Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 PM EST Wed Feb 05 2020

Valid 00Z Thu Feb 06 2020 - 00Z Sun Feb 09 2020

...Midwest through the Great Lakes/New York/New England...

Day 1/2/early day 3...

An 850 mb low tracks from the mid MS Valley this evening to
northern OH and Lake Erie tonight.  A stripe of light snow occurs
norther of the 850 mb low track across northern IL and southern
Lower MI. Significant moisture flux ahead of this system will
spread precipitation northward on warm/moist advection, with the
warm advection changing precip over to sleet/freezing rain along
the low-level frontogenesis, with a tenth to quarter inch of ice
accumulation common from west central OH to northeast Ohio. A few
ensemble members have between 0.25-0.4 inches in the axis of
icing, so significant differences remain to be resolved.

Temperatures remain below freezing for a considerable portion of
the precipitation Wednesday night through Thursday night across
interior Southern and Central New England. Freezing rain
accretions of a tenth to quarter inch are forecast for the Laurel
Highlands and the Poconos through the Catskills, Berkshires,
northern Worcester Hills, and Southern Greens.

Developing low pressures crosses eastern PA, southeast NY and
across southeast New England into the Maine coastal waters Fri.
This low is expected to intensify rapidly as it moves across
Southern New England and near the Maine coast. A maxima in mid
level frontogenesis occurs near the 700 mb low track across
northern NY, VT, NH, and western ME. Heavy snow is expected along
this axis with a foot possible in northeast NY across the
mountains of northern New England.
Probabilities are moderate to high for 8/12 inches across
northeast NY and northern New England on day 2 .

Near the 850 mb low tracks across central NY to southern
VT/NH/downeast Maine, a messy mixture of snow/sleet, and freezing
rain is expected,
with ice extending across the Monadnocks of southern NH and across
downeast Maine. The departure of the low Fri up into the Canadian
maritimes ends the synoptic portion of the event, with precip in
Maine winding down.


...Inter Mountain West...
Days 1-3...

Confluent flow across the Pacific northwest and northern Rockies
results in low level frontogenesis.  Moisture advects across the
frontal zone and
along the upper jet as it moves from the Pacific northwest across
the ID/western MT Rockies and then western WY ranges, with
secondary maxima developing in northern UT and western CO as the
jet extends into these areas Thu. Due to high snow levels in
western WA/OR,
mostly higher elevations of the WA Cascades and OR Cascades are
forecast to heave heavy snow. Probabilities for 8/12 inches focus
from central ID across southwest MT and then northwest WY.

On Day 2 Thu night-Fri, the 300 mb jet axis extending across
western MT and western WY/central CO leads to snowfall maxima in
windward terrain as divergence maxima aloft persists, with lift
aided by terrain. The heaviest snow is expected across the
Sawtooth and Bitterroots of ID, across southwestern MT/western WY
and then central CO. Here, WPC probabilities for 8/12 inches are
high. Snow amounts of 1-2 feet are anticipated in the favored
upslope areas of central ID to southwest MT and northwest WY.
Several inches are likely in the northern UT Wasatch.

On Day 3, another shortwave will move onshore across the PacNW
coast and inland into the northern Rockies. Lowering heights/temps
aloft leading to decreasing snow levels as snow continues at
higher elevations of the WA/OR Cascades.  The heaviest snow
accumulations as most likely in the PacNW on Friday night to
Saturday, where WPC probabilities are moderate for 12 inches
again, with another round of heavy snow likely developing across
the ID/western MT/northwest WY Rockies as the
upper jet extends inland and sprawls from west to east across
southern ID to WY on Sat.

3 day snow totals of 3-4 feet are possible in favored mountain
locales in the WA Cascades, and also in the ranges of central ID
to southwest MT and northwest WY. 

The probability of greater then 0.25 inches of freezing rain is
less than 10 percent Day 3.

Petersen