969
FXUS63 KLOT 111216
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
616 AM CST Thu Dec 11 2025

.KEY MESSAGES...

- A period of accumulating snow this evening into tonight may
  lead to hazardous travel conditions, primarily southwest of a
  Peru, IL to Rensselaer, IN line.

- Another clipper system may produce a few inches of powdery
  snow accumulation in areas mainly south of Interstate 80 on
  Saturday.

- Bitterly cold temperatures and potentially dangerously cold
  wind chills are expected this weekend.

- There is a growing signal for a pattern change toward milder
  (and less snowy) conditions next week.

&&

.DISCUSSION...
Issued at 352 AM CST Thu Dec 11 2025

Through Friday:

Expansive stratus early this morning has caused temperatures to
stall in the 20s across the area early this morning.
Temperatures were accordingly adjusted upward by several degrees
through the morning to account for this. Still can`t rule out
some upper teens in far northeast Illinois near the Wisconsin-
Illinois state line where some breaks in the stratus may still
occur. Continued cloudy skies today and tonight will likely
continue to reduce the influence of the typical diurnal trend,
with temperatures remaining mainly in the 20s through tonight.

Attention in the near term then turns to the potential for
accumulating snow this evening and overnight for portions of the
area. Aloft, broad upper ridging exists across the western
CONUS with a closed upper low spinning over Ontario/Quebec. A
sheared out clipper system currently over northeast Montana is
forecast to dive southeast toward the region between these
features within the mid-upper flow and produce a narrow swath
of f-gen driven accumulating snow. This is currently favored to
occur across Iowa into central Illinois and Indiana but may
extend into northeast Illinois and northwest Indiana. Subtle
variations in the track will have implications as to where any
associated fgen features and accumulating snow axes set up.

While notable differences have existed between the hi-res and
global guidance over the past 24+ hours, 6Z hi-res runs have
begun to trend toward the farther southwest global solutions
which would lead to less snowfall here locally (roughly a trace
to locally 3" amounts far southwest CWA). If the forecast
remains unchanged, a Winter Weather Advisory will likely be
needed for areas along and south of a roughly Peru, IL to
Rensselaer, IN line. However, given the potential for guidance
to trend even farther southwest (limiting impacts here
locally), it felt prudent to hold off one more cycle and let the
day shift get one final look at 12Z guidance this morning.

Will plan to issue an SPS for counties mainly southwest of the
Chicago metro to highlight the potential for hazardous travel
this evening (including part of the commute) and overnight given
temperatures in the 20s would support pavement accumulations
where any snow falls. There also remains a signal for a weaker
secondary fgen band to set up closer to the Chicago metro which
could produce locally up to 1" of snow, so felt comfortable
maintaining at least some accumulations as far northeast as
Chicago.

In the wake of the Thursday night snowfall, some hi-res
guidance does develop a lake effect band over southern Lake
Michigan which may swing back west into northeast Illinois as
the surface flow turns more northeasterly. If this occurs, it
could produce additional localized accumulations up to an inch
near the lakeshore. Meanwhile a trailing weak disturbance will
dive southeast across the area during the day on Friday which
could provide enough lift to produce scattered flurries and
isolated snow showers late morning through early evening leading
to an additional localized dusting.

Petr


Friday night through Wednesday:

The stubborn northwest flow pattern that has been in place for
the past several days will persist through this weekend, sending
a few additional clipper-type systems and steering much colder
temperatures into the Midwest. However, it is appearing
increasingly likely that we will finally be able to break free
from this pattern next week and transition to milder and less
snowy conditions.

Some snow showers or flurries may still be lingering around
come Friday night, but the bigger story will be the arrival of a
cold front denoting the leading edge of a bitterly cold,
modified Arctic air mass. Cold air advection off of brisk west-
northwesterly winds behind this front will push single digit air
temperatures into much of the area prior to daybreak Saturday.
Between these winds not letting up much, cloud cover shrouding
the region, and the snowpack in place over much of the area,
temperatures will then not be able to rise much, if at all,
during the daytime on Saturday. Accordingly, our gridded
forecast currently has Saturday`s high temperatures remaining in
the single digits across our northwestern CWA and in the low-
mid teens elsewhere.

The next shortwave trough in the active clipper wave train will
also be arriving at our longitude during the daytime on
Saturday, bringing along another swath of accumulating snow with
it. Global deterministic and ensemble guidance continues to
trend towards a more southerly positioning of the sharp
low-/mid-level baroclinic zone that this wave will interact
with, which focuses the main frontogenetical response across
central Illinois and Indiana. This would favor the bulk of the
accumulations delivered by the induced snow band remaining south
of the Illinois and Kankakee rivers, and possibly even south of
our forecast area altogether.

The latest NBM PoPs for Saturday may be a bit too high across
our northern CWA based on the latest QPF footprints from global
guidance and the expectation that there will be a pocket of dry
low- to mid-level air limiting saturation within a sizable chunk
of the few kilometers deep dendritic growth zone. However, if
this dry pocket ends up being saturated sufficiently via the
sublimation of the ice crystals descending into it from a higher
mid-/upper-level cloud deck, or if there was even just a slight
shift back northward with the shortwave, baroclinic zone, dry
air pocket, etc., then it wouldn`t be difficult to envision snow
(or flurries) occurring as far north as southern Wisconsin.
Have thus elected to stick with the PoPs output by the NBM for
now. Wherever snow does occur, the cold, dry air mass and deep
dendritic growth zone would support high snow-to-liquid ratios
and a fluffy/powdery character to the snow.

Northwesterly winds will prolong the push of cold air advection
into Saturday night, when it appears that clouds will also
begin to clear out from west to east. The end result will be a
very cold night across the region, with a large portion of our
forecast area likely to see sub-zero low temperatures and
minimum wind chills of 20 below zero or lower. Therefore, a Cold
Weather Advisory will likely be needed for most or all of the
area Saturday night into Sunday morning. The very cold start to
the morning will then limit how much temperatures will be able
to climb during the daytime on Sunday, even beneath abundant
sunshine away from portions of northwest Indiana that may
plagued by lake-induced stratocumulus. The current record daily
low maximum and record low minimum temperatures for December
14th are 5F and -9F for Chicago and 5F and -11F for Rockford,
respectively. Based on our current temperature forecast,
Chicago`s records probably won`t be broken, but it will be a
much closer call for Rockford.

Sunday night will be another very cold night across the area.
There will be less wind as a result of a strong Canadian/Arctic
surface high spreading over the region, so wind chills may not
get as low as they`ll have gotten the prior night. However, the
lighter winds (at least earlier on in the night) may afford
better radiational cooling conditions, so low temperatures
themselves may end up being similar to what they were Saturday
night/Sunday morning.

A long-awaited pattern change then looks to be in store for
next week. While one more clipper system should pass to our
north on Monday, the northwest flow aloft will begin to be
replaced by a more zonal flow pattern early next week. This will
result in a warming trend over the first half of the upcoming
workweek, with temperatures likely to climb above freezing and
possibly even reach the 40s if potential low cloud cover
doesn`t end up being too much of an inhibiting factor.

Ogorek

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 616 AM CST Thu Dec 11 2025

Key Messages:

- MVFR ceilings may occur through a large portion of the TAF
  period.

- There is a chance for snow tonight, though it is increasingly
  likely that the bulk of the snow will remain southwest of the
  terminals.

- There is also a low chance for lake effect snow to affect ORD,
  MDW, and/or GYY late tonight/early tomorrow morning.


Expansive borderline MVFR/VFR stratus remains draped over the
region this morning. It is possible that this deck may clear out
to some extent and/or lift to VFR later today, but confidence
is only low-medium in this occurring. Additionally, as flow
streamlines within the cloud-bearing layer turn more
southerly/southeasterly later today, the stratus is likely to
fill back in over the terminals late this afternoon and evening,
so opted for a more pessimistic outlook for the MVFR conditions
in the 12Z TAFs.

An upper-level disturbance is expected to slice across the
region tonight from northwest to southeast with a band of snow
accompanying it. Model guidance has come into better agreement
that most, if not all, of this snow will remain southwest of the
terminals. Didn`t have enough confidence to pull the snow
mentions from the TAFs entirely yet as it remains possible that
some flurries could still occur at the very least, but ended up
scaling back a bit on the snow mentions from the previous TAFs.
If model and observational trends continue to support this
latest thinking, then more optimistic adjustments may be able to
be made to the going TAFs.

There is also a chance that lake effect snow may affect ORD,
MDW, and GYY late tonight or early tomorrow morning. With winds
over the lake expected to be fairly light during this time,
confidence is fairly low on how the lake effect snow band(s)
will behave and whether they will end up affecting one or more
of these TAF sites. For now, opted to highlight this
possibility in a PROB30 group in the ORD and MDW TAFs.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 7 AM CST this morning for Winthrop
     Harbor IL to Gary IN.

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

&&

$$

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