Chicago, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

018
FXUS63 KLOT 171127
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
527 AM CST Wed Dec 17 2025

.KEY MESSAGES...

- Mild temperatures through Thursday should melt much of the
  remaining snow cover.

- Rain showers and drizzle will develop late tonight with a
  period of steady rain ahead with a strong cold front on
  Thursday.

- Blustery and briefly colder conditions Thursday night into
  Friday will be followed by variable but generally above normal
  temperatures into next week.

&&

.DISCUSSION...
Issued at 321 AM CST Wed Dec 17 2025

Through Thursday night:

A shortwave trough is in the process of swinging through the
region early this morning. We managed to wring out some spotty
sprinkles/light showers at the office a bit earlier from a 15
kft cloud deck. Given modeled ascent increasing through
daybreak, have added some sprinkle wording to the grids roughly
south of I-80. Thankfully, temperatures remain much warmer than
previously thought, and are currently solidly above freezing.
Seeing a few road temperature sensors still hovering around the
32 F mark, but given the light nature of things and warmer
ambient conditions, don`t have significant icing concerns before
the column rapidly dries out through 5-6 AM.

Upstream satellite trends reveal post-frontal stratus is thinning
across central Wisconsin. As a result, it looks like we`ll
probably clear out here this morning towards midday. Forecast
soundings suggests we`ll probably attempt to mix into at least
some spotty lower stratus/stratocu this afternoon, but it
remains a bit unclear just how widespread this will become.
Continue to advertise general partly cloudy/partly sunny wording
this afternoon as a result.

Dry conditions will continue through this evening, but cloud cover
will thicken up ahead of our next robust storm system. Model
guidance continues to indicate rapid saturation taking place under
700 mb during the 3 to 6 AM timeframe. This will likely manifest
as a quick blossoming of showers and drizzle across much of the
area, and PoPs commensurately increase during this time frame.
Wetbulb temperatures will be solidly above freezing, and even
pushing above 40 degrees, as precipitation develops, so no
concerns with slick spots at this point. Widespread rain/showers
will continue to expand across the region Thursday morning.
Given the dynamics at play, wouldn`t be totally surprised to see
a lightning flash approach our southern CWA mid-morning towards
midday. Any low-level instability of note appears like it`ll
remain decidedly south of the area.

An initial front/pre-frontal trough will push through the area
Thursday afternoon sending temperatures falling quickly into the
mid to upper 30s. Have worked to sharpen up this non-diurnal
trend a bit with the NBM-delivered grids not quite capturing
this sharp gradient. While we can`t totally rule out some snow
mixing in, the sharp nature of this boundary suggests that
precipitation will come to an end prior to the column cooling
enough to support a widespread changeover to snow.

The true push of deep cold advection will lag this initial
boundary a bit, but will arrive decidedly early Thursday
evening and overnight with temperatures plunging through the
teens. As the main blast of cold air arrives, saturation through
a thickening DGZ is forecast to occur. This setup looks
favorable for continued intermittent gusty snow showers and
widespread flurries through Thursday night. Continuing to keep
an eye on the Thursday evening timeframe as some guidance
(mainly the NAM) suggests a brief window for a potentially more
squall-like snow showers with notably steeper low-level lapse
rates compared to the rest of the guidance suite, but note that
this remains an outlier solution. Regardless, localized dustings
of snow will be possible before activity diminishes late
overnight into Friday morning. Wind chills by early Friday AM
will fall into the +5 to -5 range.

Regarding winds: a substantial southerly LLJ will develop on
Thursday, but not seeing any signs that this flow will manage to
meaningfully work its way to the surface during most of the
daytime hours. Once the main push of CAA arrives, steepening
low-level lapse rates will allow for some deeper mixing, likely
resulting in a period of 35 to 40 mph wind gusts before the
strongest flow weakens towards daybreak Friday.

Carlaw


Friday through Tuesday:

A weakening mid-level ridge and associated surface ridge will
cross the forecast area through the day on Friday. Stratus with
some flurries (cloud layer in the DGZ) trapped under a strong
subsidence inversion late Thursday night may linger into Friday
morning, but substantial low-level dry air advection on gusty
northwest winds in the morning should erode remaining clouds
from west to east before the ridge arrives late in the
afternoon. After highs in the mid to possibly upper 20s Friday
afternoon, increasing WAA amid weak low-level stability and a
40+ knot LLJ spreading over the area will likely result in
steady or even rising temps Friday night.

A low-amplitude wave will then cross far western Ontario on
Saturday while keeping any appreciable forcing well north of the
area. A brief warmup into the 40s and a ribbon of very light
rain may precede a passing cold front Saturday afternoon and
evening. Colder conditions with highs in the upper 20s and
mostly sunny skies are expected Sunday as another high pressure
crosses the region.

Moisture return ahead of the next storm system passing well to
the north Monday into Tuesday appears to be insufficient for
widespread precip over the area before a weak cold front passes
through on Tuesday. Looking ahead to Christmas Eve and
Christmas, a rapid return of WAA over the central CONUS is on
track to bring unseasonably warm conditions over the area. With
a long trajectory of moisture from the Atlantic and Gulf,
abundant moisture surging over the region will likely bring
periods of showers/drizzle and plenty of stratus.

Kluber

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 527 AM CST Wed Dec 17 2025

A strong inversion between 1-2kft will remain in place over
much of the region through tonight. Under this inversion,
shallow MVFR stratus from southern MN to southern Lake MI has
struggled to push southward during the early morning hours.
However, daytime mixing from filtered sun through batches of
cirrus may be sufficient to yield FEW/SCT shallow low-end MVFR
stratus under the inversion later this morning into this
afternoon. If these clouds do not materialize, VFR conditions
can be expected through at least this evening.

WNW winds up to around 10 knots at TAF issuance will become VRB
under 5 knots late morning into early afternoon as a surface
ridge crosses the area. Winds will then settle SE/SSE around 10
knots by late afternoon. A 50-60 knot low-level will then
develop within the inversion this evening. Given the strong low-
level stability in place, surface gusts may struggle to develop
until very late this evening or even overnight. Have therefore
included LLWS at 50 knots down as low as 1.2kft in the event
surface gusts do not occur or are only sporadic. Winds will
eventually shift just west of south overnight, with gusts of 25
knots or higher likely becoming more prevalent as low-level
stability weakens.

The low-level jet will produce a surge of moisture over the
area late tonight as deep-layer forcing increases. This will
result in rapid saturation and a quick transition from VFR to
IFR ceilings and SHRADZ in just an hour or two (generally 09-11Z
for ORD/MDW). These conditions should continue through much of
the morning, with a cold front shifting winds SW, ending precip,
and quickly lifting/scattered ceilings by early afternoon.

Kluber

&&

.MARINE...
Issued at 407 AM CST Wed Dec 17 2025

A strong area of low pressure near 29.1 inches will move across
the Great Lakes region on Thursday. In advance of this system,
southerly winds will increase this evening over the Illinois
and Indiana nearshore waters. While winds are forecast to
increase to near 50 kts at 1500 ft very late tonight and into
Thursday morning, strong stability over the lake is expected to
limit much of this flow from reaching the surface, curtailing
the frequency of 35 kt gales. A Small Craft Advisory is in
effect from 9 PM this evening through 6 PM Thursday.

As the surface low moves across Lake Superior, a cold front
will shift winds out of the west Thursday evening and overnight.
The potential for 35 to 40 kt gales will increase during this
period, although some uncertainty in the frequency of the
strongest winds remains. As a result, a Gale Watch has been
issued and is in effect from 6 PM Thursday through 6 AM Friday.

Winds will diminish through late Friday afternoon as an area of
high pressure briefly builds across the southern portion of
Lake Michigan.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 9 PM this evening to 6 PM CST
     Thursday for the IL and IN nearshore waters.

     Gale Watch from Thursday evening through late Thursday night
     for the IL and IN nearshore waters.

&&

$$

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