083
FXUS63 KLOT 100530
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1230 AM CDT Thu Jul 10 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 /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1230 AM CDT Thu Jul 10 2025

Forecast concerns include...

Chance of thunderstorms this afternoon through tonight.
Potential for fog early this morning, mainly south of the tafs.

Fog is beginning to form across areas south of the terminals
early this morning and is expected to continue through daybreak
and then steadily lift/dissipate. While fog is still possible
across northwest IL and at RFD, most recent guidance trends
suggest the fog may be further south. Maintained current tempo
mention for now and trends will need to be monitored.

Light easterly winds will continue through mid morning and then
a lake breeze is expected to move inland this afternoon, with
east/northeast winds for the Chicago terminals. Further inland,
winds will likely become light south/southeast and then turn
more to the south/southwest this afternoon.

Scattered thunderstorms are expected to develop over eastern IA
and northwest IL this afternoon and possibly persist into this
evening. Expanded prob timing at RFD and its possible tempo
mention may eventually be needed. Some of this activity may move
further east toward the Chicago terminals in the early/mid
evening hours, but confidence is very low and will maintain a
dry forecast for now. How this convection evolves will play a
role into the potential for additional scattered thunderstorms
early Friday morning. If these materialize, its possible they
would be right over northern IL and the terminals. Confidence
remains too low for this time period as well. However, later
forecasts may end up with a few periods of thunder for the
Chicago terminals. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement until 10 AM CDT this morning for
     ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement until 10 AM CDT this morning for
     INZ001-INZ002.

LM...None.

&&

$$

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