797
FXUS63 KLOT 181818
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
118 PM CDT Mon Aug 18 2025

.KEY MESSAGES...

- Today will be hot and humid with peak heat index values near
  100 degrees.

- A round of showers and storms is expected 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.

&&

.MESOSCALE...
Issued at 1209 PM CDT Mon Aug 18 2025

Severe thunderstorm watch will be in effect for the afternoon
for northern Illinois. We have two areas of thunderstorms we are
monitoring. The first being along the Interstate 65 corridor in
NW Indiana and to the southern tip of Lake Michigan. These are
ahead of better instability and along an elevated (above
surface) boundary, and thus lightning and brief downpours would
be the main hazards, maybe pea sized hail. Of near term concern
is the cluster/line of strong thunderstorms across NW Illinois
into southern Wisconsin. The line is currently well balanced
across Wisconsin and is still a bit outflow dominant across
northern Illinois. The atmosphere is starting to destabilize
from the southwest as southwest flow pushes warm and humid
(higher theta-e) air into northeast Illinois. Storms will likely
continue to develop ahead of the current line and even develop
overhead. There is some uncertainty as to how widespread the
coverage will be south of Interstate 88 and closer to the
lakeshore, but given the environment will be fully unstable as
the line moves east, we will be including areas down to
Interstate 80 in the watch. The main hazard will be damaging
winds. There is a non zero tornado threat for brief spin ups,
mainly north of I-88 where winds are slightly backed and we will
have a northeastward shifting warm/moist front, and shear is
also higher in these areas.

KMD

&&

.DISCUSSION...
Issued at 330 AM CDT Mon Aug 18 2025

Through Tuesday:

As is common in the summer and particularly so this summer
(seemingly), today`s convective forecast is, needless to say,
uncertain. However, we`ve been able to glean some potentially
key observational details that may help tip Mother Nature`s
hands as to how things may play out, or at least a more favored
scenario amongst a larger spectrum of still plausible outcomes.

The stubborn cluster of storms that pushed across the Rockford
metro overnight is finally losing its steam as of this writing.
Objective mesoanalysis indicates that instability is greatly
muted with northeastward extent, which should continue to be the
case past sunrise, until low-level moisture trajectories return
to southwesterly (vs. current southeasterly). While the morning
hours *should* be primarily dry, we can`t completely rule out
isolated (15-20% PoPs for a good chunk of the area) festering
showers and non- severe storms for a localized lightning strike
and downpour threat.

Our main feature of interest that will likely play a large role
in the convective evolution this afternoon and evening is a
loosely organized MCS approaching and crossing the mid MO River
Valley early this morning. Overnight CAM guidance that
adequately initialized this MCS resolved a 700 mb wind response
pointing towards a remnant MCV likely emerging as the *probably
decaying* complex tracks eastward today toward the mid and upper
MS Valley.

One thing that is of high confidence is an oppressively humid
air mass sloshing back into the area today, especially over and
near farming areas due to evapotranspiration. Dew points well
into the 70s, and possibly nearing 80F, especially well inland,
paired with temps in the upper 80s to around 90F will yield peak
heat indices near to around 100F, with some localized readings
approaching 105F in the southwest 1/3 or so of the CWA. Leaned
toward the less mixed out guidance for dew points, which has
been the way to go much of this summer (vs. the overly mixed out
HRRR depictions).

This element will be very important as to how much instability
can be realized out ahead of the MCV this afternoon and how
extensive thunderstorm coverage becomes. Despite marginal mid-
level lapse rates (6C/km or less), unlike this past Saturday and
Saturday night, 200 mb ELs in the high dew point and high PWAT
environment can still easily yield MLCAPE of 3500 J/kg or more.
In addition, the marginal mid-level lapse rates will
commensurately yield less MLCIN to erode during peak heating.

With the higher dew point, larger instability, and less capped
scenario being our current favored scenario as of this writing,
the forecast PoPs this afternoon and evening were tailored to a
an earlier convective initiation and quick increase in coverage
as outflow from the earlier mentioned key MCS (and MCV)
approaches the MS River early this afternoon. 55-60% "likely"
PoPs (numerous storms) were focused near and north of the
Kankakee River Valley between ~2pm-10pm CDT.

Strong instability, steep low-level lapse rates, extreme dew
points, and high PWATs are a favorable mix for DCAPE pushing
1500 J/kg with intense precip loaded downdrafts supportive of at
least localized damaging downburst wind damage. If effective
shear can push into the 25-35 kt range aided by the MCV wind
response, that may up the ante for upscale growth and a more
organized line of storms with scattered downburst wind damage.

If this currently more favored scenario comes to pass,
instability may be sufficiently exhausted in the wake of the
afternoon-evening activity to bring a late evening and early
overnight minima. Since we can`t rule out the alternative or
hybrid scenarios in which a larger chunk of the CWA has a less
worked over environment by the late evening, we held onto chance
PoPs to account for this possibility.

A modest southwesterly low-level jet overnight conceptually
should replenish MUCAPE most effectively with southward extent,
where evening t-storm coverage may end up a bit less. Additional
impulses should fuel an uptick in showers and storms,
particularly near and south of I-80 (where 55-60% PoPs were
focused). The high to very high PWATs in place today certainly
do lend to concern for flash flooding, particularly for the
hardest hit areas on Saturday night. With this being said, the
potential for fairly progressive convection this afternoon and
evening may keep the threat more localized. The setup overnight
may favor a better chance for training storms and flooding,
depending on where the axis of highest coverage sets up,
currently looking to be near and south of I-80.

On Tuesday, a cold front heralding a long awaited pattern change
to cooler and less humid midweek and onward will push southward.
Following the effects of lingering storms past sunrise, focus
near/southeast of I-55 by then?), the effective air mass should
be shunted southward, keeping the highest chance for renewed PM
storms primarily in our southeast 1/3 or so. A few showers and
maybe an isolated t-storm may nonetheless be possible near the
WI border from cyclonic flow aloft. Highs will be in the lower-
mid 80s I-80 and north and upper 80s to near 90F south of I-80
on Tuesday, with 70s dew points making for another uncomfortable
day.

Castro

Tuesday Night through Sunday:

Wednesday through the end of the workweek, a southeastward-
drifting high pressure system will serve as the primary
influence over the weather in the Great Lakes. The result should
be a much-needed break in active weather, with generally
seasonably temperatures (highs in the lower 80s, lows in the
lower 60s), light winds, and lower humidity levels. The only
only impactful weather of note will be a threat for hazardous
waves on Lake Michigan beaches Wednesday owing to a push of
northeasterly winds as the surface high approaches the region.
Also, would not be surprised to see a few sprinkles in the
region on Wednesday especially if low-level capping ends up
weaker than currently forecasted. Otherwise, the middle to end
of the week is shaping up to be pretty quiet.

Ensemble model guidance is advertising a strong upper-level low
barreling toward the Great Lake 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 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.

Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1245 PM CDT Mon Aug 18 2025

For the afternoon and evening we are looking for two rounds of
thunderstorm activity, one with the initial line of storms
approaching the RFD area at forecast issuance time, and then
more widespread and longer lived activity expected to develop
upstream and move through in the wake of the first line.

It is actually not completely certain that the initial line
heading toward RFD will hang together or build south enough to
reach the Chicago area terminals, though the convective
inhibition that has been in place for most of the day appears to
be eroding. Thus it would seem that convergence along the
pronounced line of cumulus stretching south out of Wisconsin
would be enough to trigger expanded development. There is also a
decent possibility of random pop up convection out ahead of
this line. Thus the strategy was to extrapolate this first line
through RFD, DPA, and ORD/MDW and add a small buffer around the
most likely arrival times.

The second round of activity would be expected to trail the
first by an hour or two, assuming it actually manages to
develop at all. Upstream across Iowa there is a broad region of
agitated cumulus along with hints of a MCV circulation showing
signs of trying to intensify. Boundary layer dewpoints in the
mid to upper 70s will certainly provide sufficient moisture,
and this is also the area with better synoptic and mesoscale
support in terms of an upper trough and more pronounced shear.

After these two rounds move out, there is one more chance
overnight into early tomorrow for a final round as the main
upper trough and low level frontal zone push through.

Lenning

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement until 3 PM CDT this afternoon for
     ILZ006-ILZ103-ILZ104.

IN...None.
LM...None.

&&

$$

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