457
FXUS63 KLOT 292332
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
632 PM CDT Tue Jul 29 2025

.KEY MESSAGES...

- Periods of thunderstorms through Thursday morning, some
  possibly strong to severe with torrential rain which could
  lead to localized flash flooding.

- Possible period of prolonged heavy rain Wednesday night into
  early Thursday morning.

- Dangerous swimming conditions at Lake Michigan beaches
  Wednesday afternoon through Friday.

&&

.DISCUSSION...
Issued at 252 PM CDT Tue Jul 29 2025

Through Tonight:

It has been another hot and humid day under mostly sunny skies,
though dew points have trended slightly lower then yesterday. Heat
index values currently in the upper 90s for most locations will top
out around 100, with some readings in the 100-105 range across the
western CWA amid higher low-level moisture.

Cumulus had struggled to develop until the past hour or so due to
modest warming/capping in the 6-8kft layer. Mixing heights have
started to rise into that layer, though satellite imagery and visual
appearance of stunted cumulus suggest capping is still holding as of
3pm. A weak front/boundary drifting southward over southern
Wisconsin as well as a lake breeze oozing inland from the Illinois
shore will begin to interact with a more pronounced cumulus field
over the northern Chicago metro during the next few hours. Have
maintained slight chance PoPs/thunder in this area into early evening
in the event the cap does break. If this were to occur, modest deep-
layer shear for late July would be sufficient for an organized
discrete cell or two with locally strong winds and torrential
rainfall.

Dry conditions are expected this evening, and possibly well into the
night, as winds synoptically veer north or northeasterly while high
pressure builds over the far northern Great Lakes. Phasing of a pair
of waves that will be convectively enhanced by substantial
convection over Nebraska and Iowa tonight will shift eastward across
Iowa through sunrise Wednesday. A resulting low-level contraction of
the baroclinic zone along the southern extent of the high will
settle somewhere over the forecast area late tonight. Cannot rule
out isolated convection along this boundary across the western CWA
very late tonight, though weak low-level flow nearly parallel to the
front will likely provide insufficient forcing to overcome existing
MUCIN remaining over northern Illinois overnight.

Kluber


Wednesday through Tuesday:

Primary forecast concern remains severe thunderstorm and heavy
rain potential through Thursday morning with several rounds of
thunderstorms.

Thunderstorms may be ongoing Wednesday morning across the
western cwa, possibly as a slowly weakening line. Its also
possible that while the line may weaken during the mid morning,
that it maintains enough organization to reintensify as it
moves across the Chicago metro area midday. Some of these storms
could possibly become strong/severe along with heavy
rain/localized flooding.

The cold front will be moving south across the area Wednesday
and the morning/midday storms may aid the front pushing into
the southern cwa into the afternoon. Additional thunderstorms
are expected to develop Wednesday afternoon. Depending on where
the front ends up when storms develop, there is the potential
for supercell thunderstorms across the southern third or so of
the cwa Wednesday afternoon, which would then allow for all
modes of severe weather. Despite the northeast winds further
north, if any organized thunderstorms can develop/persist, there
would remain at least an isolated damaging wind potential
further north.

There is now a consensus for additional rain, most likely heavy
rain Wednesday night into Thursday morning across much of the
area, particularly along/north of I-80. A slow moving secondary
low is expected to move across the area allowing for a prolonged
period of rain. Precipitable water values during this time are
in the 2 to 2.25 inch range with several models showing QPF
amounts anywhere from 2 to 4 inches, much of this falling in 6
to 12 hours. Consideration was given to a flood watch for this
time period but it is still 30-36 hours out and there remains
quite a bit of uncertainty. Have increase QPF during this period
but it may not be enough if the current trends materialize.
While scattered thunderstorms would remain possible Wednesday
night, the heavy rain and possible flooding remains the main
threat during this time period.

Northeasterly winds steadily increase Wednesday afternoon and
may be gusting into the 30-35 mph range Wednesday night. Did
increase speeds/gusts, but they may need to be increased further
with later forecasts. These northeast winds will allow for high
waves and dangerous swimming conditions at Lake Michigan beaches
through Friday.

Much less humid conditions are expected through Sunday and then
humidity levels begin to increase again early next week. Highs
in the 70s are expected Thursday/Friday and then highs will
likely reach 80 Saturday and then back into the 80s Sunday into
early next week. cms

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 632 PM CDT Tue Jul 29 2025

- Periods of showers and thunderstorms Wednesday into Wednesday
  evening.
- Strengthening northeast winds Wednesday afternoon

There could be some patchy shallow ground fog again tonight,
especially at GYY and DPA, but confidence is too low to
introduce into the TAFs. Otherwise, VFR conditions expected
until after sunrise when the first wave of SHRA/TRSA is expected
to impact the terminals. There could be a break before another
period of SHRA/TSRA later in the day with potential for steady
rain to develop Wednesday evening. Confidence in timing the best
chances for heavier TSRA and breaks in the rain is very low at
this distance, so opted to make very few changes from the
previous forecast. As storms organize to our west over the
Cornbelt tonight, should be able to refine timing for TSRA and
likely breaks in rain by the 09z TAF update.

Other than the waves of SHRA/TSRA, look for northeast winds to
increase Wednesday afternoon. Closer to the lake (ORD, MDW, GYY)
look for gusts over 20kt to develop Wednesday afternoon with a
period of even stronger gusts possible, especially Wednesday
evening. Should see CIGS trend to prevailing MVFR Wednesday and
probably stay there into Wednesday evening.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from Wednesday afternoon through
     Friday afternoon for ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement from Wednesday afternoon through
     Friday afternoon for INZ001-INZ002.

LM...Small Craft Advisory from 1 PM Wednesday to 9 AM CDT Friday
     for the IL and IN nearshore waters.

&&

$$

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