Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
314 AM EST Fri Feb 10 2023

Valid 12Z Fri Feb 10 2023 - 12Z Mon Feb 13 2023

...Northeast...
Day 1...

Surface low moving through the St. Lawrence Valley this morning
will take the last area of precipitation with it by the afternoon
as the triple point low becomes the dominant feature over Nova
Scotia. A few more inches are likely across far northern Maine
(the very last place to warm to near freezing) this morning with
some light icing just south of there. In the wake of the system, a
mid-level trough will swing through the eastern Great Lakes this
afternoon/evening to enhance some lake-effect snow. This will be
focused east of Lake Ontario into the Tug Hill as well as across
northern NY/VT this evening as it swings through. WPC
probabilities for at least 4 inches of snow after 12Z Fri are
generally <50%, highest in the Tug Hill.


...West...
Days 1-3...

Splitting mid-level trough just off the WA/OR/NorCal coast this
morning will continue eastward into early Saturday, with generally
light snow over the Cascades. Southern portion of the trough will
close off into an upper low near San Francisco with a weak surface
reflection. Moisture will be limited (no subtropical moisture
connection), but combination of height falls and upslope into the
terrain will yield several inches of snow over the
Shasta/Siskiyou/Trinity ranges into the northern Sierra D1 that
continues into D2 into the central Sierra as the upper low sinks
southward. WPC probabilities for at least 4 inches of snow
maximize in the 00Z Sat - 00Z Sun period near 50% in the central
Sierra. By D3, the upper low will turn the corner over
SoCal/Northern Mexico with a slight moisture tap from the eastern
North Pacific into the Southwest. Light snow will break out of the
Mogollon Rim by very early Monday and continue beyond this
forecast period. Back into the PacNW, another trough will dive
southeastward with additional snow for the WA Cascades.


...Southern Appalachians...
Days 2-3...

Deep closed mid-level low (500mb heights around -3 sigma) over the
northern Gulf early Saturday will track ENE off the Carolina coast
late Sunday. Modest divergence aloft between two jet streaks
(northeast Gulf and over the Mid-Atlantic) will provide broad lift
for a developing area of low pressure at the surface that will
move northeastward along the Southeast coast. Warm conveyor belt
will bring plenty of moisture from the Gulf/Caribbean northward
ahead of the cold front and up and over the warm front into the
Southeast/southern Appalachians. Temperatures will be marginal
despite the bridged high pressure to the NW and N that will weaken
with time, so dynamical cooling will favor higher elevations in
western North Carolina into southwestern VA for appreciable snow.
Convergent upslope flow will favor the Smokey Mountains as the
mid-level center passes just to the southeast of there early
Sunday, where more than 6 inches are likely above about 4000ft.
SLRs will start low due to the high moisture content and lack of
cold air in the column, but will rise as the QPF starts to wane.
WPC probabilities for at least 4 inches of snow are >50% above
around 3000ft in western NC/far eastern TN along the Appalachians
into southwestern VA.


Fracasso