309
FXUS63 KLOT 251837 CCA
AFDLOT

Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
137 PM CDT Wed Jun 25 2025

.KEY MESSAGES...

- Isolated to widely scattered showers and storms are possible this
  afternoon and evening. The strongest storms will be capable
  of strong to locally damaging wind gusts.

- Hot and humid conditions will continue through the weekend
  with peak heat indices in the mid 90s to near 100 much of the
  week.

- Periodic shower and storm chances exist through early next
  week with many dry hours in between rounds.

&&

.UPDATE...
Issued at 1010 AM CDT Wed Jun 25 2025

12Z surface analysis has a quasi-stationary front draped across
the northern part of the forecast area. Visible satellite
imagery shows a thicker cloud deck north of the boundary with
clearer skies to the south. However, satellite imagery is also
showing an outflow boundary moving southeastward of the
Mississippi River through western Illinois. As it crossed the
river, it helped develop scattered showers that are still moving
east northeast over northern Illinois. There was some thunder
associated with these showers closer to the river, but as they
moved into a more stable/capped environment over the forecast
area, lightning trends diminished. Scattered showers are
expected through the late morning morning as they gradually
lift north of the area into Wisconsin.

There is yet another outflow boundary in eastern Iowa that is
slowly moving east. This outflow should provide enough forcing
to allow for isolated to widely scattered showers and
thunderstorms this afternoon. Confidence is lower on exact
timing and location of development, although with more sunshine
south of the stationary front, thinking is that this would
favor areas south of Interstate 88 for better shower and storm
coverage.

DK

&&

.DISCUSSION...
Issued at 340 AM CDT Wed Jun 25 2025

Through Thursday:

A quasi-stationary boundary is currently draped across the area
early this morning with showers and embedded thunderstorms
ongoing along it. This activity will continue generally along
and between the I-80/88 corridors through the pre-dawn hours
then lift northeast out of the area by mid morning. As it does
so, fog along the Wisconsin Lake Michigan shore may ooze south
along the Illinois shore this morning leading to brief
visibility reductions as low as 1 mile along the lakeshore
before eroding mid to late morning.

In contrast to Tuesday, the better mid-level forcing/vort lobes
are expected remain largely north of the area during the day
today. This suggests that additional shower/storm coverage
during the daytime hours will largely be diurnally driven with
an added boost by any surface convergence axes and near the
vicinity of the lake breeze. Have accordingly capped PoPs at
35-40% during the afternoon with the expectation that coverage
will end up mainly isolated to widely scattered at best. As a
result, it is expected that many areas will end up dry during
the daytime hours. Similar to yesterday, the main concern with
any storms that manage to develop will be precip-loaded strong
to locally severe downburst wind gusts thanks to ample
instability (2000+ J/kg MUCAPE), weak mid-level lapse rates, and
high PWATs (potentially >2"). The more favorable deep layer
shear remains well north of the area so organized severe
convection is not anticipated. However, there is a little bit of
flow in the low-levels again late in the afternoon (~25kt of
0-3km shear over northern IL) which could support brief mini
supercell-like structures and possibly a funnel cloud.

Onshore winds will help keep temperatures much more comfortable
near the lakeshore today with highs potentially only in the
70s. Inland areas of northeast Illinois will also be cooler
overall with highs in the 80s with lower 90s expected south of
I-80. Dewpoints in the 70s will make for another hot and humid
day (away from the lakeshore), with heat indices in the upper
90s to near 100 forecast south of a Dixon to Joliet to
Valparaiso line.

Greater coverage of storms is anticipated well northwest of the
area during the day closer to the surface front draped across
northwest IA and southeast MN paired with better mid-level
forcing. Heading into the evening hours some of this activity
may manage to hold together long enough to drift across far
northern Illinois or at the very least result in a gusty
northwest wind shift from remnant outflow into the early
overnight hours.

Thursday`s shower/storm potential looks fairly similar to today
with isolated to widely scattered showers and storms possible,
focused along any subtle surface convergence axes/boundaries.
Once again storms are expected to develop northwest of the area
which could move through the area during the evening hours (more
on this in the long term section below). Offshore winds will
allow temperatures to be warmer closer to the lakeshore with
highs in the upper 80s to lower 90s making for another hot and
humid summer day.

Petr


Thursday Night through Tuesday:

The upper high driving the extreme heat over the eastern half of
the CONUS, including during our recent heatwave, will move off
the Mid-Atlantic coast early Friday. This will make room for a
series of upper disturbances to scoot across the region late
this week into early next. However, the core of the polar jet
will remain to our north for the next several days keeping the
mid-summer heat pumping into the Midwest. The continued heat
will give us access to surface-based instability and at least a
chance for freely convecting storms each day into next week, but
there will be a few triggering features to watch for as well.

The long term period picks up late Thursday when a disorganized
surface low looks to track across southern WI. Instability
pooled south of the low track will bring storm chances to
Thursday night as the the system moves nearby. It`s cold front
will trail behind the circulation and drape east to west in or
north of our area early Friday and remain pretty stationary
through the day. Models suggest scattered showers and storms
could be churning along this front for most of the day on
Friday, especially during the afternoon and evening after
instability builds up. The best storm chances will be focused in
the unstable air along and south of the front but its location
isn`t entirely clear, although that looks to include a majority
if not all of our CWA during the day. The front will drop south
Friday night and hang out in our area through Saturday, probably
somewhere south of I-80, although it`s looking pretty diffuse
by Saturday morning and there`s no real apparent gradient in
temperature guidance for Saturday. Drier air feeding in behind
the front will help inhibit precip development to an extent, but
scattered chances again look pretty good near and south of the
front during the day.

High temperatures are forecast in the upper 80s to around 90
both Friday and Saturday but depending on shower and storm
coverage some spots could end up cooler. Heat indices both days
look to be in the 90s to locally near 100. Onshore flow behind
the cold front on Saturday will keep areas near the lake several
degrees cooler. Lower 90s are expected on Sunday as some warmer
air feeds in ahead of an approaching wave. Heat indices on
Sunday could exceed 100 degrees around a majority of the CWA.

Early next week, an upper trough will deepen as it swings from
the northern Plains into the Midwest. The surface response will
be a potent baroclinic zone that will move across the CWA early
next week bringing additional rain and thunder chances. That
looks to take place sometime between late Sunday and midday
Monday. A quieter and relatively cooler synoptic pattern then
looks to intervene for at least a couple of days during the
middle of next week.

Doom

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 128 PM CDT Wed Jun 25 2025

Key Messages:

- Isolated to scattered showers and storms expected at times in
  the area during the TAF period.


The main forecast concern for this TAF cycle is the potential
for isolated to scattered showers and thunderstorms through most
of the TAF period. The warm and humid air mass over the region
will continue to support shower and thunderstorm development
from this afternoon through tomorrow, but the relatively
nebulous large-scale forcing and expected presence of multiple
surface boundaries in the area make it difficult to pinpoint
exactly when showers and storms may affect the terminals over
the next 24-30 hours, or if they will even affect the terminals
at all.

That said, it appears that the greatest potential for showers
and storms to pass over or near the terminals appears to be from
late this afternoon through this evening as a slightly more
unstable air mass located over central Illinois at press time
gradually sloshes northeastward into northern Illinois and
northwest Indiana. Opted to highlight this time window of
slightly greater confidence with a targeted PROB30 group in the
TAFs. Confidence in showers and storms affecting the terminals
outside of this time window is lower, so have continued to
withhold on introducing additional formal precipitation mentions
into the TAFs at this time. However, tactical introductions of
such mentions may ultimately be needed in forthcoming TAF
issuances/amendments -- particularly if confidence increases in
an outflow boundary drifting southward into northern Illinois
from a thunderstorm complex tracking across central/southern
Wisconsin tonight.

Otherwise, VFR conditions are largely expected to prevail
through the TAF period, though BKN MVFR cumulus could still be
observed at times at a few of our TAF sites through mid-
afternoon today. Southwesterly winds could gust to around 20
kts tomorrow afternoon, though should otherwise largely remain
at or below 10 kts.

Ogorek

&&

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

&&

$$

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