996
FXUS63 KLOT 150557
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1257 AM CDT Tue Jul 15 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 /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1257 AM CDT Tue Jul 15 2025

Key Messages:

- Visibility reductions from fog possible at GYY, RFD, and DPA
  through about 13Z or 14Z.


A few ASOS/AWOS sites have already reported visibility
reductions prior to 06Z tonight from what is likely shallow
ground fog, and with clear skies and calm winds expected to
persist through the night, suspect that these visibility
reductions will continue overnight. Observations suggest that
GYY and RFD appear to be most likely to be affected by this
fog tonight out of our five TAF sites, though it`s possible
that visibility reductions could occur at DPA as well. Would
think that the dry low-level air should prevent the fog from
growing too deep or becoming dense on a widespread basis, so
settled on an MIFG mention in the GYY and RFD TAFs (at least for
now). Unrestricted visibilities should then return by mid-
morning as the rising Sun burns off the fog.

Otherwise, winds today should generally remain at or below 10
kts and mainly out of a southerly to south-southwesterly
direction, though there could be a period of time late this
afternoon into this evening where they turn east of 180 degrees.
There is also a low chance (about 15-20%) for showers to affect
one or more of our TAF sites late this afternoon or evening and
an even lower chance (about 10%) of an isolated thunderstorm or
two developing near the terminals. Neither of these
probabilities are high enough to warrant a formal precipitation
mention in the TAFs at this time, but model and observational
trends will need to be monitored.

Ogorek

&&

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

IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.

LM...None.

&&

$$

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