Berwyn, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

794
FXUS63 KLOT 211113
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
513 AM CST Sun Dec 21 2025

.KEY MESSAGES...

- One more chilly day is in store today before a warm-up begins
  Monday and lasts through the end of the week.

- The upcoming pattern will not be supportive of widespread
  soaking precipitation events in our region. With that said,
  there will be occasional opportunities for light rain and/or
  drizzle, particularly Wednesday PM through Thursday.

&&

.DISCUSSION...
Issued at 230 AM CST Sun Dec 21 2025

Through Monday:

High pressure continues to build into northern IL and northwest
IN this morning and will linger through this evening. As a
result, the current breezy northwest winds will gradually
subside through daybreak before becoming southwesterly with
light speeds (around 5-7 mph) towards midday. Despite the sunny
skies and southerly winds, temperatures this afternoon look to
top out in the low to mid-30s (upper 20s near the IL-WI line)
due to persisting cold advection aloft.

The surface high will begin to move east as another shortwave
trough pivots through the Upper Midwest and northern Great Lakes
tonight. In the wake of the high, a tightening pressure
gradient will allow the south-southwest winds to advect in
warmer temperatures so overnight lows tonight will likely occur
prior to midnight (lows in the low to mid-20s) before
temperatures gradually warm towards the lower 30s by Monday
morning. While there is a signal for some precipitation to
develop along a stalled baroclinic zone in WI, it seems the dry
sub-cloud layer over northern IL and northwest IN will keep
conditions dry tonight. Though, there is a small chance for some
light rain/drizzle to develop across the northern 1/3rd or so
of the area (namely areas along and north of I-88) Monday
afternoon as some better low-level moisture arrives and
interacts with the stout warm advection. Given that moisture
depths are a bit marginal (around 2000-3000 ft deep), confidence
on rain/drizzle occurring is low and thus have just maintained
some silent (10-14%) POPs in the aforementioned area for now.
Regardless, cloud cover will increase tonight with overcast
skies prevailing through the day on Monday. Despite the clouds,
highs on Monday should top out in the upper 30s to lower 40s.

Yack


Monday Night through Saturday:

On Tuesday, a low-level thermal ridge with 850mb temperatures
of +10 to +12C will slide into the Great Lakes. Even as winds
turn northwesterly in the wake of a weak cold front, the
expectation is for high temperatures to rise into the mid to
upper 40s, warmest away from the Wisconsin state line. A surface
high pressure system will meander through the region Tuesday
night, setting the stage for a quiet night with calm winds,
passing upper-level clouds, and overnight lows in the upper 20s
to lower 30s.

Wednesday into Thursday/Christmas, upper-level shortwaves
emanating out of aggregate troughing from the Pacific Coast will
propagate eastward atop a broad baroclinic zone draped across
the Lower Great Lakes. At this point, ensemble model guidance
favors the first shortwave arriving in the Wednesday afternoon
to early Thursday morning, and another sometime in the Thursday
afternoon to early Friday morning timeframe. In both instances,
periods of low clouds, drizzle, and rain showers appear likely
as warm air advection drives isentropic ascent atop the broad
baroclinic zone, as confirmed by probabilities from both the EPS
and GEFS for measurable precipitation now exceeding 60%. So,
will go ahead and increase PoPs beyond the NBM (which suffers
from a dry bias in situations with low QPF), featuring nearly
continuous 15 to 60% chances for drizzle/rain from Wednesday
afternoon through Thursday afternoon.

Temperatures both days will be dependent on the ultimate
arrival time and track of both aforementioned shortwaves, as
each will provide their own period of warm-air advection to
locally "kink" the baroclinic zone northward. With ensemble
model guidance favoring the first shortwave to move directly
over our area Wednesday evening, continue to favor a non-diurnal
temperature curve (e.g., non-NBM guidance) with daytime
temperatures largely in the 30s and 40s areawide and a surge of
lower-50 degree readings occurring overnight into early Thursday
morning. The thinking follows that another surge of warm air
will arrive Thursday afternoon ahead of the next shortwave,
helping drive temperatures into the lower 50s near the Wisconsin
state line to the lower 60s near the IL-9 corridor.

With all of that said, am noting an emerging signal
particularly in the EPS suite for the shortwaves be a bit
stronger, track a hair further to the south, and arrive later
than depicted in GEFS/CMC mean fields. The net results would be
a higher coverage of drizzle and showers from Wednesday night
through all of Thursday as well as little to no (or at least a
delay in the) northward progression of the baroclinic zone. More
specifically, temperatures north of I-80 could end up stalling
in the upper 30s to lower 40s Wednesday night through much of
Thursday, some 10 to 15 degrees lower than currently advertised.
With this in mind, opted to grant influence of the emerging EPS
signal into our temperatures forecast Wednesday through
Christmas day via the manual lowering of temperatures by a few
degrees areawide.

Forecast confidence decreases beyond Christmas owing to
ensemble spread in the treatment as a more formidable upper-
level trough gets shed from the Pacific troughing to our west,
leading to phasing opportunities with northern-stream troughs
emanating from western Canada.

Borchardt

&&

.AVIATION /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 513 AM CST Sun Dec 21 2025

A surface high pressure system will meander through the Lower
Great Lakes region today, facilitating generally light (near
5kt) winds backing from northwest this morning to westerly this
afternoon and then southwesterly this evening. A few sites (such
as RFD) may back toward southeasterly after sunset, as well.

The next storm system anchored by yet another southwesterly
low-level jet will approach the Upper Great Lakes region toward
the end of the TAF period. With flow at 2kft progged to increase
to 45-50 kt, a short (3-4 hour) window of LLWS may materialize
between daybreak and mid-morning before mixing allows for
southerly surface gusts up to about 25 kt. For now, will not
advertise LLWS in the outgoing TAF package for ORD/MDW, but will
keep an eye on model trends for adjustments.

Finally, clear skies today will eventually get covered by mid-
to upper-level clouds tonight.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until noon CST today for Gary to Burns
     Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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