Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 AM EST Mon Feb 04 2019

Valid 12Z Mon Feb 04 2019 - 12Z Thu Feb 07 2019

...Western States...
Days 1-3...

Anomalously deep upper low with 500mb height anomalies of -3
standard deviations below the climo mean will drop slowly along
the Pacific NW coast today and then continue southeast into
California while filling on Tuesday. To the south of this feature,
a Pacific Moisture plume will continue to spread inland on robust
700mb SW flow and an ejecting upper level jet streak. As the upper
low digs southward, it will drive an arctic front beneath it, and
snow levels will fall to near the surface as far south as central
CA, northern AZ, and northern NM by day 3. Drier air will flood
into the region from the north as the upper low continues to lose
latitude and squelches the moisture plume, but a secondary impulse
is progged to quickly drop into the Four Corners region and phase
with the dissipating initial upper feature to renew precipitation
across Colorado on Wednesday.

Snow amounts will be extremely impressive in the high terrain of
the Sierra Nevada, Southern Wasatch and Uintas of Utah, as well as
the ranges of Colorado and NW Wyoming. While the Sierras have a
high probability of 3-4 feet or more during the forecast period,
the other ranges also have high probabilities for 2 feet. As the
snow levels fall, accumulating snow is likely down to the valleys
of much of the inter-mountain West, and WPC probabilities are high
for 2 inches from Oregon through northern New Mexico. This
includes the city of Portland, OR as well as the mountains just
outside of San Francisco, CA. Despite lowering snow levels through
the 3 days, probabilities for 8 inches or more are high only in
the terrain above 5000 feet throughout the west.


...Northern Plains and Great Lakes...
Days 1-3...

An active period of will bring 3 distinct storm systems with
wintry precipitation to the area.

The first will be today as mid-level impulse and strengthening
surface low spread moisture from the Dakotas eastward into the
Upper Great Lakes. Moderate snowfall is likely in the cold air NW
of the surface low, where the best height falls and synoptic lift
due to jet level diffluence produce ascent. The heaviest snow is
likely across the U.P. of Michigan where the synoptic ascent is
aided by more mesoscale features to produce heavier snow rates.
WPC probabilities are low for 4 inches outside of the U.P. South
of the snow, a swath of freezing rain will likely produce
significant accretion as WAA drives a warm nose atop surface cold
air, and even colder ground temperatures. A multi-model consensus
exists for 0.25" of accretion across the eastern U.P. of Michigan,
and it is in this area where WPC probabilities are over 50% for
this amount. Elsewhere, from northern WI and the western U.P., 0.1
inches of accretion is possible. Complicating this forecast are
the very cold ground temperatures which may permit freezing of
rain despite 2m temps above 0C. This has been factored into this
forecast such that despite the potential for heavy precip rates
which usually don't accrete efficiently, this significant freezing
rain amounts are still possible.

On Day 2 /Tuesday/ a mid-level impulse wave will lift across the
region quickly bringing a brief period of robust ascent through
upper diffluence, WAA, and isentropic lift atop cold high
pressure. The temporal duration of this event is limited, but
strong omega is forecast as an 850mb LLJ advects warm moist air
and produces intense isentropic lift. This will produce a swath of
snow from southern MN through WI and into the U.P. of MI, where
briefly heavy snow rates of 1/2" to 1" per hour are possible.
Despite the short duration event as drier air quickly works in
from the west, WPC probabilities are moderate for 4" of snowfall,
centered across Wisconsin. South of this area, another round of
freezing rain is likely, centered in a stripe from southeast IA
through southern MI. The aforementioned LLJ will drive 850mb temps
above 0C, but surface cold air will remain locked in as high
pressure behind an arctic front persists cool E/NE surface winds.
Precipitation rates are forecast to reach up to 0.25"/hr, which
will likely not accrete efficiently due to runoff and thanks to
warmer ground temps in this area from Monday. Still, WPC
probabilities show a high risk for 0.1 inches of accretion, with a
30% chance for 0.25 inches.

Finally, on Day 3, Wednesday, a more robust surface low will
develop along the stalled front to the south bringing yet another
round of snow and ice to the same general areas. The system on
Wednesday is currently progged to be a bit further NW than Day 2,
with the best chance for 4 inches of snow aligned from SD through
northern MN, with moderate probabilities for 0.1 inches of
freezing rain SE of there and into Michigan once again. While
currently no individual event from day 2 or 3 is expected to bring
0.25" of freezing rain, it is possible some locations receive much
more than that during the 2 days depending on the low tracks.


...Upstate New York and New England...
Day 3...

The same storm system impacting the Midwest on Day 3 will drive a
warm front northward into New York and New England late Wednesday.
Surface temperatures will initially be cold, but the surge of warm
air will begin to erode this without a strong high to the north. A
period of freezing rain is becoming more likely, with the terrain
of the Catskills, Adirondacks, and New England featuring the
highest probabilities for 0.1 inches by Thursday morning.


Weiss