Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Thu Oct 29 2020

Valid 12Z Thu Oct 29 2020 - 12Z Sun Nov 01 2020



Day 1...
...New York/Vermont/New Hampshire...
An open mid level wave is forecast to accelerate northeastward
across the Appalachians today to the Mid-Atlantic coast and
coastal waters by Friday morning.  Moisture lifting to the north 
into an associated low-to-mid level frontal zone, is expected to
support a stripe of precipitation across upstate New York to
central New England Thursday night into Friday morning.  Cold air
advects south across interior NY/Vermont/New Hampshire into the
area of precipitation tonight, causing the rain to change to snow.

There are model differences regarding precip amounts, timing and
length of precip after rain changes to snow.
The models continue to show a brief 3 or 4 hour period of snow
before drying aloft and sinking motion cause the snow to taper off
and end.

The risk for impactful snowfall seems to be more focused on the
terrain/elevated regions. There is still high confidence in
accumulating snow in the northern Catskills of NY/southern
Adirondack Mountains, southern Greens in VT and southern
Whites/Monadnocks in NH, where WPC probabilities for 4 inches are
moderate.

Once the secondary wave of low pressure moves off the coast and
out to sea Friday, the snow should end.

Day 2...

...WA Cascades...

The models show a well defined cold front progressing onshore and
inland across western WA state.
Widespread precipitation amounts roughly averaging half an inch
liquid equivalent are forecast in the WA cascades. 
It should be cold enough at higher elevations for the precip fall
as snow, with confidence that it would be cold enough at lower
elevations dropping at lower elevations. Rapid drying aloft the
latter half of Fri night as the 70 mb wave moves east out of WA
leads to the snow tapering and ending by Sat morning.

The probability of heavy snow is less than 10 percent Day 3.

The probability for significant ice (0.25" or greater) is less
than 10 percent all 3 days.

Petersen