Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 AM EST Tue Mar 03 2020

Valid 12Z Tue Mar 03 2020 - 12Z Fri Mar 06 2020


...Southern Rockies of New Mexico...
Day 1...

Closed upper low moving eastward along the NW Mexican coast will
shift towards the Big Bend of Texas Wednesday. Robust moist
advection will transport anomalous PW of +2 standard deviations
above the climo mean in conjunction with prolonged high 700-500mb
RH. Enhanced jet-level diffluence combined with the height
falls/PVA invof the upper low in the saturated environment will
produce heavy precipitation across far southern New Mexico. Snow
levels during this time will be generally 6000-8000 ft, which will
confine the heavy snow to the Sacramento Mountains. WPC
probabilities today and tonight are high for 8 inches in this
range, with isolated amounts potentially exceed 12 inches before
forcing and moisture shift eastward.


...Pacific Northwest and Northern Rockies...
Days 1-3...

Through Wednesday, confluent mid-level NW flow with embedded
shortwaves will persist across the Pacific Northwest and into the
Northern Rockies. Most of these impulses will remain north of the
international boundary into Canada, but enough moist advection,
height falls, 700mb omega will produce snowfall in the high
terrain of the Cascades and Olympics of Washington, into the
ranges of western MT, northern ID, and northwest WY. Although the
forcing is modest, it is prolonged, and moisture surges identified
by PW plumes of more than 1 sigma above the climo mean will drive
rounds of snowfall. WPC probabilities are high for 4 inches both
day 1 and day 2, generally above 4000 ft, and snowfall may exceed
1 foot in the Cascades and Northern Rockies.

On day 3 /Thursday/ a more significant mid-level low approaches
from the Gulf of Alaska. This turns the flow from NW to SW,
advecting more significant moisture into the Northwest, but
raising snow levels as well. It appears the majority of this
moisture will move onshore beyond this forecast period, but WPC
probabilities do increase once again on D3 in the higher terrain
of the Olympics and Cascades of Washington, featuring a high risk
for 4 inches on Thursday.


...Northeast...
Day 2...

Mid-level northern stream trough will dig across the Great Lakes
tonight into Wednesday while taking on a modest negative tilt.
This shortwave will interact with increasing upper diffluence in
the LFQ of a poleward advancing jet streak to induce surface
cyclogenesis across upstate New York. Features remain progressive,
so this low will move quickly across Northern New England and
central Maine, so the thermal profile of the column will initially
support primarily liquid (rain) on WAA ahead of the low. As the
system pulls away to the east, CAA wrapping behind the low and
increased mid-level deformation will enhance ascent and drive
dynamic cooling of the column to transition p-type over to snow.
The heaviest snow is likely across northern Maine where WPC
probabilities are high for 4 inches. Less substantial snowfall is
possible in the other high terrain of the Adirondacks and ranges
of Northern New England.


...Northern Great Lakes...
Day 3...

Potent shortwave will race eastward in the fast northern stream to
move across the Great Lakes late Thursday into Friday. This
feature will attempt to close off near the Great Lakes, with the
associated height falls and modest jet streak diffluence leading
to weak surface low development. This low will move quickly
eastward with a stripe of synoptic snowfall on WAA late Thursday
into Friday. This leading snowfall is not expected to be
significant, but behind the departing system Thursday night and
early Friday, a brief period of lake enhancement snow is likely,
focused in the U.P. of Michigan as northerly fetch develops across
Lake Superior. The temporal duration of this is short, and
inversion heights are low, so the potential for heavy LES is
modest. However, WPC probabilities are high for 4 inches in the
favored N/NW snow belts of the U.P. with isolated amounts over 6"
possible.


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

Weiss