Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EST Thu Jan 02 2020

Valid 12Z Thu Jan 02 2020 - 12Z Sun Jan 05 2020

...Pacific Northwest through the Northern Rockies...
Days 1-3...

A pronounced Pacific jet streak rotating over top of a Pacific
ridge will dive quickly from British Columbia into the Central
Rockies Thursday, bringing with it deep layer moisture and modest
synoptic ascent. Forcing into the terrain where snow levels are
low enough for accumulating snow will shift east through the first
half of D1 in response to ridging advecting from the west, but WPC
probabilities indicate a high risk for 6 inches of snow in the
Washington Cascades and the Northern Rockies.

Low pressure lifting onshore British Columbia Saturday will drive
a warm front, and subsequent cold front, Friday and Saturday into
the Northwest. This will be accompanied by increasing jet-level
diffluence, height falls and PVA, with snow levels dropping behind
the cold front as well. Moisture streaming into the area is
forecast to drive PWATs to 1-1.5 standard deviations above the
climo mean, which will be wrung out as heavy snow in the Cascades
of Washington and Oregon, with spillover into the Northern Rockies
likely by D3. WPC probabilities are high for 12 inches in the
Cascades and Olympics of Washington, and 2-day snow totals could
exceed 4 feet. Snow levels are forecast to fall to 2000-3000 ft
east of the Cascades, and as low as 500 ft by the end of the
forecast period into ID/MT.


...Eastern Great Lakes through New England...
Day 3...

Northern stream and southern stream energy will try to phase near
the east coast Sunday morning, but timing and intensity detail
differences lead to lowered confidence by day 3. A surface low
pressure moving from near Arkansas into the Ohio Valley will
occlude to a triple point and secondary low development near NJ.
This secondary low development will be intensified through
interaction of a 170+kt jet streak lifting into SE Canada, and
rapid height falls as the northern/southern stream energies phase
to a negatively-tilting longwave trough. A blend of the ECENS,
CMC, GEFS, and NAM are preferred for the placement and evolution
at this time, which suggests the surface low will track quickly
eastward as the capture occurs too late to bring a robust system
up the coast towards Maine. There could be a deformation band or
accumulating snow showers in the vicinity of the northern stream
upper low/vort from IN/OH into NY state, with more significant
snowfall possible across parts of New England late in the period.
Thermal profiles ahead of the low suggest primarily rain at onset,
but as the low begins to pull away cold air should rapidly work
southeastward to change p-type from rain over to snow. WPC
probabilities for 4 inches are highest in the terrain of upstate
NY and into VT/NH, where they are greater than 50%. Further south
into the central Appalachians, a period of robust upslope forcing
should produce moderate to heavy snow in the mountains of WV,
where WPC probabilities are also moderate for 4 inches.


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

Weiss