517
FXUS63 KLOT 112350
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
650 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.

&&

.UPDATE...
Issued at 1210 PM CDT Fri Jul 11 2025

The main focus continues to revolve around the severe thunderstorm
and heavy rain threat later this afternoon and evening.

The remnant boundary from the early morning convection is now
beginning to lift back northward into northern IL and IN. South
of this boundary, thinning cloud cover is resulting in strong
heating and destabilization of a very moist low-level airmass (low
70s dewpoints). As this continues over the next few hours,
continued heating and destabilization will contribute to the
development of a very unstable airmass, with MLCAPE values of
2,000-3,000 j/kg.

This will certainly be priming the area for another round of
showers and thunderstorms later today into this evening. Our main
focus for the initial development of these storms will be to our
west across IA this afternoon. Over the past couple hours elevated
storms have been developing across northern parts of IA along the
nose of a remnant southerly low-level jet. This activity is
generally shifting to the east-northeast, and thus is likely not
impact our area in the near term. However, additional near surface
based storms are likely to develop farther south across
southwestern into south central parts of IA into early this
afternoon. This will occur as surface heating and destabilization
of the moist low-level airmass in continues in the vicinity of the
surface frontal boundary and just in advance of the main mid-
level trough axis and surface low.

The kinematic environment in which these storms develop will
feature clockwise turning hodographs amidst 700 to 500 mb
southwesterly flow of 30 to 40 kt. This will be supportive of
organized severe storms including supercells. As the afternoon
progresses, amalgamating outflows from individual storms is
expected to support a quick upscale growth into a forward
propagating QLCS across eastern IA into northwestern IL mid to
late this afternoon (likely after 4 PM). The main threat with this
QLCS across northern IL will be strong and damaging wind gusts
(potentially in excess of 75 mph). However, a tornado threat also
will exist into this evening, particularly across northwestern
portions of IL and eastern IA. With the increasing confidence in
this severe QLCS the SPC has increased the severe threat to a
level 3 of 5 across portions of northwestern IL. While a gradual
weakening trend in the QLCS is anticipated later this evening into
tonight with eastward extent across northern IL and northwestern
IN, the severe wind threat with these storms is likely to extend
through the Chicago metro area this evening (7 to 11 pm
timeframe).

Very heavy rainfall will also accompany these storms, and may
result in some renewed flash flooding across some of the same
areas that were hit with the very heavy rainfall in and around the
Rockford area last night. A flash flood watch thus continues for
northwestern IL through this evening.

KJB

&&

.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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