175
FXUS63 KLOT 080531
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1231 AM CDT Tue Jul 8 2025

.KEY MESSAGES...

- Dangerous swimming conditions continue at Lake Michigan
  beaches, but improving late this evening.

- Chance (30-50%) for scattered showers and thunderstorms on
  Tuesday across primarily northern and central IL.

- Low shower/storm chances continue on Wednesday. Better chances
  return Friday/Friday night.

&&

.DISCUSSION...
Issued at 226 PM CDT Mon Jul 7 2025

Tonight:

Expect partly cloudy skies and quiet conditions with seasonable
overnight lows in the lower to mid 60s outside of Chicago and
upper 60s in the city. The Beach Hazards Statement is set to
expire at 7pm for Illinois beaches and 9pm for Indiana beaches,
which looks reasonable per web cams and observations as of this
writing. There could be some patchy fog tonight near/south of
the Kankakee River overnight.

Castro


Tuesday through Monday:

Forecast evolution for Tuesday remains a bit unclear, with some
timing differences in the handling of low-amplitude disturbances
noted in today`s guidance. At the surface, a moist axis which is
currently displaced west of the Mississippi River will slosh
eastward into the region as low-level southwesterly flow returns.
Aloft, several vorticity maxima will be in the vicinity as a more
notable shortwave translates well to our north across the
Minnesota Arrowhead Region. Have expanded some 20-30 percent
PoPs a bit farther east across the region Tuesday morning as
very modest ascent within a moistening airmass could yield
clusters of showers and a few storms.

Through the rest of the day, upper 60s/near 70 degree dewpoints
are forecast to return, and most guidance commensurately
suggests capping will erode through the late-morning and
afternoon, even with muted insolation and the potential for
pockets of morning showers/storms tied to a 700 mb wave in the
region. While surface convergence isn`t strongly defined
tomorrow, the presence of a moist/destabilizing airmass and
presence of several disturbances aloft suggests mid-range chance
PoPs remain appropriate to cover the potential for afternoon
and early-evening shower and thunderstorms. Mid-upper flow looks
fairly weak, so storms should remain disorganized and posing
primarily a locally heavy rain and gusty wind threat.

Some potential that activity festers a bit through Tuesday
evening, but should see a gradual diminishing trend as drier
low-mid level air spreads in from the north. Through the day on
Wednesday, forecast soundings look like they become increasingly
capped across the northwest half of the region as a subsidence
inversion pushes eastward. Deeper boundary layer moisture is
evident with south and east extent through the area, however,
resulting in more limited capping. A lack of defined upper-level
features suggests that any diurnally-driven activity may remain
more sparse in coverage, and tied generally to surface
boundaries.

Speaking of boundaries (on Wednesday): today`s guidance is
starting to hit the wind response on the lake a bit harder as a
backdoor front drops down the lake, which conceptually fits the
set up of a near 1020 mb high pushing east across the UP of
Michigan. Gusty northerly wind response is not ubiquitous in the
model guidance today, but did boost winds a bit on the lake,
and will need to keep an eye on this window for a period of
moderate or greater swim risk.

Thursday could end up being largely dry in the wake of the
aforementioned front. The surface high won`t build across the
entire area, and lingering moisture could hold on in our far
southern locales, enough to justify hanging onto some very low
(15%) PoPs south of the Kankakee River vicinity, but even these
could end up being a bit too high.

On Friday-Saturday, forecast confidence begins to diminish as a
fairly sharp shortwave trough slicing across the Northern
Plains starts to interact with remnant MCVs/vorticity clusters.
Deterministic GFS continues to advertise some severe weather
threat across northwest Illinois but currently remains a bit of
a south and east outlier across the guidance suite.

Carlaw

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1231 AM CDT Tue Jul 8 2025

Forecast concerns include...

Chance of thunderstorms this afternoon.
Low chance for showers through mid morning.
Potential for fog early Wednesday morning.

Current isolated showers across northwest IL are expected to
dissipate over the next few hours before reaching the Chicago
terminals, though an isolated shower will remain possible. A few
showers across southeast MN/western WI may continue to fester
through the overnight hours, bringing a chance for showers to
RFD after daybreak. If these were to materialize, they may
persist north of the terminals through the mid/late morning
hours.

There remains a chance of thunderstorms for all of the terminals
this afternoon, though coverage remains uncertain and maintained
prob mention for now. A large complex of thunderstorms is moving
southeast across southwest IA into northern MO. How these storms
evolve over the next 12 hours, will impact storm trends locally
this afternoon. The most favored areas appear to be west central
IL with activity possibly dissipating further to the northeast.
Overall, low confidence for trends, which will need to be
monitored for possible later forecast changes. After this
potential precipitation, the rest of this evening into early
Wednesday morning is looking mainly dry.

Winds are light and variable and will become light southerly by
daybreak then become southwest by mid morning with speeds
increasing into the 10-12kt range. The forecast soundings
suggest there may be a period in the late morning, when gusts
into the 15-20kt range will be possible. Wind directions this
evening are uncertain. If there are storms, winds may favor a
light southeast direction.

Forecast soundings also show hardly any wind aloft early
Wednesday morning as weak frontal boundary sags into northern
IL. This would easily support fog development. However, fog may
be dependent on how widespread any precipitation becomes and the
best location for fog appears to be northwest IL, which would
be after the current 24 hour RFD taf period. But some fog
mention may be needed with later forecasts. cms

&&

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

&&

$$

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