279
FXUS63 KLOT 031145
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
645 AM CDT Wed Sep 3 2025

.KEY MESSAGES...

- Period of rain showers expected with a strong cold front this
  afternoon and early evening. A few embedded non-severe
  thunderstorms are also possible.

- Another period of rain showers and a brief period of strong
  winds will be possible Thursday night into Friday morning as
  another cold front passes through the area.

- Below to well below normal temperatures are expected from
  Thursday through this weekend.

&&

.DISCUSSION...
Issued at 332 AM CDT Wed Sep 3 2025

Through Tonight:

Outside from a few widely scattered warm air advection induced
showers ongoing early this morning across far northwestern IL
into southern WI, all is quiet across the area. This will change
in a big way as we head through the day, however, as a strong
cold front currently across southeastern MN quickly sweeps
southward through the area. The main weather story with this
front will be the turn towards some rather unseasonably chilly
temperatures tonight through the remainder of the week. However,
as we transition into this much cooler airmass today, shower
chances (along with some non-severe thunderstorms) will ramp up
with and behind this front as increased forced ascent along the
tightening lower-level baroclinic zone coincides with good deep
layer moisture. The best timing for showers and storms will come
mid to late this morning through early afternoon across far
northern and northwestern parts of IL, then spread across the
Chicago metro area and points south and east through the
afternoon and early evening hours. The rain should cutoff fairly
quickly from northwest to southeast late this afternoon into
this evening, with all of our area looking to be dry prior to
midnight.

Winds in advance of the quickly approaching cold front will
become breezy from the west-southwest later this morning, then
shift northwesterly with the frontal passage this afternoon.
Temperatures will be the warmest today south of I-80, where the
rain and thicker cloud cover will hold off until later in the
day. In these areas, temperatures are expected to warm well into
the 70s to around 80. Farther north, however, expect readings
to fall into the 60s this afternoon as the cooler airmass begins
to work southward with the showers. While the thermodynamic
environment today will not be supportive of severe
thunderstorms, it still appears that lapse rates will be
sufficient to support some instances of charge separation in the
clouds and hence the presence of isolated to scattered
thunderstorms embedded within the area of showers. Some locally
moderate to heavy downpours may accompany this activity, but
with a rather fast southward pace of the frontal zone, total
precipitation amounts today should be less than an inch in most
areas.

Robust cold air advection in the wake of the cold front,
especially after dark, will drive a notably chilly airmass into
the region by early Thursday. Low temperatures are currently
forecast to fall into the lower to mid 40s across interior
northern Illinois, with mid 40s to around 50F elsewhere (low-mid
50s downtown Chicago and Indiana shore). In addition, brisk
northwesterly winds on the lake will result in hazardous
conditions (large waves and strong currents) developing and
lingering into Thursday for the Indiana shore in particular.

KJB


Thursday through Tuesday:

In the wake of today`s cold frontal passage and the arrival of
an upper-level trough of polar origin into the Upper Midwest, an
unseasonably cool air mass will settle into the region for the
latter half of the week. While plenty of sunshine is expected on
Thursday, the cool start to the day (early morning temperatures
in the 40s and low 50s) will limit just how warm we`ll be able
to get, and daytime high temperatures are accordingly forecasted
to range from the mid 60s to low 70s.

Another pronounced upper-level trough located over the Yukon
this morning will largely retrace the footsteps of the leading
polar trough and dive southeastward into Great Lakes late
Thursday into Friday. The strong dynamics associated with this
disturbance will induce another round of precipitation across
the Midwest as it moves through, though there is fairly good
agreement amongst the latest ensemble and deterministic guidance
that the bulk of this precipitation will remain to our north in
Wisconsin. Still, the southern tail of the precipitation shield
should extend far enough south ahead of an inbound reinforcing
cold front for at least some of our northern locales to see a
period of rain showers Thursday night before the cold front
clears the area Friday morning.

Winds will become gusty Thursday night into Friday as low-level
pressure gradients tighten in response to a surface low passing
to our north. Initially out of the southwest, winds will veer
westerly behind the front and likely gust above 25 mph with
regularity during the daytime on Friday in the steep low-level
lapse rate/cold air advection regime behind the front. An
examination of GFS forecast sounding evolution in BUFKIT
suggests that there could also be a brief "pop" of much stronger
winds (potentially in excess of 40 mph) immediately ahead of
and/or behind the front towards daybreak Friday, though wind
gusts of this caliber coming to fruition will likely require 1.)
the aforementioned surface low tracking farther south across
Wisconsin than most other guidance depicts, and 2.) any rain that
falls to not suppress mixing heights appreciably. High
temperatures on Friday look to largely depend on the extent of
post-frontal stratocumulus and how easily it will be able to be
scoured out, but at this time, we favor Friday`s high temperatures
to more or less be in the same ballpark as Thursday`s highs.

Behind this reinforcing disturbance, surface high pressure
looks to settle back into the region over the weekend, favoring
another period of dry conditions with continued below normal
(though gradually warming) temperatures. Ensemble consistency
regarding the overall synoptic pattern evolution then breaks
down a bit going into next week, though there does appear to be
increasing support for the warming trend to continue through at
least Tuesday.

Ogorek

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 645 AM CDT Wed Sep 3 2025

Key Messages:

- A period of showers is expected today as a cold front drops
  through the area. A few embedded thunderstorms are expected to
  occur as well.


A cold front will drop southeastward through the area today,
bringing a period of showers with a few embedded thunderstorms
to the terminals. While a brief sprinkle could be seen prior to
14Z this morning, the main time window for the bulk of the
shower activity will come this afternoon at the Chicago metro
terminals and during the late morning and early afternoon at
RFD. Thunderstorm coverage still looks to be scattered at best,
so have maintained the going PROB30 groups for TSRA. While VFR
conditions are generally favored to prevail during the main time
window for showers, the most robust showers or any embedded
storms will be capable of knocking visibilities down to MVFR or
IFR levels. Could also see a brief period of MVFR ceilings as
well during and after the steadiest rainfall, but otherwise,
cloud bases are likely to remain above 3000 ft AGL.

Southwesterly winds will also become increasingly gusty out
ahead of the cold front this morning before turning westerly,
then northwesterly behind the front later on in the day. Gusts
into the 20-25 kt range will be possible at times from mid-
morning through this evening.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Beach Hazards Statement from this evening through Thursday
     afternoon for INZ001-INZ002.

LM...Small Craft Advisory from 7 PM this evening to 3 AM CDT
     Thursday for Northerly Is. to Gary IN.

     Small Craft Advisory from 7 PM this evening to 3 PM CDT
     Thursday for Gary to Burns Harbor IN-Burns Harbor to
     Michigan City IN.

&&

$$

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