Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
237 PM EST Tue Jan 02 2024

Valid 00Z Wed Jan 03 2024 - 00Z Sat Jan 06 2024

...California/Great Basin/Four Corners...
Days 1-2...
A frontal band associated with an amplifying upper trough is
forecast to move into Northern California this evening.  Deep
onshore flow and strong upper forcing ahead of the trough will
support heavy precipitation, including heavy snow, in portions of
Siskiyou and Trinity counties this evening.  HREF guidance shows
rates increasing to 1-2 in/hr this evening as snow levels dip to
around 4500 ft.  WPC PWPF indicates that accumulations of 8 inches
or more are likely in the higher terrain.

As the upper trough moves inland overnight, energy moving through
the base of the trough will continue to amplify the trough and
shift the focus for heavy snow farther south into the northern
Sierra.  The HREF shows rates around 2 in/hr moving south from the
northern into the central Sierra by daybreak.  This will be a
fast-moving system, with rates forecast to quickly drop off as the
trough axis moves inland during the day.  Snow levels beginning
around 5000 ft are expected to drop to around 4000 ft.  Even
through this will be a fast-moving storm, the intense snowfall
rates are expected to produce a long stripe of accumulations of 8
inches or more for areas above 6000 ft, including the major passes.

Widespread but lighter snow will spread into the Nevada mountains
beginning Wednesday and continuing into Thursday.  WPC PWPF
indicates that accumulations of 4 inches or more will be common in
the north-central to the northeastern mountains, especially for
areas above 5000 ft.

By late Wednesday, the models are in generally good agreement
depicting a new closed low developing in the southern stream over
the Great Basin that will dig further south into the
Southwest/Four Corners Wednesday night.  This will bring areas of
light to moderate snow from southern Utah, northern and central
Arizona through the Four Corners into the southern Rockies late
Wednesday into Thursday.  While widespread heavy accumulations are
not expected, several areas including the southern Utah,
east-central and northeastern Arizona mountains, the San Juan and
Sangre de Cristo mountains are likely to see accumulations of 4
inches or more.

...Great Lakes... 
Days 1-2... 
Canadian cold front just north of the border today will move
through the upper/western Great Lakes tonight and early tomorrow
and the lower/eastern Great Lakes tomorrow night into early
Wednesday as the main mid-level trough pivots around the deep
upper low near the Hudson Strait. 850mb temperatures will fall to
around -10 to -15C (about -0.5 sigma) but water temperatures are
above freezing (nearly 100% open with basically no ice coverage).
As winds veer from WSW to W to NW and N, multi-band streamers will
expand in coverage as instability grows with steeper lapse rates
(sfc-850T delta increases). Mid-level trough will dig a bit more
into southern Ontario and then NY where a single band east of Lake
Ontario will set up. There, the highest totals are forecast and
WPC probabilities for at least 6 inches of snow are moderate
(~40%), though these values will likely rise with the inclusion of
more hi-res guidance in the background ensemble variance.
Elsewhere, totals will be light/moderate at best owing to a
limited residence time of favorable winds and a lack of
single-band accumulation except for maybe near/south of Buffalo on
Wednesday ahead of the front on SW flow. Portions of the Michigan
U.P. and northern-most tip of Michigan's Mitten have moderate
chances (40-70%) for >4" of snowfall during the next couple of
days.

...Southern to central High Plains...
Day 3...
The upper low moving across the Southwest on Thursday is expected
to move east of the southern Rockies and into the High Plains
Thursday night. As it does, snow is forecast to develop over
northeastern New Mexico and southeastern Colorado into the TX/OK
Panhandle region and western Kansas.  Precipitation is expected to
spread farther east on Friday, but with the rain/snow line
shifting north as the low ejects to the northeast.  Some models do
suggest that upper jet forcing and low-to-mid level frontogenesis
may support a band of heavier snow developing on the northwest
side of the surface low as it moves into the TX/OK Panhandle
region Friday morning.  However, probabilities for accumulations
of 4 inches or more are relatively low at this point, likely due
in part to the significant spread in model solutions.

...Northwest...
Day 3...
Active period continues for the Pacific Northwest as a series of
fast-moving mid-level vort maxes push through the region. Into day
3, Friday, a surface cold front will bring in colder air to the
region which will lower snow levels from around 4000 ft to
2500-3000 ft as a secondary front comes into the region by the end
of the period. 700mb temperatures will fall to below -10C (about
-0.5 sigma) with a potentially colder air mass to follow. Though
each front may have an increase in moisture into the region,
source region from the higher latitudes favors near to below
normal levels. Upper level divergence and upslope enhancement into
the Cascades will drive much of the snowfall, with more modest
amounts into the northern ID ranges/NW Montana and into the OR
Cascades. Relatively low snow levels will mean greater impacts at
the WA Cascades passes -- WPC probabilities for at least 4 inches
of snow are at least 50% above ~3000ft.
     

The probability of significant ice (> 0.25 inch) across the CONUS
is less than 10%.

Pereira/Fracasso