553
FXUS63 KLOT 140454
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1054 PM CST Mon Jan 13 2025

.KEY MESSAGES...

- Period of accumulating snow and slippery travel expected
  Tuesday morning areawide, though highest accums are expected
  around the Interstate 80 corridor

- Wind chill values as low as -15F are possible across northern
  Illinois late tonight into Tuesday morning.

- A storm system may bring a mix of rain and snow to the general
  region in the late Friday through early Sunday timeframe.

- Another push of Arctic air will arrive by early next week.
  Temperatures early next week may be the coldest thus far of
  the winter season.

&&

.UPDATE...
Issued at 914 PM CST Mon Jan 13 2025

Despite quiet, clear, and chilly conditions at present, some
sizable tweaks to the forecast were made on this shift. There
has been a notable northward trend in high res guidance for
tomorrow morning`s snow.

Surface obs around the South Dakota/Iowa/Minnesota tri-state
area are showing light snow. So far, most model guidance has
struggled with the development in the northern Plains and is
only now catching up to it. But even using the "eye test" on
satellite, the system can be extrapolated with an eastward
progression through Northern Illinois (potentially staying just
south of the Wisconsin state line?). Previous model runs were
keeping this system`s snow south of Interstate 80. However, the
gridded forecast was adjusted by shifting and expanding PoPs
northward to encompass the entire forecast area. Given the high
likelihood that most areas will receive at least a dusting,
categorical PoPs were introduced to the forecast.

In terms of amounts, there is still some uncertainty where the
strongest f-gen forcing will be as the wave moves west to east
over the domain. But the northward shift of the better forcing
is suggesting increased snowfall rates (up to around a third of
an inch per hour) around the Interstate 80 corridor, though
there is a chance these could extend as far north as Interstate
88.

With higher snow ratios (around 20:1), it is plausible that
much of the area could see a half an inch to up to an inch of
snow, but it would not be surprising if there were localized
reports over an inch (particularly around and south of
Interstate 80). The northern tier of counties in Illinois
(Winnebago to Lake) will likely have lower snow totals to just a
dusting due to the expectation of a strong gradient to no
snow/flurries in Southern Wisconsin. The last thing to mention
is that is that the speed of this system should make it only a
short window (three hours or less) for snow fall. It will most
likely enter the CWA to the west around 3 AM and exit to the
east long before noon. The downside here is the fact that the
northward shift and the timing of the system has it moving
through the Chicago Metro during the morning commute with the
risk of creating slippery travel conditions.

Temperatures through the night will be cold with parts of the
area in the single digits. Westerly winds switch to the
northwest and increase behind the snowfall helping to create
sub-zero wind chills during the commute. As the system exits to
the east during the morning, skies should clear but temperatures
will only climb into the teens.

DK

&&

.DISCUSSION...
Issued at 258 PM CST Mon Jan 13 2025

Through Tuesday Night:

Quiet weather conditions continue late this afternoon as a
surface ridge brushes the area to the south. Stronger gradient
flow north of the ridge will continue to result in west winds
gusting to 25 mph through this evening. Even with the gusty
winds, initial clear skies early this evening should allow temps
to quickly lower to around 10F or lower before temps steady as
cloud cover increases ahead of an upcoming wave.

A disturbance currently over the Dakotas will amplify while
rotating around a broader upper-level low centered over the
Boundary Waters through tonight. Low to mid-level moisture
associated with the wave will be modest at best as it shifts
across central Illinois late tonight into mid-morning Tuesday.
Though overall saturation remains in question given the amount
of available moisture, low to mid-level forcing by way of weak
WAA followed by a mid-level front should support at least some
light snow south of I-80. Much of the saturated layer will be
colder than the DGZ, so SLR values may settle closer to 15:1
with dendrites less favored. Overall, the expectations is for a
range of some flurries over the southern Chicago metro southward
to up to 1" south of the Kankakee River Valley. Slippery
conditions can be expected with temps generally in the upper
single digits to low teens during the snow.

CAA and dry air advection will take hold behind the system by
late Tuesday morning, which will counteract the expected
increasing sunshine to keep temps reaching only the mid teens
Tuesday afternoon. A surface high will then pass across central
Illinois Tuesday night, allowing for good radiational cooling
over the existing snowpack over the southeast 2/3 of the CWA.
Have undercut guidance by a few degrees, with lows expected to
be in the low single digits above zero north to sub-zero south.

Kluber


Wednesday through Monday:

The mean upper-level longwave trough and accompanying Arctic
air mass stationed over the Great Lakes at the beginning of this
week will give way to a ridge building over the central CONUS
by the end of the week. This, in tandem with return flow on the
backside of a departing surface high as well as a good deal of
sunshine, should cause a warming trend to ensue over the latter
half of the week, likely culminating in the first 40+ degree
readings being observed in parts of our forecast area this year
on Friday. This should occur in spite of an upper-level
shortwave diving southeastward out of Canada late Wednesday into
early Thursday and temporarily reinforcing the mean troughing
and cold air aloft over the Great Lakes. While a period of
flurries can`t be ruled out as this reinforcing wave clips our
area, westerly to northwesterly flow trajectories over Lake
Michigan should keep any lake effect snow confined to our east.

The main atmospheric features of importance for our
precipitation forecast for Friday into the weekend are a
northern stream upper-level disturbance currently located near
the Aleutian Islands and a soon-to-be cut-off upper-level low
that is currently bringing a renewed period of strong winds and
fire danger to southern California. During the middle of the
week, the former feature will crest a Rex block that will form
as a result of the California low retreating southwestward over
the Pacific Ocean. The block will break down shortly thereafter,
and both features will proceed into the CONUS, where they will
start to interact with one another.

The majority of the past several ensemble and deterministic
model runs favor the two waves undergoing a relatively
disjointed phasing or not phasing much at all, which is good
news if you`re looking to avoid a winter storm and bad news if
you`re yearning for one. Such an upper-level pattern evolution
could still bring precipitation to the area, particularly
southeastern portions of our forecast area, but many EPS and
GEFS members depict the bulk of the precipitation remaining to
our southeast altogether or not even developing at all until the
driving forcing mechanisms are east of our longitude. What will
ultimately happen will be dependent on if, how, when, and where
the two waves end up phasing, which is something that will
likely not come into focus for at least a few more days. Given
these uncertainties and the existing spread in ensemble
guidance, keeping PoPs capped below 55% (the cut-off between
`chance` and `likely` PoPs) seemed appropriate for now.

With the milder temperatures that will be in place on Friday,
the type of precipitation that will be observed (assuming that
it occurs) is also somewhat uncertain and will depend on when it
arrives with respect to both time of day and the arrival of a
push of colder air accompanying the northern stream wave. The
latest NBM temperature and PoWT grids depict rain as the
dominant precipitation type at onset before a transition to a
rain/snow mix or all snow is advertised before the precipitation
ends. Depicting such an evolution in our forecast grids continues
to seem fine for now, but confidence in this actually occurring
is quite low at this time given the existing uncertainties
regarding the two aforementioned waves and what influence they
may have on the progged borderline thermal profiles.

Regardless of whether or not we see any precipitation Friday
into the weekend, confidence is high that our short-lived warm-
up will come to an abrupt end over the weekend as temperatures
plunge back down with the arrival of our next air mass of Arctic
origin. Just how cold it will get will, in part, depend on
whether a snowpack will be in place over the region (and how
deep that snowpack is), but the 12Z deterministic ECMWF and GFS
suggest that even areas with no snow on the ground stand a good
chance to see sub-zero air temperatures during the nights/mornings
from Sunday night/Monday morning through mid-week. If winds
remain elevated enough during this time frame, then we could be
looking at potential Cold Weather Advisory issuances, but that
is something that is still several days down the road at this
point.

Ogorek

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1055 PM CST Mon Jan 13 2025

The main aviation weather concerns are:

- Brief period of accumulating snow late tonight into early
  Tuesday morning.

VFR conditions will continue for a few more hours this evening
before snow overspreads the region late tonight. Latest model
guidance and observations suggest no significant changes are
needed to the timing in the current TAFs, with snow starting at
RFD around 9z, and the Chicago-area terminals 10-11z (flurries
could start a little earlier). The time window for snow will be
brief, likely about 3 hours, during which vsbys will lower to
IFR. Vsbys could even drop as low as a half mile, but suspect
this would be for a very brief period-- too brief in fact to
justify inclusion in the TEMPO group.

Snow amounts look to end up near about an inch, perhaps a
smidge higher if any more persistent banding develops. Ratios
will be fairly high (dry/fluffy) near 20 or even 25:1. Any
lingering snow will rapidly clear the region through mid-morning
on Tuesday. VFR conditions will prevail thereafter, with
blustery northwest winds gusting 20-25 knots. Winds will ease
Tuesday evening.

Carlaw

&&

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

     Small Craft Advisory until 9 PM CST Tuesday for Gary to Burns
     Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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