Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 AM EST Sat Dec 23 2023

Valid 12Z Sat Dec 23 2023 - 12Z Tue Dec 26 2023

...Rockies...
Days 1-2...

A northern stream and southern stream shortwave will move across
the Rockies today into Sunday, rotating past each other on either
side of Colorado tomorrow morning. The southern shortwave will
continue to help bring additional snowfall to eastern AZ (White
Mountains), the San Juans in southwest CO, and Sangre de Cristos
in CO/NM before it lifts into the Plains. The northern stream
shortwave over Idaho this morning will dig further into western
WY/CO as the surface cold front continues its march to the east
and southeast. By early Sunday, last push of the vort will spread
generally light snow across the CO Rockies and Front Range before
diminishing. WPC probabilities for at least 4 inches of snow are
highest across the higher peaks of the Rockies from southwestern
MT, western WY, much of CO above about 7000ft, and into northern
NM.

...Northern/Central Plains to the Upper Midwest...
Days 2-3...

A complex and multi-part/phase system (or systems, really) will
transpire Sunday through Monday into Tuesday east of the Rockies,
across the Plains, and into the Upper Midwest. Lead shortwave on
Sunday will lift through the Plains, helping to spread snow (rain
to snow) across NE into SD on the northwest side of the front and
weak area of low pressure. WPC probabilities for at least 4 inches
of snow are low from central NE northward into SD with the lead
system. After a bit of a break, the conglomeration of mid-level
vorts will amalgamate into a sizable upper low nearly cut off from
the westerly flow, allowing a surface low to wrap northward and
northwestward over the Corn Belt. The impressive negative tilt
will likely draw in eastern Gulf moisture northwestward into the
Midwest/Corn Belt via the WCB into a TROWAL. Northern stream front
moving through Canada will promote its own moisture surge, with
the combination supporting PW anomalies of +3 to +4 near the
rain/wintry precip transition zone. The guidance remains uncertain
in the critical details, with a zone of maximum precipitation type
uncertainty (mostly SN/ZR vs IP) from near the IA/SD border
northward and northeastward across northern MN. In the colder air
to the northwest/west of the surface low, strong FGEN deformation
may support heavier snowfall but placement will be refined over
the next couple of days. For now, WPC probabilities of at least 4
inches are moderate (40-70%) from northeastern NE northward into
southeastern SD. To the north, arcing surface high from ND into
southern Canada may maintain some low-level cold air beneath the
southeasterly warm flow aloft to support freezing rain
accumulation near the ND/SD/MN border and eastward to the
Arrowhead. WPC probabilities for at least 0.10" icing are low
(10-20%) at this time.

...Pacific Northwest...
Day 3...

A quickly-moving shortwave will move into British Columbia late
day 3 (late Monday into Tues) with a moderate but diminishing
moisture surge (initially +1 to +2 sigma) downstream of a split
upper jet. Upper dynamics will favor a weakening of the system
early Tuesday, but enough moisture on SW flow will lift into the
Cascades and materialize as light to moderate snow. Lower snow
levels at precip onset of around 2500-3000ft will rise to over
4000ft as precipitation continues into early Tuesday. Some initial
colder valleys may support some freezing rain until milder air
scours it out. WPC probabilities for at least 4 inches of snow are
moderate (40-70%) over the northern Cascades, generally above
5000ft.


Fracasso