339
FXUS63 KLOT 140808
AFDLOT

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

.KEY MESSAGES...

- 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,
  though the weekend doesn`t look to be a washout.

&&

.DISCUSSION...
Issued at 308 AM CDT Mon Jul 14 2025

High pressure over northern Illinois early this morning has
brought with it some lower dewpoints into the area, though the
higher dewpoints are lurking to the south as close as central
Illinois. Light winds and the recent rains have allowed for
patchy dense fog to form well south of I-80 within that more
humid air mass. The patchy dense fog will quickly dissipate
after sunrise. Otherwise, look for a good deal of sunshine and
seasonably warm temperatures today. Light winds will allow for
an afternoon lake breeze with slightly cooler temps adjacent to
the lake. The HRRR smoke model shows decreasing smoke column
integrated smoke concentrations today, so not anticipating smoke
to be a significant factor today.

A mid-upper level shear axis, the remnants of an upper low and
upper trough, from the Red River Valley northeast into the
Ozarks today is progged to move northeast into Illinois Tuesday.
As it does, light southerly flow will begin to draw the higher
dewpoints downstate northward into our CWA. This shear axis was
the focus for isolated-widely scattered thunderstorms Sunday
afternoon and is expected to continue to do today across the
Ozarks, with isolated afternoon convection likely to develop
northeast across Illinois Tuesday afternoon. Greatest coverage
Tuesday afternoon should be across our southern CWA, though some
chances will exist north across the CWA. It looks like we`ll be
on the north side of this shear axis, so any storms that do
form will likely be moving slowly toward the west!

This shear axis is progged to move away from the area Tuesday
night into Wednesday, but a very warm, humid, moderately
unstable, and nearly uncapped air mass should be in place
Wednesday. While the stronger synoptic forcing should remain to
our north, closer to the main belt of westerlies, we will need
to watch for any MCVs riding east along the southern flanks of
the mid level westerlies. Precise timing/track of MCVs this far
out isn`t possible, but MCV that does affect the area Wednesday
or Wednesday night will have ample moisture and moderate
instability to work with, so the threat would exist for locally
heavy rainfall and perhaps a localized flash flood threat. In
addition, strong to severe storms could be a threat,
particularly where any MCV augments the low-mid level flow
resulting in more favorable shear profiles.

Synoptic upper trough is progged to move across the area
Thursday along with the attendant surface front. Prior
convection could easily end up modulating where the highest
convective chances are Thursday, but the general idea of
increasing shower and thunderstorm chances Thursday looks
reasonable. Any MCV augmented shear could lead to a corridor
with a greater threat for severe weather, especially damaging
winds, somewhere in the region Thursday, but where will
certainly need to be refined in coming days. Non-convectively
disturbed air mass in advance of the front will be very moist
leading to thunderstorms capable of producing torrential
downpours and a continued localized flash flood threat where
storms do occur Thursday.

Temperatures Tuesday through Thursday look to be very warm to
potentially rather hot depending on coverage of storms and/or
possible convective debris. With some sunshine during peak
heating, temperatures should get close to, if not into, the
lower 90s, which when combined with dewpoints in the low to mid
70s would result in peak heat indices nearing 100 degrees. Any
of these days that feature more convective debris or convection
during peak heating will probably see highs more in the mid to
upper 80s with still very humid conditions and heat indices well
into the 90s. Behind the front, Friday looks to be a bit cooler
and noticeably less humid with generally dry conditions
expected. Will need to watch for the potential for increased
beach hazards for Lake Michigan beaches Thursday night into
Friday if guidance depicting the stronger northerly flow in the
wake of the front pan out.

Next weekend, it looks like the front could try and work its
way back northward, but the eventual positioning of the front
will probably be affected by convection. We look to be on the
fringe of the stronger belt of mid-upper level westerlies and
certainly within a broad general that could be affected by an
MCS or two next weekend into early next. The chance pops offered
up by the NBM reflect this threat quite well at this distance.

- Izzi

&&

.AVIATION /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1229 AM CDT Mon Jul 14 2025

Some patchy fog is likely to reduce visibility in spots this
morning, particularly at GYY and DPA. Otherwise, expect light
winds through the TAF cycle which should allow for a lake breeze
to move through ORD, MDW, and GYY this afternoon. Winds will
shift to light east or northeast behind this lake breeze.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory until 9 AM CDT this morning for ILZ032-
     ILZ033-ILZ039.

IN...Dense Fog Advisory until 9 AM CDT /10 AM EDT/ this morning for
     INZ010-INZ011-INZ019.

LM...None.

&&

$$

Visit us at weather.gov/chicago