837
FXUS63 KLOT 162009
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
309 PM CDT Wed Jul 16 2025

.KEY MESSAGES...

- Scattered to numerous thunderstorms into the early evening
  from the Chicago metro and points south and east. There is a
  level 2/5 severe weather risk for most of the area with
  damaging winds as the primary severe weather hazard.

- MUCH cooler and less humid Thursday with gusty northerly winds
  and dangerous swimming conditions at Lake Michigan beaches.

- Periods of warmth, humidity, and thunderstorms expected this
  weekend into early next week, followed by an increasing signal
  for potentially dangerous heat arriving mid next work week.

&&

.MESOSCALE...
Issued at 235 PM CDT Wed Jul 16 2025

Strong to severe storms continue to sweep across the area this
afternoon along the southeastern flank of an MCV moving across
southern Wisconsin. With a gradient of shear from north to south
across the Lower Great Lakes, behavior of storms across
northern Illinois has been outflow dominant thus far with
pockets of damaging winds as high as 70 mph within the past
hour. Going forward, the expectation is for the band of storms
to continue moving east, sweeping across the remainder of the
area this afternoon with a continued threat for locally damaging
winds. With the storm mode being an outflow dominant squall
line, not concerned about damaging hail or tornadoes in our area
going forward.

The Severe Thunderstorm Watch has been expanded to include the
rest of our area, though did clear Winnebago/Ogle/Lee out behind
the storms.

Borchardt

&&

.DISCUSSION...
Issued at 309 PM CDT Wed Jul 16 2025

Tonight:

After the strong to severe storms end early this evening in the
wake of the cold front, the southward progress of the outflow-
augmented front will slow. A strengthening low-level jet
overriding the boundary could foster the development of training
convection just to its north. This could include areas roughly
near and south of US-24 if the boundary stalls just south of the
CWA. Most of the latest guidance brings the front far enough south
to result in a scenario with training convection with torrential
downpours and an associated flood threat into areas near and south
of US-24 as a less likely outcome at this time (but still
certainly one worth monitoring for). Aside from this low-end
potential, cooler air will filter into the area from the north
behind the front with lows down into the 60s near and northwest of
I-55.

Thursday through Wednesday:

As alluded to above, fairly strong high pressure (~1020mb) for
mid- summer combined with push from earlier convection should send
the composite outflow boundary/cold front south of our area
Thursday. In the wake of the front, look for much cooler and less
humid air to filter into the region. Morning stratus should break
up into a scattered to broken stratocumulus deck during the
afternoon, with this cloud cover likely aiding in keeping temps
cooler. Thursday afternoon temps should be around 20F cooler in
the Chicago area with readings in the low to mid 70s with a brisk
(for July) wind off the lake. That brisk wind off the lake will
also help build up larger waves and lead to hazardous/dangerous
and potentially life-threatening swim conditions at unprotected
Lake Michigan beaches Thursday into Thursday night. A Beach
Hazards Statement will be issued for this, in effect into early
to mid Friday morning.

Friday will be warmer, but with still comfortable humidity
levels. Boundary that will have pushed south of our area
Thursday should stall near the Ohio River before heading back
north as a warm front Friday night into Saturday. There`s been a
pretty consistent signal in bringing a convectively enhanced
shortwave across the region later Friday night into Saturday.
Still several days out, so exact timing of this feature and
where exactly it tracks may need to be refined in coming days,
but the general idea of an MCS moving across the mid-Mississippi
Valley in this time frame, possibly affecting portions or all
of our area, continues.

Medium range guidance has trended toward slightly more
amplified northern stream troughing over the Great Lakes into
New England later in the weekend. This would allow for a
transient surface high pressure to move across the Great Lakes
Sat night into Sunday. This, combined with the prior MCS, could
push the effective boundary and rain/storm chances south of our
area. Confidence is too low to significantly reduce or remove NBM
POPs for Sunday, but there is an increasingly plausible scenario
where the second half of the weekend could end up dry and a bit
cooler and less humid, particularly near and north of I-80.

Looking ahead to the first half of next week, there continues to
be a consistent signal in medium range guidance that a strong
upper level ridge and associated dome of heat will build over
the central and eastern portions of the nation. In this time
frame, we`d expect that the front that pushes southward some
Sunday will lift back north bringing the potential zone of MCS
activity with it back into our area for a time early next week.

By the middle of the week, the upper ridge is progged to become
anomalously strong and centered over the lower and middle
Mississippi Valley. Assuming MCS activity doesn`t significantly
impede the northward movement of the boundary, our area could
get into a bout of some potentially dangerous heat during the
middle and end of next week. Operational runs of the GFS and
ECMWF both are explicitly forecasting 80F+ dewpoints
accompanying the heat, which given we`ll be in the heart of
evapotranspiration season, may not be unreasonable. Still plenty
of uncertainty regarding if convection can temporarily impede
the northward building heat, but concern is growing that
potentially dangerous heat wave could develop next week in or
near our area.

- Izzi/Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1226 PM CDT Wed Jul 16 2025

Key messages for the 18Z TAF period include:

* A broken line of thunderstorms will impact the TAF sites this
  afternoon through early evening. Some storms may be strong to
  severe.

* MVFR cigs will fill in tonight behind the storms and stick
  around through Thursday.

A broken line of thunderstorms is moving across northwest IL
early this afternoon and is expected to impact the TAF sites
later today. Storms should first arrive at RFD between 18 and
19Z and approach the Chicago sites closer to 21-22Z. Some of
these storms could be strong to severe and feature damaging
winds and perhaps some hail. The severe storm threat should only
exist for an hour or two over any given site, but additional
isolated storms will be possible through about 01Z.

Following the storms, MVFR cigs are expected to move overhead
tonight. MVFR looks to arrive at RFD around 04Z and Chicago
around 06Z or not long thereafter. The MVFR is anticipated to
last through most of the daytime tomorrow with a majority of
guidance suggesting late afternoon/early evening for a return to
VFR.

Doom

&&

.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.

     Beach Hazards Statement from Thursday morning through Friday
     morning for ILZ006-ILZ103-ILZ104.

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

     Beach Hazards Statement from late tonight through Friday
     morning for INZ001-INZ002.

LM...Small Craft Advisory from 9 AM Thursday to 4 AM CDT Friday for
     Winthrop Harbor IL to Gary IN.

     Small Craft Advisory from 4 AM Thursday to 4 AM CDT Friday for
     Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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