204
FXUS63 KLOT 190550
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1250 AM CDT Tue Aug 19 2025

.KEY MESSAGES...

- Several rounds of showers and storms are likely this
  afternoon and evening. Some storms may be severe with damaging
  winds and torrential downpours.

- Additional showers and storms may develop overnight, primarily
  along and south of I-80. The main threat with any additional
  storms would be torrential downpours.

- A pattern change toward cooler and less humid conditions is
  expected by midweek. Even cooler and less humid conditions may
  arrive over the weekend.

&&

.DISCUSSION...
Issued at 239 PM CDT Mon Aug 18 2025

Through Tuesday:

Two rounds of thunderstorms late this afternoon and evening
still appear to be in store, though the gap that was expected
between these may be more short lived than previously
anticipated. The initial line of storms stretching south out of
WI has been very slow to expand across northern IL but the last
few radar scans finally show more organization, and even the
potential development of a rear inflow jet in the storms along
the I-39 corridor. In the wake of the somewhat disorganized
earlier afternoon activity, more widespread and longer lived
showers and storms are expected to develop upstream and move
through this evening.

Plenty of boundary layer support for convective development is
already in place with widespread dewpoints in the mid to upper
70s across the CWA, and any convergent outflow from new cells
should also help to trigger new storms in the near term. Support
for severe storms remains somewhat marginal and would mostly be
driven by thermodynamics given that the more favorable shear
axis is farther to the north over WI. Rainfall rates up to 2
inches per hour are certainly possible.

The second round of activity, initially expected to trail the
first by an hour or two, is also showing signs of developing and
expanding in coverage across eastern IA into northwest IL along
with better background synoptic and mesoscale support from an
upper trough and more pronounced shear. The additional concern
with this second round would include heavy rain and the
possibility for flooding. This would arise from a combination of
seasonably high PWATs, the relatively slow expected cell
motions, and the overall extent of expected coverage.

After these two rounds move out, we continue to monitor for the
chance overnight into early tomorrow of a final round of
showers and thunderstorms as the main upper trough, developing
surface low, and trailing frontal zone push through. There also
appears to be some support from a low level jet toward the
predawn hours ahead of the upper trough. With the high
precipitable water values and repeated rounds of rain, a flood
watch for flash flooding is in effect through late tonight.

After highs today in the mid 80s, temperatures tomorrow may be
similar to today`s, especially ahead of the front, but given the
morning convection and possible lingering cloud cover along
with dewpoints a few degrees lower, heat indices should pose
less of a concern.

Lenning

Tuesday Night through Monday:

High pressure moving overhead in the wake of the frontal passage
should bring an end to the hot, humid, and active pattern we
have been experiencing these past few days. Temperatures return
to a more seasonable level and humidity levels also fall. The
main concern from mid- to late-week would be potential wave
growth on Lake Michigan and an elevated swim risk with onshore
flow.

Ensemble model guidance is advertising a strong upper-level low
barreling toward the Great Lakes this weekend, which should
usher in a seasonably cool and relatively dry airmass. With
meager moisture return ahead of the principal cold front on
Saturday, we currently favor a dry frontal passage. Falling
850mb temperatures toward the upper single digits (+7 to +9C)
will support highs in the mid to upper 70s and overnight lows in
the 50s. Perhaps even more noticeable will be the lower humidity
levels, as dew points will be poised to fall into the 50s. The
cool and dry pattern should continue into early next week before
a period of warmer, and potentially stormy, conditions return
to close the month of August.

Lenning/Borchardt

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1250 AM CDT Tue Aug 19 2025

Forecast concerns include...

Scattered thunderstorms early this morning.
Ifr, possible lifr cigs through mid morning.
Fog potential around sunrise.
Wind shift to northerly Tuesday morning.

Scattered thunderstorms have redeveloped over the Chicago
terminals and will continue for the next few hours, slowly
moving east. Isolated thundertorms will remain possible through
daybreak ahead of a cold front that will move across the area.
Confidence is low for possible coverage with any additional
convection in the predawn hours and near daybreak. Prob thunder
mention still seems reasonable at this time.

Prevailing winds will start light southerly but are expected to
shift south/southwest over the next few hours then to the west/
southwest prior to daybreak. As the cold front moves across the
area, winds will shift northwest and then are expected to become
mainly northerly, with speeds potentially into the 10-15kt
range. Eventually, winds will become north/northeast for ORD/MDW
and only medium confidence for this timing. Winds will likely
become north/northeast at MDW before ORD and trends will need to
be monitored.

Patches of lifr cigs began to develop a few hours ago, prior to
this latest round of thunderstorms. Guidance is in very good
agreement with low clouds developing ahead of the cold front and
then slowly lifting Tuesday morning. Prevailing ifr cigs seem on
track based on current model trends, which will then lift
through mvfr and likely scatter out late Tuesday afternoon or
Tuesday evening.

Fog is also possible around daybreak, most likely northwest of
the Chicago terminals and may be brief, associated with the cold
front. Confidence for fog is low, but trends suggest ifr vis at
RFD/DPA and have included tempo mention with this forecast.
There is a small chance that dense fog may develop but that
seems unlikely, if widespead ifr cigs develop. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch until 4 AM CDT early this morning for ILZ003-
     ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-
     ILZ019-ILZ020-ILZ021-ILZ023-ILZ103-ILZ104-ILZ105-ILZ106-
     ILZ107-ILZ108.

IN...Flood Watch until 4 AM CDT early this morning for INZ001-
     INZ002.

LM...None.

&&

$$

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