289
FXUS63 KLOT 091045
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
545 AM CDT Wed Jul 9 2025

.KEY MESSAGES...

- Dangerous swimming conditions will develop at Lake Michigan
  beaches Wednesday afternoon into early Thursday, especially
  east of Gary, Indiana.

- There is a low chance (20 to 30%) for isolated thunderstorms
  today, primarily near the lakeshore.

- Chances for showers and thunderstorms return Friday and
  Saturday with the best chance (50-70%) on Saturday. Strong/
  severe storms and heavy rain/localized flooding are possible.

&&

.UPDATE...
Issued at 545 AM CDT Wed Jul 9 2025

Did end up having to hoist a Dense Fog Advisory for much of
northwestern Illinois this morning as visibilities uniformly tanked
to 1/4 to 1/2 of a mile. The expectation is for fog to
gradually erode in the next few hours as the sun angle
increases. The Dense Fog Advisory is in effect through 9 AM CDT.

The leading edge of the backdoor/lake enhanced front is quite
evident in satellite data, and is currently just north of
Sheboygan, Wisconsin. Based on the current speed, the front
should reach Waukegan by 11 AM and Chicago by 1 PM. There is
quite the expansive stratus bank behind the front, so clouds
appear poised to fill quickly across the area this afternoon and
evening.

One final note - would not be surprised to have to add central
Cook and Lake (IL) counties to the Beach Hazards Statement
pending how quickly winds and waves pickup behind the front.
Will let the incoming day shift take a close look at upstream
observations to inform any adjustments to the product during the
scheduled morning update time.

Borchardt

&&

.DISCUSSION...
Issued at 222 AM CDT Wed Jul 9 2025

Through Thursday:

A broad upper-level trough is evident across the Great Lakes
this morning in water vapor imagery atop a very weak/baggy
surface low pressure system centered over Lake Michigan. An
embedded shortwave within the broad trough is forcing an
expansive area of upper-level clouds from Lower Michgian though
Southern Illinois, with the back edge more or less oriented
along I-57 at press time. Meanwhile, a surface high pressure
system is building southward toward Lake Superior along the
backside/inflection point of the upper-level trough.

With nearly calm winds, clearing skies, and pockets of wet soil
from showers and storms yesterday, areas of fog are developing
early this morning. So far, the highest coverage of fog has been
near and west of I-39 (where clearing occurred earliest this
morning). Going forward, would expect fog to continue gradually
expanding and become locally dense with visibility less than 1/2
of a mile. For now, will plan to handle the locally dense fog
with a Special Weather Statement. However, will continue to
evaluate trends to determine if a targeted Dense Fog Advisory
will be needed instead.

After sunrise, any remaining fog should erode. Attention then
turns toward a lake-enhanced front due to race down and inland
from Lake Michigan from late morning through early afternoon.
Based on surface observations across the Upper Peninsula of
Michigan, the backdoor front appears to be approaching the
northern tip of Lake Michigan at press time. High res guidance
appears ubiquitous in showing the front zooming south throughout
the morning and reach northeastern IL in the 10 am to noon
window, and northwestern Indiana in the 11am to 1 pm window. The
front will then spread inland through the remainder of the
afternoon. Thanks to the seasonably warm lake water temperatures
in the upper 60s to lower 70s, the marine airmass won`t be
particularly cool. With that said, highs will range from the mid
80s well inland to the mid 70s lakeside this afternoon.

Of greater interest will be in increase in wave heights behind
the front, with an attendant threat for rip currents
particularly this afternoon through Thursday morning.
Experimental WW3 model data run using winds exclusively from the
NAMNest (which typically performs well in these types of
regimes) shows waves hitting 3 to 4 feet by this evening along
beaches from Gary to Michigan City, Indiana. Considering
conditions will be rather pleasant this morning, would like to
get the message out for rapidly changing conditions this
afternoon. So, will go ahead and issue a Beach Hazards Statement
for Lake and Porter county beaches in northwestern Indiana (and
hold at a "Moderate" swim risk for northeastern Illinois
beaches).

As mentioned in previous forecast discussions, will have to
watch for the isolated to perhaps scattered coverage of showers
and storms along the front from late morning through early
afternoon. Owing to mid-level subsidence in the wake of the
departing upper-level trough, capping is expected to develop
around 600mb by early afternoon. as a result, what should be a
chunky cumulus field along the front may struggle to sprout more
than an isolated storm or two. All things considered, will
maintain the inherited 20 to 30% chances for showers and storms
through the evening hours, with the highest values along the
front.

The southward build of the Great Lakes high pressure system
will set the stage for a quiet night. A few patches of fog may
develop toward daybreak Thursday, mainly south of I-80 (south of
the remnant lake breeze/front). Overnight lows are expected to
fall into the low to mid 60s.

On Thursday, the upper-level pattern will be undergoing a
transition as a series of upper-level shortwaves propagate from
the central US toward the Great Lakes, south of a deeper trough
approaching from the Pacific Northwest. There is a growing
signal for one such shortwave to approach northern IL in the
late afternoon, though will have to think that the instability
gradient positioned well to our west would guide any upstream
convection safely around the perimeter of our area.
Nevertheless, this will be a trend to monitor. Thursday
otherwise looks like a nice day with highs in the low to mid 80s
and partly cloudy skies.

Borchardt


Thursday Night through Tuesday:

Primary forecast remains thunderstorm chances Friday through
Saturday.

There has been little change to the overall pattern/trend for
late week with an upper trough moving across the northern Plains
and upper midwest with a wave ejecting out of the central
Plains with an eventual surface low developing somewhere nearby
or over the Lakes region. There seems to be a bit better
consensus that much of Friday and Friday evening could be dry
for the local area with better chances northwest of the area,
with perhaps some decaying activity late Friday night/Saturday
morning across northwest IL. Then on Saturday when the the main
front moves through and a possible surface low is shifting east
of the area, this would be the best chance of thunderstorms.
Blended pops for Friday evening and Friday night are now high
chance, low likely and these seem too high based on the latest
trends but made no changes. Pops have increased to high chance
and likely on Saturday. Quite a bit of ensemble support for the
Saturday precip timing, especially from the ECMWF.

With timing looking a bit better for Saturday and perhaps early
to mid afternoon with the heating of the day, some stronger
storms are possible along with heavy rain and possible localized
flooding as precipitable water values will be around 2 inches
ahead of the front. Any thunderstorms on Friday and Friday night
would also have the potential to be strong and produce heavy
rain.

High pressure appears to move far enough into the area Saturday
night and Sunday to end the precipitation chances with Monday
also possibly being dry. High temps look to warm back into the
upper 80s/lower 90s Monday and Tuesday. Then by midweek, current
trends would suggest another larger trough/cold front moving
across the region. cms

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1232 AM CDT Wed Jul 9 2025

Mvfr cigs, possible ifr cigs around daybreak.
Ifr/mvfr vis with fog northwest IL this morning.
Wind shift to northeast early this afternoon.
Chance of showers this afternoon.

An isolated shower will remain possible, but the rest of tonight
through late morning appears to be dry. A cold front will move
inland from Lake Michigan early this afternoon, switching winds
to the northeast and providing the potential for a few showers.
An isolated thunderstorm is also possible but current trends
would suggest any precipitation later today will be isolated
and current prob shower mention seems on track.

Light south/southwest winds will turn westerly or northwesterly
this morning, then shift northeast with the cold front. There
may be a short period where gusts into the 20kt are possible
with and behind the front. East/northeast winds will then
slowly diminish into this evening.

A few locations are reporting mvfr vis with fog/mist currently.
Fog is most favored across northwest IL and the Fox Valley. Have
included tempo mention at RFD/DPA for now. Guidance is also now
showing the potential for ifr cigs across northwest IL and mvfr
cigs across much of the rest of the area, around daybreak this
morning, which may limit how widespread any fog becomes. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory until 9 AM CDT this morning for ILZ003-
     ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-ILZ012-ILZ019-ILZ020-
     ILZ021-ILZ032.

IN...Beach Hazards Statement from 1 PM CDT this afternoon through
     Thursday morning for INZ001-INZ002.

LM...None.

&&

$$

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