629
FXUS63 KLOT 101121
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
521 AM CST Fri Jan 10 2025

.KEY MESSAGES...

- Accumulating snow is expected today with totals generally in
  the 1-3 inch range especially along and southeast of I-55,
  resulting in slippery travel for the morning commute.

- There is potential for a brief period of freezing drizzle
  north of a Dixon to Waukegan line early this morning, which
  could result in a glaze of ice in spots.

- Isolated to scattered snow showers are possible (20% chance)
  this evening and overnight which may lead to additional snow
  accumulations.

- A shot of Arctic air will move through the region Monday
  through Wednesday. Tuesday will be the coldest day of the
  week.

- Temperatures should thaw toward the end of the week (though a
  return to below-average temperatures may very materialize in
  the January 19 to 24 timeframe).

&&

.DISCUSSION...
Issued at 333 AM CST Fri Jan 10 2025

Through Saturday:

A pair of upper-level shortwave troughs continue to phase over
the Mississippi Valley this morning with an associated cold
front stretching from central WI into northern MO. While
precipitation along the front so far has been just some spotty
snow showers and flurries due to limited moisture, forecast
soundings continue to support the idea that some patchy freezing
drizzle may develop across portions of northwest IL (mainly
areas north of a Dixon, IL to Waukegan IL line) as the phasing
trough interacts with some low-level clouds hovering in the -7
to -8 celcius range. Since the freezing drizzle should be of
relatively short duration (less than 2 hours at any one
location) confidence is still low as to whether or not any ice
accumulation would actually materialize, but pockets of a light
glaze are possible.

Any freezing drizzle that does develop will quickly transition
to snow as the aforementioned cold front moves into the area
towards daybreak. As this occurs, the broad area of snow
currently spreading north across central IL will converge with
the front resulting in accumulation snow across all of northeast
IL and northwest IN through this evening. However,
accumulations will vary from north to south today due to
differences in where the better moisture and forcing will
overlap. That said, the highest accumulations are forecast to
occur along and southeast of I-55 where totals in the 1-3 inch
range can be expected while elsewhere totals will generally be
in the 1-2 inch range or less. While these snow amounts are not
terribly impressive, they will be sufficient to result in some
slippery travel for the morning commute so plan to exercise
caution when driving.

The phased upper trough and associated front will begin to push
into northern IN this evening allowing snow to taper from west
to east. Though, a vorticity maximum is expected to pivot into
northern IL behind the trough this evening which should allow
for additional snow showers to develop. Forecast soundings show
that these snow showers will likely be convective due to steep
low-level lapse rates and shallow moisture meaning coverage will
likely be in the isolated to widely scattered category. Meaning
not everyone will see the showers tonight, but those that do
could see additional accumulations up to a quarter inch
especially on grassy and untreated surfaces.

All snow will come to a conclusion by daybreak Saturday morning
as the trough and vort shift east and high pressure begins to
build into the region. While we will see some sun for our
Saturday, temperatures are still expected to remain on the
chilly side with highs in the mid to upper 20s.

Yack


Saturday Night through Thursday:

On Sunday, a compact upper-level shortwave will propagate
across the Great Lakes region along the southern flank of
expanding upper-level cyclonic flow centered in southern
Canada. Ensemble model guidance appears to have a fair handle on
the track of the shortwave and depicts the surface reflection
passing north of our area from southern Minnesota toward the UP
of Michigan. As a result, our area will be within the WAA-regime
of the system which will be characterized by low clouds and
southerly low-level winds. Even with respectable upper-level
forcing by virtue of DCVA, limited time for moisture return
between the cold front crossing our area today and the system
arriving on Sunday supports a high PoP/low QPF scenario.
Forecast soundings from deterministic guidance depict varying
degrees of saturation through the depth of the profile,
including an apparent scenario in which mid- level moisture is
scant and temperatures within the remaining low-level cloud
layer remain entirely warmer than -10C (lack of ice nuclei,
which would in turn support freezing drizzle rather than snow).
With surface temperatures near or just below the freezing mark
and the expectation for low liquid equivalent amounts (<0.1"),
the impact of any freezing drizzle, or snow for that matter, is
somewhat unclear. For now, opted to set the precipitation type
as equal chances for snow and freezing drizzle in our forecast
in favor of adjustments in later forecast packages.

Along the backside of the system on Sunday evening, an arctic
cold front will rush across the Great Lakes. Forecast ensemble
median 850mb temperatures plunge toward -20C by early Tuesday,
which will set the stage for a period of cold temperatures to
start the workweek. Highs Monday through Wednesday will be held
to the teens (F), while lows Monday and Tuesday nights and will
fall into the single digits. While the surface pressure gradient
doesn`t look overly tight during the period of cold
temperatures, at least some wind (5 to 10 mph) during nighttime
periods will force wind chills below zero (-5 to locally -15F).
Finally, with our area solidly on the cyclonic shear side of the
upper-level trough axis during the period of coldest
temperatures, would have to think that any threat for clipper
systems would be confined to our southwest. With that said, any
snow that falls would be prone to causing slick travel as road
treatments are often less effective with such cold temperatures.

From the middle to end of next week, ensemble model guidance
favors upper-level ridging becoming reestablished across the
northern US allowing for temperatures to rebound (highs
returning toward the lower 30s by Thursday). The thaw may be
short-lived, however, as ensemble meteograms paint a picture of
a return to below-average temperatures in the week 2 period. In
fact, the CPC advertises "slight" risks (corresponding to 20-40%
chances) for both "hazardous" temperatures and "heavy" snow
across broad swaths of the central US in the January 19-24
timeframe. Time will tell if such threats end up materializing.

Borchardt

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 521 AM CST Fri Jan 10 2025

Snow will prevail at DPA/ORD/MDW/GYY and possibly RFD from the
start of the TAF through early afternoon. The lowest
visibilities, which may be as low as 3/4SM at times, are
expected intermittently through 16 to 17Z as snow rates
occasionally flare to as much as 0.5"/hr. Cigs will also range
from IFR to MVFR. Opted to restructure the inherited TAFs to
feature TEMPO groups for the lowest visibility. Snow rates
should begin to taper by early afternoon before ending from
northwest to southeast. Total snow accumulations will range from
0.5" near RFD to locally 2" at ORD/MDW/GYY.

A cold front will move through the terminals this afternoon with
a wind shift toward the northwest (though speeds will remain
less than 10 kt). SCT to BKN MVFR cigs should prevail behind the
front. As an upper-level wave moves overhead overnight, a period
of snow showers is expected across the general region. The
chance that one hits any given terminal is about 30%, so will
feature PROB30 groups in each TAF. Snow showers should clear by
daybreak tomorrow, though MVFR to low-end VFR cigs may linger.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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