714
FXUS63 KLOT 141921
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
221 PM CDT Mon Jul 14 2025

.KEY MESSAGES...

- There is a chance (20%) for thunderstorms south of I-80
  Tuesday afternoon and evening.

- Humidity along with periodic thunderstorm chances return
  Tuesday through Thursday, with some threat for localized flash
  flooding and severe weather late Wednesday through Thursday.

- After a brief break Friday, humidity and the chance for
  occasional bouts of storms expected to return next weekend and
  stay through next week.

&&

.DISCUSSION...
Issued at 221 PM CDT Mon Jul 14 2025

Through Tuesday Night:

A few showers have developed across the far southern cwa early
this afternoon, likely in the vicinity of a differential heating
boundary leftover from the fog this morning. With very little
wind through the column, these showers are just drifting. While
they are tiny, there may be some brief heavy rain. These will
dissipate later this afternoon into early this evening.

A weak wave will move across central IL and central IN Tuesday
into Tuesday evening bringing a low chance (20%) for showers and
a few thunderstorms, mainly for areas south of I-80. A lake
breeze is expected to being moving inland Tuesday afternoon and
this may allow for a few showers across the Chicago metro area
but for now have remained dry north of I-80 Tuesday. Whatever
does form Tuesday afternoon, which is expected to be isolated,
should be on a slow downward trend Tuesday evening. Depending on
how fast this wave departs, its possible a few showers may
continue Tuesday night into early Wednesday morning across the
southeast cwa, but confidence is fairly low.

High temps this afternoon will likely top out in the mid 80s for
most locations with dewpoints in the 50s for parts of the
Chicago metro area. Highs on Tuesday are expected to reach the
upper 80s to lower 90s with increasing low level moisture.
Dewpoints look to get back into the upper 60s/lower 70s in the
afternoon, which would push heat index values into the mid 90s
for most areas with perhaps a few locations reaching the upper
90s. cms


Wednesday through Monday:

Toward the middle of the week, the upper-level pattern is
expected to be characterized by zonal flow along the US/Canadian
border. Within and along the southern extent of the zonal flow
will be embedded shortwaves, which will likely be influenced by
convective episodes in the northern Plains. A broad low-level
frontal boundary will slowly slip south through the Great Lakes
and act as a highway for episodic clusters of showers and
storms. Taken together, a quintessential July pattern is shaping
up for the middle of the week.

There remains a signal that the first convectively-augmented
wave and associated surface MCV will move into the Lower Great
Lakes region sometime in the Wednesday to Wednesday night
timeframe. Exactly when and where it arrives/goes will be
subject to refinement as we get closer. Regardless, augmented
flow/shear along the periphery of the circulation and PWATs
climbing toward 2" may spell trouble for both severe weather and
flash flooding wherever the MCV tracks on Wednesday. Our
gridded database will feature mid-range chances (40-60%) for
storms Wednesday afternoon and evening, which seems appropriate
at this point in time. WPC and SPC-advertised level 1/5 threats
for both flash flooding and severe weather in our general region
remain fair as well, though certainly can envision embedded
corridors where (much?) higher probabilities/threat levels will
be needed in later forecasts. If convective coverage ends up
maximized after peak heating Wednesday, daylight hours will be
stuffy with the combination of heat (highs in the lower 90s) and
humidity (dew points in the mid 70s) making it feel like the
upper 90s to lower 100s.

Another shortwave is poised to move into the general region on
Thursday, presenting another opportunity for showers and storms.
As is typical in these patterns, the placement of the broad
frontal boundary by then will dictate the threat zone for the
next round of storms. For now, will feature mid-range chance
PoPs (40-60%) along/south of both I-55 and I-80 keeping in mind
adjustments are all but likely once we get an idea of the
convective footprint on Wednesday evening/night. Outside the
threat for thunderstorms, our area may be bisected by relatively
comfortable temperature/humidity levels north of the front and
continued hot and humid conditions to the south. We`ll also have
to watch for a quick uptick in wave heights Thursday evening as
a surface high builds into the region and reinforces northerly
flow down the spine of Lake Michgian.

The front should be well south of our area on Friday, leading
to a much quieter and more comfortable day. Depending on the
strength of northerly winds Thursday night, beach conditions may
remain choppy through much of Friday. If planning to head to
the beach toward the end of the week, stay up to date on the
forecast.

Looking toward the weekend and into next week, ensemble model
guidance is exhibiting an unusually strong signal for building
heat and humidity levels as well as episodic (severe) MCSs in
the broad Midwest region. Stay tuned.

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1216 PM CDT Mon Jul 14 2025

VFR with no significant concerns - Main items of note:

- Winds today near the lake.

- Potential for a lake breeze wind shift to east-southeast at
  ORD and MDW on Tuesday afternoon.

- Small (<15%) chance for an isolated SHRA on the lake breeze
  Tuesday afternoon.

Recent radar scans indicate a lake breeze slowly pushing inland
early this afternoon, already within 5 mile range of MDW, and
~7-8 miles from ORD. High resolution guidance has been insistent
on the lake breeze`s progress slowing and full passage not until
early this evening. With a very light wind field overall and
weaker lake breeze this time of year, maintained light VRB (SW
winds at MDW) and previous easterly wind timing (23z) at both
ORD and MDW.

On Tuesday, south to south-southwest winds near 10 kt should
develop inland of the lake breeze. While there`s a chance for
the lake breeze to reach ORD and MDW in the afternoon,
confidence is low, and prevailed southerly winds in the TAF.
Finally, an isolated shower developing on the lake breeze can`t
be completely ruled out, but the chance of occurrence is far too
low for explicit TAF mention. Isolated TS will be possible well
south of the terminals.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Air Quality Alert until midnight CDT Tuesday night for ILZ005-
     ILZ006-ILZ012-ILZ013-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-
     ILZ106-ILZ107-ILZ108.

IN...Air Quality Alert from midnight tonight to midnight CDT
     Tuesday night for INZ001-INZ002-INZ010-INZ011.

LM...None.

&&

$$

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