771
FXUS63 KLOT 101141
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
641 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.

&&

.UPDATE...
Issued at 426 AM CDT Thu Jul 10 2025

Dense fog continues to thicken across areas along and south of
the Kankakee River Valley with visibilities ranging from 1 mile
to less than 1/4 of a mile. Based on current and expected
trends, opted to expand the Dense Fog Advisory to Benton,
Jasper, and Newton counties in northwestern Indiana. Will also
issue an SPS for areas north of the Dense Fog Advisory for
patches of dense fog.

Borchardt

&&

.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 00Z and 06Z
NAM3 runs are 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 focus 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 /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 641 AM CDT Thu Jul 10 2025

Primary forecast concern and challenge is thunderstorm chances
through the period.

In the short term, areas of dense fog south and southwest of the
terminals will steadily lift/dissipate over the next few hours.

Light and variable winds will become northeasterly for the
Chicago terminals this morning and then a lake breeze will allow
winds to become easterly, to perhaps 10kts for ORD/MDW/GYY.
Further inland, winds will turn south/southeast this morning and
then likely turn to south/southwest this afternoon. Light
southeast winds are expected tonight, becoming southwest Friday
morning.

Thunderstorm chances are quite uncertain for the Chicago
terminals through Friday morning. Scattered thunderstorms are
expected across northwest IL this afternoon into this evening
though still some uncertainty for timing and location and thus,
maintained a longer duration prob thunder at RFD, that will need
to be refined as trends emerge. Some of this activity may make
it to the Chicago terminals this evening, but all current trends
would suggest it will be weakening before or as it arrives. For
now, have only included prob shower mention for the the Chicago
terminals. How this convection evolves will likely determine
how much, if any, thunderstorms develop overnight. There is some
guidance showing thunderstorms developing near the IL/WI state
line overnight which could also be a bit further north or a bit
further south, near or at the terminals. Confidence is too low
to include any mention with this forecast but later forecasts
may need to include thunder mention for the Chicago terminals.
Any thunderstorms that do form will have the potential to
produce torrential rain with brief ifr/lifr cigs/vis. cms

&&

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

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

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

LM...None.

&&

$$

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