999
FXUS63 KLOT 092350
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
650 PM CDT Wed Jul 9 2025

.KEY MESSAGES...

- Dangerous swimming conditions are expected at Lake Michigan
  beaches through early Thursday

- Scattered showers and isolated thunderstorms possible through
  this evening

- Multiple potential opportunities for showers/storms/localized
  flooding Thursday afternoon through Saturday evening. Saturday
 PM currently appears to have the highest chance for a threat for
  more than isolated strong to severe thunderstorms.

&&

.DISCUSSION...
Issued at 322 PM CDT Wed Jul 9 2025

Through Thursday Morning:

A lake enhanced cold front has worked its way down the Illinois
lakeshore and continues to sag southward. Due to sustained
onshore flow, the High Swim risk for waves up to 5 feet for both
the Illinois and Indiana nearshore and Beach Hazard Statement
will be maintained through early Thursday morning.

This cold front has also kicked off showers in Cook County and
southeastern Wisconsin. There have been isolated lightning
strikes in a few of the tallest cells. Additionally, better
coverage of scattered showers have cropped up along a ribbon of
better moisture southeast of Interstate 57. There is plenty of
uncapped MLCAPE to allow for thunderstorms to bubble up.
However, due to a growing subsidence inversion with drier mid
level air, cells will likely struggle to get the proper charge
separation need for lightning.

Have maintained shower chances (20% to 30%) through sunset,
however, the chances for thunder were capped at 20% to message
the isolated nature of it. Lastly, with over 1.5 inches of
precipitable water and "very" slow storm motions, the main risk
with these showers will be the threat of localized downpours.
Flash flooding is not expected given the lower coverage, but
having rain rates around and inch per hour or more is certainly
possible and could lead to localized ponding on roadways.

Shower chances quickly diminish after sunset with the forecast
remaining dry through midday tomorrow. The only other concern
that will be monitored through the overnight will be fog trends.
Compared to this morning, the set-up for widespread dense fog
looks more murky. Model soundings are showing drier air near the
surface with only mid level condensation that would favor more
stratus than fog. The forecast was capped at "patchy" mention,
and it would mainly be for areas west of the the Fox Valley and
south of Interstate 80.

DK


Thursday Afternoon through Wednesday:

Convection emerging out of Iowa and southern Minnesota late
tonight into Thursday morning may have yet another lower
predictability MCV associated with it. In a change from most
previous guidance, several recent CAM solutions simulated a
convective footprint in advance of the MCV slowly spreading
east-southeastward Thursday afternoon and evening, possibly
lingering into the overnight.

Forecast soundings valid early Thursday afternoon even near/west
of I-39 feature notable dry air at the mid-levels as well as a
subsidence inversion around 600 mb. If (likely decaying)
convection advances eastward across the MS River Thursday
afternoon, it will plausibly outpace the rather sharp west to
east instability gradient across the area due to aforementioned
antecedent mid-level dry air and capping issues. If this occurs,
dissipating showers and isolated storms crossing the I-39
corridor may serve to stabilize the environment for any attempts
at robust redevelopment and/or intensification late Thursday
into Thursday evening. With this being said, a tongue of more
favorable 800-600 mb RH is forecast to spread steadily eastward
Thursday afternoon and evening in advance of the MCV. Even with
sub-marginal deep layer shear, as is common this time of year,
large DCAPE/steep low-level lapse rates and precip loading may
present a threat for localized strong to severe downburst winds
into Thursday evening (level 1 of 5 severe threat).

Given an already low-predictability setup and the change from
previous forecast cycles, we took an initial measured step of
introducing chance PoPs near/west of the I-39 corridor Thursday
afternoon (3pm onward) and then brought these east-southeastward
through Thursday night. Seasonably high PWATs in the 1.5" to
1.8" range will also spread eastward Thursday night, in tandem
with a modest low-level jet. As we saw with Tuesday evening`s
MCV associated surprise flash flooding in Chicago, inherent slow
storm motions (and any training convection) spurred by these
features could yield a localized non-trivial flash flooding
setup. WPC`s day 2 level 1 of 4 (marginal ERO) flash flood
threat near/west of the Fox River valley appears reasonable at
this time.

On Friday, depending on how things evolve Thursday night,
lingering showers and isolated embedded storms may serve to slow
the diurnal warming some. Assuming any morning convection
fizzles out, Friday afternoon-evening looks like a classic
diurnal pulse convection setup amidst weak forcing, very warm
and humid conditions, and little/no capping of a moderate to
strongly unstable profile. Isolated to widely gusty scattered
storms (20-35% PoPs/coverage) appear probable, with an IL shore
hugging lake breeze potentially serving as a foci. A bonafide
advancing mid-level trough Friday night into early Saturday
could aid in scattered convection lingering deeper into the
overnight, though as usual uncertainty abounds. Any slow moving
storms Friday PM through Friday night will likely produce
torrential downpours capable of causing at least localized flash
flooding.

Saturday`s afternoon-evening thunderstorm forecast will in all
likelihood be modulated by the extent of convection lingering
into Saturday morning. Synoptically speaking and mesoscale
unknowns aside, though, Saturday has more apparent ingredients
that could yield a scattered (level 2 of 5 type) severe wind
threat. Mid-level short-waves embedded in a trough axis
extending back to northwest Ontario will be accompanied at the
surface by a weak surface low and cold front progressing across
the region through Saturday evening. The front will encounter a
very warm and moist (exactly how unstable TBD) air mass with dew
points well into the 70s. 25-35 kt of deep layer bulk shear
will support a bit more storm organization and longevity,
especially if linear/upscale growth occurs just ahead of the
cold front. For the reasons above, we`re advertising scattered
thunderstorms Saturday afternoon (60-70% convective coverage)
with this afternoon`s forecast package.

Depending on the exact timing of convective initiation and cold
frontal approach and passage late day Saturday, some storms
should be in progress early Saturday evening, especially I-55
and southeast. Thunderstorms (and any wind and flooding
threats) should then quickly end by the late evening as the cold
front clears the area. The front should push far enough
southeast into Sunday, along with very dry mid-level air and
neutral to positive mid-level height tendencies, to result in a
mostly (if not entirely) dry period at least through Tuesday
morning. Thunderstorm chances may then again uptick midweek and
beyond. Temps will continue to average solidly above normal
with moderately humid to muggy conditions common to mid July.

Castro

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 650 PM CDT Wed Jul 9 2025

Key messages for the 00Z TAFs:

- Scattered SHRA/isolated TS for RFD early this evening along
  cold front.

- IFR ceilings persist off the lake at GYY this evening before
  improving to VFR.

- Low chance for patchy/shallow fog late tonight, mainly at RFD.

- Low chance for TSRA to affect RFD Thursday afternoon, with
  chances decreasing toward the Chicago area. Additional TS
  potential into the evening, though timing/coverage of low
  confidence.

Combined cold front/lake breeze boundary has pushed slowly
inland to about a RFD-JOT-IKK line as of 23Z. Convergence along
this boundary has initiated isolated SHRA/TSRA, with main area
of TSRA threat now well south of the terminals in northwest IN.
SHRA continue to develop along the boundary over RFD however,
and expect a period of SHRA there for the first couple of hours
of the TAF. Can`t completely rule out TS with this, though
eventual loss of diurnal heating combined with fairly warm mid-
level temps/poor lapse rates should limit that threat. Have
indicated period of SHRA at RFD, though will monitor for
any indication of increasing TS potential early this evening.

Overnight, light northeast winds are expected to persist, with
otherwise quiet aviation weather conditions. There is some
potential for shallow/patchy fog to develop after midnight
mainly southwest of the frontal boundary location, though can`t
rule out some fog at RFD. Surface winds expected to be east to
east-northeast across the metro Thursday, while shifting south-
southeast and eventually south-southwest toward RFD during the
afternoon.

Scattered thunderstorm potential increases upstream of the area
across MN/IA by Thursday afternoon, with CAM guidance suggesting
current convective activity over the Dakotas will develop toward
the region along with a mid-level disturbance. Confidence lowers
with eastward extent across northern IL however, with stronger
capping aloft farther to the east. Have included a PROB30 for
TSRA at RFD in the afternoon, but maintained dry forecasts for
the Chicago metro sites for now where confidence decreases.
Additional scattered TS threat will exist into Thursday night,
though again low-confidence in coverage/timing for now.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement through Thursday morning for ILZ006-
     ILZ103-ILZ104.

IN...Beach Hazards Statement through Thursday morning for INZ001-
     INZ002.

LM...None.

&&

$$

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