355
FXUS63 KLOT 100750
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
250 AM CDT Thu Jul 10 2025

.KEY MESSAGES...

- Patchy dense fog will continue mainly south of Interstate 80
  through mid-morning.

- Dangerous swimming conditions will continue at Lake and Porter
  county Indiana Lake Michigan beaches through mid-morning.

- Several waves of showers and storms are possible from this
  afternoon through Saturday across the area. Some storms may be
  severe with damaging winds and flash flooding.

- Provided low coverage of thunderstorms, Friday may be hot and
  humid with heat indices near 100 degrees.


&&

.DISCUSSION...
Issued at 250 AM CDT Thu Jul 10 2025

Through Friday:

Through Mid-Morning:

A baggy surface pressure gradient has once again allowed for
patchy dense fog to develop early this morning. This time
around, the fog is focused along and south of the Kankakee River
Valley where mid-level clouds are less prevalent. A Dense Fog
Advisory is in effect for Livingston, Ford, Iroquois, and
Kankakee Counties until 9 AM this morning. As fog evolves,
counties may be added to the Dense Fog Advisory through the
next few hours.

Meanwhile, waves continue to gradually subside along Lake
Michigan beaches. Will maintain the inherited Beach Hazards
statement for Lake and Porter counties in Indiana through 10Z
given the northerly flow trajectory. Though, will cancel the
statement for Illinois locations given lower waves.

Borchardt

Mid-Morning Onward:

The main forecast concern from mid-morning onward is the threat
for rounds of thunderstorms lasting through at least Friday.
The overarching pattern will be defined by weakly-defined upper-
level shortwaves parading along a diffuse baroclinic zone
draped across the Lower Great Lakes. Such patterns can be
unforgiving to forecast for basically all variables
(temperature, cloud cover, QPF, etc) with dependencies on and
cascading consequences from episodic thunderstorm clusters
parading within the baroclinic zone. While confidence is high
that our general region and indeed parts of our local area will
see episodes of storms over the coming days, confidence is
notably lower on pinpointing the exact locations and favored
time windows.

Early morning radar and water vapor imagery shows broken lines
of thunderstorms from western Minnesota southwestward through
southeastern Nebraska. An MCV is becoming quite apparent behind
one line segment moving along the Iowa/Minnesota border, with
recent radar imagery from KFSD indicating nearly 50 to 60kt of
flow at 10kft on the western side of the circulation (i.e.,
impressively strong). As the morning progresses, the expectation
is for thunderstorms to continue developing across central and
eventually eastern Iowa along eastward-suring outflows and along
the southeastern influence of the eastward-drifting MCV. As
consolidated outflow crosses the Mississippi River by late
morning, thunderstorms may continue to fester (if not
reintensify given the warming boundary layer), and reach the
I-39 corridor by early afternoon. Should this occur, DCAPE of
850-1000 J/kg and PWATs of 1.5" may support torrential downpours
and gusty winds in the strongest storms. With instability
decreasing with eastward extent, coverage and intensity of
storms may tend to wane as they approach Lake Michigan this
afternoon. Will carry mid-range (30 to 40%) chances for storms
from mid-morning to mid-afternoon for this potential, focused
mainly along and north of I-80.

Another round of showers and storms may develop late this
afternoon through the evening hours across northwestern Illinois
as the aforementioned MCV moves toward southwestern Wisconsin.
Now, the intensity of the storms during the second round appears
dependent on (1) earlier shower and storm coverage being at a
minimum to preserve instability; and (2) the strength of the MCV
maintaining integrity to provide enhanced veering 850 to 500mb
flow across eastern Iowa and northwestern Illinois. If both
dependencies are realized, have quite a bit of concern that
there could be a focused area with a threat for damaging winds
and even tornadoes northwest of the I-39 and I-80 corridors back
into northeastern Iowa and southwestern Wisconsin. The NAM3 is
probably overly aggressive in decaying early afternoon storms
and beefing up the strength of the MCV, but does provide a
glimpse at a reasonable worst case scenario with a string of
supercells moving into northwestern IL this evening. The more
favored scenario appears to be for just another round of gusty
and torrential-rain producing thunderstorms this evening across
northwestern IL. Of course, trends will need to be monitored closely
as the MCV approaches.

Tonight, the MCV should track across southern Wisconsin
providing a focus for showers and storms through the overnight
hours. Coverage of showers and storms may end up highest just
north of the state line, though feel anyone along and north of
I-80 will be within striking distance. With a southwesterly
low-level jet expected to develop in the mid-Mississippi River
Valley overnight, will have to watch for any signs of
backbuilding to provide a threat for flash flooding somewhere in
general region. With that said, the orientation of the low-
level jet and approach of another shortwave may instead a
separate area of storms firing across Iowa, disrupting moisture
transport into the Lower Great Lakes. All things considered,
will advertise mid-range chance PoPs (30-50%) for showers and
storms during the overnight hours, highest near the Wisconsin
state line.

On Friday, subsidence in the wake of the departing MCV should
promote a period of dry weather at least through the first half
of the day. Confidence in a dry forecast lowers substantially by
the afternoon as another shortwave (or several) track
northeastward toward the Lower Great Lakes. It`s easy to
envision scenarios where the afternoon is completely dry or
completely stormy depending on the arrival time of the waves.
With PWATs nearing 1.75" and hints of enhanced midlevel flow
provided by each shortwave, any storm in the general region Friday
could be severe with damaging winds and torrential downpours.
For now, will carry low-end (20-30%) chances for storms through
the entire day keeping in mind they can essentially vanish or
double as we get closer. Note that if storms are at a minimum
Friday afternoon, the combination of heat and humidity will make
it feel like 100 degrees.

Borchardt

Friday Night through Wednesday:

Thunderstorms may be ongoing Friday evening, especially across
the northwest cwa but overall confidence is low for timing as
each potential wave of thunderstorms is dependent on the
previous. If thunderstorms are not ongoing or develop Friday
evening, that may lead to a better chance of thunderstorms late
Friday night into early Saturday morning. The main cold front
will then move across the region Saturday afternoon and this
still appears to be the best chance of thunderstorms for the
entire area. Precipitable water values are generally in the 1.5
inch range Friday night, increasing to around 2 inches Saturday,
allowing for heavy rain and localized flooding. Dewpoints may
pool in the mid 70s ahead of the front on Saturday, allowing for
even more available moisture. Thunderstorm evolution Friday
night into Saturday morning will determine where storms may form
on Saturday, but current trends would suggest that the best
chance of strong/severe would be along/east of I-55, likely
moving east of the area by early Saturday evening.

Sunday is still looking dry for most of the area, but the GFS
has been showing a wave lifting through central IL the past
few days and as a result, there are now blended low chance pops
through I-80 or so and maintained those for now. Monday is
looking dry but by Tuesday, the airmass is again quite moist and
given how uncertain the pattern has become, low chance pops
seem reasonable. Another stronger cold front is expected to move
across the region midweek, likely in the Wednesday time period
with a chance of thunderstorms, possibly strong/severe along
with heavy rain/localized flooding. Quite a bit of ensemble
support for at least a few dry days, near normal temps and lower
humidity for late next week. cms

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1230 AM CDT Thu Jul 10 2025

Forecast concerns include...

Chance of thunderstorms this afternoon through tonight.
Potential for fog early this morning, mainly south of the tafs.

Fog is beginning to form across areas south of the terminals
early this morning and is expected to continue through daybreak
and then steadily lift/dissipate. While fog is still possible
across northwest IL and at RFD, most recent guidance trends
suggest the fog may be further south. Maintained current tempo
mention for now and trends will need to be monitored.

Light easterly winds will continue through mid morning and then
a lake breeze is expected to move inland this afternoon, with
east/northeast winds for the Chicago terminals. Further inland,
winds will likely become light south/southeast and then turn
more to the south/southwest this afternoon.

Scattered thunderstorms are expected to develop over eastern IA
and northwest IL this afternoon and possibly persist into this
evening. Expanded prob timing at RFD and its possible tempo
mention may eventually be needed. Some of this activity may move
further east toward the Chicago terminals in the early/mid
evening hours, but confidence is very low and will maintain a
dry forecast for now. How this convection evolves will play a
role into the potential for additional scattered thunderstorms
early Friday morning. If these materialize, its possible they
would be right over northern IL and the terminals. Confidence
remains too low for this time period as well. However, later
forecasts may end up with a few periods of thunder for the
Chicago terminals. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory until 9 AM CDT this morning for ILZ023-
     ILZ032-ILZ033-ILZ039.

IN...Beach Hazards Statement until 10 AM CDT this morning for
     INZ001-INZ002.

LM...None.

&&

$$

Visit us at weather.gov/chicago