models/ensembles that high pressure eventually
settles across the western Lakes region by Monday-Tuesday of
next week. Once this occurs, it will push our stalled frontal
zone to the south and provide us with a much needed break in
this active weather pattern.

KJB

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 639 PM CDT Wed Aug 12 2026

Forecast concerns for the terminals...

- Areas of fog and low ceilings are expected to develop late
  tonight into Thursday morning.

- A decaying line of showers (perhaps an isolated storm) is
  expected to move through the terminals late Thursday morning
  into Thursday afternoon.


A rather tranquil night weather wise is expected for northern
IL and northwest IN tonight as the area resides in the
subsidence of this morning`s thunderstorms. However, there is a
lake breeze that is traversing across the Chicago area terminals
which will result in a couple hour period of breezier east-
northeast winds. Otherwise, expect light and somewhat variable
winds tonight before directions settle into an easterly
direction Thursday morning. With skies expected to remain mostly
clear tonight, areas of fog are expected to develop and result
in MVFR to IFR visibilities especially outside of the Chicago
terminals. In addition to the fog there is also a signal for
some lower ceilings to develop, generally in the 1500-2500 ft
range, after sunset. Fog and the lower ceilings will erode by
mid-morning Thursday.

Heading into Thursday, the developing cluster of thunderstorms
in near the WY/NE border is expected to be traversing across the
plains tonight before moving through northern IL and northwest
IN late Thursday morning into Thursday afternoon. With the
better instability axis currently in south-central IL, it seems
the complex should be in a weakening state as it arrives with
the greatest thunder threat mainly in IA and north-central IL.
Though, the humid air mass should still allow a period of
showers at the TAF sites. In the wake of the morning activity, a
mid-level convergence axis is forecast to set up across
northeast IL and northwest IN which looks to serve as the focus
for additional showers and storms Thursday afternoon. Latest
forecast trends do show this axis setting up south of the
terminals which should keep the threat for thunder south of the
TAF sites. However, given some modest instability cannot fully
rule out an isolated rumble of thunder (10-20% chance). The axis
of shower/storms is forecast to gradually shift southward
Thursday evening which should allow drier weather to move into
the terminals prior to the end of the 30-hour TAF period, but
confidence on when exactly is low. So for now have just kept a
VCSH mention going through the end of the forecast period.

Otherwise, expect easterly winds to prevail through the day on
Thursday with speeds around 8-10 kts.

Yack

&&

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

&&

$$

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