Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
419 PM EDT Thu Oct 25 2018

Valid 00Z Fri Oct 26 2018 - 00Z Mon Oct 29 2018

Days 1 through 3...

...New England...
Significant nor'easter will lift up the Atlantic coast Friday
night through Sunday bringing considerable precipitation to New
England. High pressure to the north will gradually retreat as the
low approaches, so although column temperatures will initially be
cold enough for all snow across northern NH and ME, as well as the
higher terrain of VT and upstate NY, warm advection between 850mb
and 700mb will quickly cause precipitation to transition to sleet
and freezing rain. A few inches of snow are possible in the
highest terrain of northern New England, where WPC probabilities
feature a slight risk for 4 inches of snow.

The more significant hazard may be freezing rain, and guidance has
trended upward in its accretion amounts this afternoon. Isentropic
lift along the 300K surface will become intense Saturday night, in
conjunction with the warm nose rising to 0C and above between
850-700mb. Surface temperatures below freezing will climb slowly
as cool ageostrophic flow from the high to the north weakens, but
a period of freezing rain is likely. Heavy precipitation rates do
not typically accrete efficiently, and there remains considerable
uncertainty into how long surface temperatures will remain below
freezing as well as how quickly drier air will advect in from the
south. Still, there is an increased risk for significant freezing
rain accretion across northern VT, NH, and ME. WPC probabilities
feature a slight risk for 0.25 inches of accretion, with
uncertainty in duration of any one precipitation type precluding
any higher probabilities at this time.


...Northern Rockies...
Two distinct shortwaves with accompanying Pacific Jet energy and
moisture will drive into the Pacific Northwest through the period.
The first will move into Washington State and British Columbia
Friday night into Saturday, with a more potent impulse lifting
onshore Sunday. Increasing 1000-500mb relative humidity will be
squeezed out by lift due to positive vorticity advection and 300mb
left exit region diffluence. Friday night into Saturday, 700mb
temperatures are marginally warm and the lift/moisture are
transient. This will create only moderate snowfall across the
highest terrain of the Washington Cascades, Northern Rockies of
Montana, and the Absarokas of Montana/Wyoming where WPC
probabilities show a slight risk for 4 inches of snow. More
significant snow is likely Sunday with a more potent shortwave
with stronger lift and a longer duration of appreciable column
moisture. Temperatures are much cooler above 850mb and some
upslope enhancement is likely into the Washington Cascades. WPC
probabilities are high for 4 inches of snow, with up to 8 inches
possible in the highest terrain.


The probability for significant icing (0.25 inches or more) is
less than 10 percent all 3 days outside of New England.


Weiss