Chicago, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

829
FXUS63 KLOT 202020 CCA
AFDLOT

Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
220 PM CST Sat Dec 20 2025

.KEY MESSAGES...

- After a see-saw in temperatures this weekend, a warming trend
  will take hold next week and peak on Christmas with high
  temperatures potentially in the 50s to locally lower 60s.

- 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 into Thursday.

&&

.DISCUSSION...
Issued at 216 PM CST Sat Dec 20 2025

Through Sunday:

A cold front will move across the area this afternoon and there
remains a small chance for a few sprinkles or flurries through
early evening, when the cloud cover clears out. Mostly clear
skies are then expected overnight with mostly sunny skies
Sunday.

After highs in the upper 30s north and lower 40s south, temps
will slowly fall into the 20s this evening with lows in the
lower teens north to around 20 south by Sunday morning. The air
behind this front is not too cold and with sun expected Sunday,
its possible high temps might rebound back to around 30 north
to lower 30s south.

Winds have diminished some this afternoon but periodic gusts
will remain possible. Winds are expected to increase again this
evening when a short period of gusts into the 30-35 mph range
will be possible, especially across northern IL. Winds will then
diminish overnight with light winds expected on Sunday as high
pressure moves across the area. cms

Sunday Night through Saturday:

On Sunday evening, surface high pressure just to the east,
mostly clear skies, and temporarily calm winds will result in
temperatures briefing to the upper teens to lower 20s (mid 20s
in Chicago). This will be followed by rising temperatures
overnight from increasing southerly winds as the surface high
slides farther east, along with mid and high level overcast.

The advertised pattern change will commence on Monday as
anomalous blocking south of Alaska generates aggregate troughing
across the Pacific Coast and broad quasi-zonal flow along the
US/Canadian border. With our area expected to be positioned
south of the energetic jet stream, we should largely escape
opportunities for widespread precipitation events and instead
have the opportunity to enjoy a period of above-average
temperatures. Confidence is high in well above normal
temperature departures for the upcoming stretch, but much of it
will likely be cloudy.

On Monday, strong low-level warm air advection on the backside
of the departing high pressure system and ahead of an upper-
level wave propagating along the US/Canadian border will
facilitate 850mb temperatures rising some 8 to 10C/24 hours to
+2C or so into the Lower Great Lakes. So, in spite of expected
widespread mid to upper-level cloud cover, do expect
temperatures Monday afternoon to warm into the upper 30s to low
40sF (locally mid 40s south). A wedge of dry low-level air left
in the wake of the high pressure system should prevent any
widespread precipitation falling from mid-level echoes that
develop within the persistent the warm air advection/isentropic
ascent regime. Can`t completely rule out flurries or even brief
ice pellets near the WI state line early to mid Monday morning.

A large bank of low stratus should be drawn northward later
Monday-Monday night by continued southwesterly low-level warm
and moist advection. If the low-level saturation depths are
sufficiently deep, there`s a play for patchy drizzle
development, but there`s not enough overall guidance support for
explicit mention in the gridded forecast. The clouds and
southerly winds Monday night will prevent temperatures from
dropping much, if at all. A weak cold front will move across the
area on Tuesday. Despite mid-level subsidence as well as dry
air advection noted down to the 925-850 mb level in the wake of
the frontal passage, there`s reason for skepticism that an
existing stratus deck will be readily eroded. Given the complete
lack of cold air advection and the magnitude of warmth aloft
(850 mb temps very mild at +10 to +12C), the low-level inversion
will be very steep and moisture (ie. stratus based below
1.5-2kft AGL) may very well become trapped.

While there is still a decent signal for a ~24 hour scouring out
of low clouds, the global guidance doesn`t often handle these
regimes well during the lowest sun angle time of year, so
trended a bit more pessimistic with the sky cover forecast.
There will be bust potential either way depending on how the
cloud cover plays out, though either way, temperatures will be
solidly above normal. The presence (or lack thereof) of low
clouds will then also modulate Tuesday night`s lows given light
winds with surface high pressure overhead. With a lean toward
the clouds hanging on, nudged Tuesday night temps a bit milder
than previous forecasts.

A weak surface trough extending toward the mid-MS Valley will
gradually draw a warm front northward on Wednesday. With this
progged pattern and overcast skies, expect a non-diurnal
temperature trend through Wednesday night, which will result in
temps only slowly rising into the 40s prior to sunset on
Christmas Eve. The true unseasonably mild and moist air mass
will not be ushered north until after dark and there`s also
uncertainty on how far north the warm front is able to progress.

With increasing low-level saturation depths underneath
lingering mid-level dry air, the isentropic ascent regime
Wednesday afternoon and night will likely be supportive of
periods of light rain/showers and/or drizzle (~40-60% PoPs).
It`s unlikely to be a washout, though. The drizzly regime may
even be prolonged into Christmas Day as dew points rise well
into the 40s and even 50s. There was a considerable increase in
EPS (ECMWF ensemble) membership depicting measurable QPF into
Thursday, and as such, added in a chance of light drizzle
mention. If this comes to fruition, temperatures likely won`t be
quite as warm as they`d be without low clouds and drizzle, but
dew point values should still push high enough to result in
highs from the low 50s to locally lower 60s (warmest far south).
This would give it a shot as ending up as a top-5 warmest
Christmas Day at Chicago and Rockford.

Even in the wake of another weak cold front passage Thursday
night into Friday, the positioning of the jet stream north of our area
supports a continuation of above average temperatures into the
weekend. There may be a brief cool-down back closer to seasonable
in the Sunday-Monday timeframe (beyond current day 7/Sat 12/27),
but mild conditions should again return heading towards New
Year`s Eve and New Year`s Day 2026.

Castro/Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1122 AM CST Sat Dec 20 2025

Key messages for the 18Z TAF cycle:

* A marginal LLWS setup this evening

Westerly winds will gust to around 20 kt through the afternoon,
becoming less frequent with time. As a low level jet builds
down toward the surface, W gusts will open up for the evening to
25 to 30 kt, occasionally higher. 45 to 50 kt of flow will be
found at 2kft AGL, but given the expected gustiness at the
surface, decided to omit LLWS from the TAFs. Winds will
gradually step down during the overnight. Beyond early Sunday
morning, expect largely sub-10 kt flow out of the W, backing to
SSW for the afternoon.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 4 AM CST Sunday for Winthrop Harbor
     IL to Gary IN.

     Small Craft Advisory until noon CST Sunday for Gary to Burns
     Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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