900
FXUS63 KLOT 120040
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
740 PM CDT Fri Jul 11 2025

.KEY MESSAGES...

- Level 2 to 3 of 5 severe thunderstorm threat late this
  afternoon and evening.

- Heavy rainfall will accompany the storms and could result in
  flash flooding, particularly in the areas in and around
  Rockford that were hit with very heavy rainfall last night.

- Showers and thunderstorms are possible again Saturday
  afternoon and evening, though the severe threat and coverage
  of storms should be lower than those expected this evening.

&&

.MESOSCALE...
Issued at 739 PM CDT Fri Jul 11 2025

Storm clusters have pushed into the region and are approaching
the I-39 corridor at this time. Environment ahead of the
incoming complex is quite unstable, with SPC mesoanalysis/RAP
fields suggesting 3000+ J/kg of MUCAPE with low to mid 70s
dewpoints continuing to push northward through the area. 500 mb
flow, per AMVs and the DMX VWP, is around 40 to 45 knots,
resulting in roughly 40 kts of deep layer shear in the vicinity
of the main QLCS. At present, it seems like the main limiting
factor for a more widespread/significant severe threat across
our area relates to the orientation of the deep-layer shear,
which is forecast to remain more northeasterly. With this
roughly paralleling the main arcing QLCS, outflow has had a
tendency to outpace stronger cores, limiting the longevity and
intensity of thunderstorm wind gusts.

Going forward, suspect that the main severe threat will remain
tied to deeper cores which can sustain and move northeast, atop
the eastward-surging outflow. At present, the deepest/strongest
cores are developing into Marshall, Putnam, and eastern Bureau
counties, and will be watching these for signs of
intensification as they continue northeast into La Salle and De
Kalb counties through 8-830 PM.

Main severe threat remains from locally strong to damaging wind
gusts with any northeast-oriented surging bowing segments.
Brief QLCS tornado spin-ups also remain a possibility, but cores
will need to intensify a bit beyond where they are currently,
and also keep pace with, or remain ahead of the outflow.

Carlaw

&&

.DISCUSSION...
Issued at 301 PM CDT Fri Jul 11 2025

Through next Friday:

The thinking for the severe thunderstorm threat into this evening
has not changed. Please reference the earlier issued discussion
above for more info.

Following the severe weather threat into this evening, some
widely scattered showers and storms may persist across the area
overnight into early Saturday morning just in advance of the
approaching mid-level trough moving into northern IL. The only
lingering threat with any of this overnight activity would be
some locally heavy downpours.

The threat of scattered showers and thunderstorms will continue
on Saturday as the area remains in a very warm and moist low-level
airmass in advance of an approaching cold front and a second
northern stream impulse. Accordingly, we anticipate widely
scattered showers and storms to once again develop in the
afternoon with the front as the low-level airmass destabilizes.
This will particularly be the case across roughly the
southeastern half of the area. Fortunately, the coverage of
storms looks to remain lower than that expected this evening, so
Saturday is not expected to be a washout. The severe threat
Saturday also looks to be lower than this evenings, owing to the
presence of weaker deep layer shear. Nevertheless, some
potential for locally strong wind gusts will be possible with
the strongest storms Saturday afternoon, these are just expected
to be few and far between at this point.

Following the passage of the cold front Saturday evening, surface
high pressure is slatted to shift into the region for Sunday and
Monday. This will promote a drier period of weather with lower
humidity both days. Seasonable temperatures in the mid to upper
80s are anticipated both days, but with lower humidity heat
indices will not deviate much from the actual temperature.

Unfortunately, humidity levels will begin to increase again into
the midweek period following the eastward departure of the
surface high. Also, as is typical this time of year, with the
return of low-level moisture also comes the return of shower and
thunderstorm chances. This looks to be especially the case around
midweek as the upper level weather pattern turns a bit more
unsettled across the central CONUS.

KJB

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 650 PM CDT Fri Jul 11 2025

Key messages for the 00Z TAFs:

- Line of strong/severe thunderstorms to arrive RFD by 00Z,
  Chicago terminals from roughly 0120Z to 0200Z.

- SHRA/VCTS likely to persist for a few hours behind the
  initial squall line.

- MVFR ceilings possible late tonight/Saturday morning.

- Scattered SHRA/TSRA development along cold front Saturday
  midday/early afternoon, mainly for Chicago terminals.


Strong/severe thunderstorm squall line currently from southwest
WI into western IL, tracking east at around 40-45 mph. RFD
expected to be impacted with severe storms around 00Z, with 50+
kt gusts and a period of IFR/LIFR vis in heavy rainfall. Current
timing of line brings it to DPA toward 0130Z, MDW by around
02Z. Storms may be a bit less intense as they reach the Chicago
area, but still potentially strong. After initial squall line
passes, expect a few hours of rain and embedded thunder to
persist into late evening. Winds will likely become somewhat
chaotic behind the initial gust front, probably developing some
east-northeast component for a time before eventually veering
clockwise to southwest later tonight. Rain/occasional thunder
in stratiform precipitation behind the thunderstorm complex
should eventually taper off/end late tonight, though some
uncertainty on how quickly that occurs.

Model guidance supports a period of MVFR ceilings across the
area Saturday morning before eventually lifting/scattering
behind a cold front later in the day. Scattered SHRA/TSRA are
expected to develop along the front by midday/early afternoon,
likely southeast of the RFD area. Have included a prob30 mention
for the Chicago sites early afternoon before this activity
shifts southeast of the area. Surface winds are expected to be
west-southwest during the morning, eventually shifting west-
northwest in the afternoon behind the cold front. May be a
little breezy, with some gusts 15+ kts or so.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch until 10 PM CDT this evening for ILZ003-ILZ004-
     ILZ008-ILZ010.

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

&&

$$

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