Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
316 AM EST Fri Mar 05 2021

Valid 12Z Fri Mar 05 2021 - 12Z Mon Mar 08 2021


...Pacific Northwest/Northern California...
Days 1-3...

A closed mid-level low off the WA coast will shift north of
Vancouver Island today as an amplifying trough digs off the
northern CA coast. A pronounced southerly flow will advect warm
air and moisture northward along the coast of northern CA and
across western WA/OR. A prolonged period of low level pre-frontal
convergence combines with the warm/moist advection ahead of the
front to produce prolonged low elevation rain and mountain snow
(snow levels around 4000ft) across the OR/WA Cascades and Olympics
today. The focus shifts south down the coast today, with precip
pushing into far northern CA later this afternoon and continuing
through tonight. Mountain snows occur for the Klamath/Shasta
Siskiyous/Trinity Alps, into the southern OR Cascades, and
northern CA Sierra Nevada with snow levels decreasing from around
5000ft to around 3000ft through tonight. Day 1 WPC probabilities
are moderately high for 8 or more inches in the Olympics and the
Klamath/Shasta Siskiyous/ with low percents in the southern OR
Cascades and the highest WA Cascades. 20 to 30 percent changes for
8 or more inches are present for the northern Sierra Nevada on Day
1.5.

Veering flow behind the cold front cuts off Pacific moisture and
mountain snows on Saturday with greatly diminished precip rates
expected as the forcing shifts inland over the Great Basin and
northern Rockies.

The cold front associated with the next in the series of slow
moving lows drifts crosses the Pacific Northwest coast Saturday
night. With the associated trough not as amplified/meridional as
the previous one, the next round of 850-700 mb warm/moisture
advection is weaker with lower precip rates expected ahead of the
cold front that spreads inland Sunday morning. Despite lower snow
levels than the precip today, the lower rates result in low snow
probabilities for Day 2/3 with high values for 6 or more inches
limited to the Olympics and moderate for the highest WA Cascades
on Day 2.5.


...New York/Vermont/Pennsylvania...
Days 1/2...
Deep low pressure drifts stalled over eastern Quebec through
Saturday, continues to allow some moisture to wrap around the
circulation and into northern New York and Vermont today with LES
resuming off Lakes Ontario and Erie late this afternoon and
continuing in earnest at times through Saturday. Day 1.5 snow
probabilities for 4 or more inches are confined to the Syracuse
area from northwesterly flow off Lake Ontario with streamlines
stretching back across Georgian Bay.


For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.


Jackson