150
FXUS63 KLOT 130012
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
612 PM CST Fri Dec 12 2025

.KEY MESSAGES...

- Accumulating snow is expected for much of the area on
  Saturday, with several inches of accumulation possible south
  of Interstate 80.

- Very cold temperatures and bitterly cold wind chills are
  expected this weekend with wind chills below zero.

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

&&

.DISCUSSION...
Issued at 305 PM CST Fri Dec 12 2025

Through Sunday night:

An active, wintry period of weather is expected to affect the
forecast area through the weekend, with accumulating snow
Saturday and very cold temperatures and bitterly cold sub-zero
wind chills.

The upper-level pattern continues to feature broad troughing
across eastern North America, downstream of an upper ridge along
the west coast. An upper level low was also evident over
central Manitoba, north of a 150+ kt 300 mb jet max translating
from the Canadian Rockies to the Northern Plains. This jet
streak was embedded within the main axis of the jet stream,
which extends along the southern periphery of the upper trough
into the Midwest and mid-Atlantic regions. The evolution of
these features and their surface reflections over the next 60
hours will be the main drivers of our period of wintry and
colder weather through the weekend.

In the near term, a more subtle mid-level short wave was
analyzed at midday within the fast northwest flow aloft across
MN/IA. This is accompanied at the surface by a strengthening
cold frontal trough stretching from western WI into IA. 12Z
RAOBS from MPX and TOP indicate shallow low-level moisture
pooling ahead of the front, with surface dew points in the
low-20s. Forecast soundings indicate somewhat deeper moisture
farther to the north, where regional radar mosaic depicts an
area of light snow/snow showers across southeast MN/western WI.
Forecast shallower moister depth into northern IL suggests
mainly flurries or spotty light snow showers here toward
evening, though shallower moisture also raises the question of
lack of cloud ice at onset (moist layer >10C) which could
support some low-confidence brief, very light patchy freezing
drizzle. WFO ARX has noted some of this in their area late this
morning. Eventually by early this evening the column cools
behind the cold front such that cloud ice is likely (and present
in thickening mid-cloud deck which may provide feeder-seeder
for snow flakes), which should support just flurries. Intrusion
of arctic air behind the cold front tonight will send overnight
temperatures into the single digits and low teens across the
area and wind chills in the negative single digits and teens by
morning.

By late tonight/early Saturday, a stronger mid-level short wave
(currently coming ashore over British Columbia) is forecast to
track quickly southeast across the Northern Plains. Warm
advection develops quickly into the region in advance of this
disturbance Saturday morning, impinging on a fairly strong west-
east oriented baroclinic zone from the IA/MO border region into
central IL/IN, and beneath the divergent right entrance of the
upper level jet across northern IL/IN/LM/MI. Guidance is in
generally good agreement in depicting snow blossoming into the
forecast area from the west during the morning and persisting
through the afternoon before ending, with the primary axis of
highest accumulations (5-6") along and south of the WFO LOT/ILX
border. Impressively deep DGZ noted in forecast soundings,
though the strongest forcing for ascent appears to be focused
near/above the top of this layer. Given the cold column behind
this evening`s cold front, SLRs approaching and even exceeding
20:1 are likely, supporting a low-density, fluffy snow. This
looks to yield amounts in the 3-5" range across our southern
tier or two of counties (locally higher along the ILX border),
tapering off into the 1-3" range farther north into the I-80
corridor and lesser amounts north of that. With the high ratios
however, can`t rule out some slightly higher amounts up through
the I-88 corridor especially if any (even weaker)
frontogenetically driven banding develops that far north. Based
on these expectations, have elected to issue a Winter Weather
Advisory for the southern tier of LOT counties (included LaSalle
but mainly for the far southern part) from 8 am through 8 pm
Saturday, for hazardous travel conditions with the accumulating
snow.

As snow ends late Saturday/early Saturday evening, very cold
temperatures and bitterly cold wind chills become the main
story. Persistent advection of cold arctic air combined with
10-15 mph winds is expected to produce wind chills as low as 20
to 25 below across most of the forecast area overnight and into
Sunday morning. Therefore have also issued a Cold Weather
Advisory for these wind chills for all but Porter county from 8
pm Saturday evening through noon Sunday. Overnight air
temperatures will sink into the negative single digits, and will
only recover into the positive single digits midday/afternoon
on Sunday despite sunshine. For Porter county, a slight wind
component off the lake and lake effect clouds are expected to
keep readings a little higher and wind chills above advisory
criteria. A period of lake effect snow showers is also likely
for northern Porter county Saturday night into Sunday morning,
with some additional accumulations possible there.

Winds will ease Sunday afternoon and evening as strong arctic
surface high pressure builds into the region. Temperatures will
likely tank after sunset with the ridge axis over the area, then
become steady or slowly rise somewhat overnight as the ridge
shifts east and south-southwest winds develop with associated
warm advection.

Ratzer

Monday through Friday:

No changes to previous discussion.

Strengthening warm advection on Monday will help send
temperatures back into the teens and 20s. Thankfully, not seeing
any signs of this intensifying WAA and attendant ascent driving
any meaningful mid-level saturation, with precip-free
conditions to start the week.

Medium range guidance, to varying degrees, depicts a low-
amplitude disturbance in the general region Tuesday night into
Wednesday. Intensifying southwesterly flow may help drive an
increase in low-level moisture, with some guidance (GFS and CMC
in particular) looking like they want to squeeze out some
drizzle at times. At this range, this signal remains a bit
nebulous, but air and dewpoint temperatures look to be warming
to near/above freezing.

An abrupt transition to quasi-zonal flow is advertised towards
the end of next week/next weekend with a strong signal that the
deep arctic airmass will--at least briefly--get shoved north of
the region.

Carlaw

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 612 PM CST Fri Dec 12 2025

Forecast concerns for the terminals...

- 20-30% chance for a period of flurries and/or freezing drizzle
  this evening.

- Breezy west-northwest winds tonight with gusts around 20-25
  kts.

- Accumulating snow expected Saturday which will result in MVFR
  to IFR visibilities.


A cold front is advancing towards the area this evening which
will result in winds turning west-northwest in its wake and
increasing with gusts in the 20-25 kt range expected overnight.
Additionally, there has been some mist and freezing drizzle
developing just ahead of the front in eastern IA. With moisture
depths on the more marginal side (2000 ft deep at best),
confidence is low on how far east the freezing drizzle will get
so for now have foregone a formal mention at the Chicago TAFs
but did put a PROB30 in at RFD for this potential. If any
freezing drizzle occurs it should only last about an hour or so
at most before switching to flurries behind the front with all
precipitation expected to end by 06z.

Heading into Saturday morning, the gusts will subside with
northwest winds around 10-12 kts expected through Saturday
afternoon. While ceilings will also lift to VFR for a period,
our next clipper system will begin to move into the area around
15-16z which will result in accumulating snowfall at the
terminals. Given the expected track of the clipper it appears
the highest snow totals should occur south of a VYS to VPZ line,
but some 1-2 inch amounts look likely at the TAF sites. Though,
locally higher amounts are possible near MDW and GYY if better
forcing extends further north. Regardless, a period of 1-3 SM
visibilities can be expected with the snow along with MVFR
ceilings.

As snow ends Saturday evening, VFR conditions will quickly
return as skies scatter out. Though, winds will become gusty
once again with northwest gusts in the 20-25 kt range through
the end of the TAF period.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Cold Weather Advisory from 8 PM Saturday to noon CST Sunday
     for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-
     ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-
     ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

     Winter Weather Advisory from 8 AM to 8 PM CST Saturday for
     ILZ019-ILZ032-ILZ033-ILZ039.

IN...Cold Weather Advisory from 8 PM CST /9 PM EST/ Saturday to
     noon CST /1 PM EST/ Sunday for INZ001-INZ010-INZ011-
     INZ019.

     Winter Weather Advisory from 9 AM to 9 PM EST Saturday for
     INZ019.

LM...Small Craft Advisory from 9 PM this evening to noon CST Sunday
     for Winthrop Harbor IL to Gary IN.

     Small Craft Advisory from 9 PM this evening to 9 PM CST Sunday
     for Gary to Burns Harbor IN-Burns Harbor to Michigan City
     IN.

&&

$$

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