Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
510 AM EDT Wed Apr 08 2020

Valid 12Z Wed Apr 08 2020 - 12Z Sat Apr 11 2020

...Interior Northern New England/Northeast...
Days 2-3...

A northern stream low pressure system shifting through Ontario
today will be reinforced by a strong trough that swings across the
Great Lakes tonight and reaches New England on Thursday. Rapid
coastal low development ahead of this reinforcing,
negatively-tilted trough axis will occur along the western Maine
coast and the low will pivot up the rest of the Maine coast into
New Brunswick into Friday. Although the low-level thermals will be
marginal for wintry precip outside of northern NH/Maine, the
combination of a rapidly strengthening low and at least subtle
wedging of Canadian high pressure suggests cold air will quickly
wrap southward into the low and throughout Maine, offsetting the
initial WAA/precip from Thursday evening. While much of the
coastal portions of New England, and all of Southern New England,
will be too warm for snow, parts of central New England and much
of northern New England from the White Mountains of NH northeast,
will receive impactful snow Thursday afternoon through Friday
evening. Where it remains all snow, heavy snow rates are likely as
a TROWAL wraps cyclonically into New England, and mesoscale
banding pivots across Maine. Snowfall rates of more than 1 to
2"/hr are likely, particularly when the atmospheric forcing is
maximized over the terrain of the White Mtns as is expected from
the increased confidence in the low track. Day 2 WPC probabilities
are moderately-high for 12 inches from northern NH and across
inland northern Maine. It is becoming more likely that many areas
of northern and central Maine will receive double-digit snow
totals, and where the most intense bands occur, 24 inches of snow
are possible, despite the relatively quick motion of this system.
Continued shortwave impulse advection around the parent low will
locally enhance wrap around snow and likely bring light, but
measurable snow beyond the typical upslope areas and toward the
coast Friday. Day 3 snow probabilities for 2 or more inches are
limited to the higher terrain of the Adirondacks, Catskills,
Green/White Mtns as well as far northern Maine.


...Southwest U.S....
Days 1-2...

A cutoff low drifting toward the southern CA coast will
stall/occlude over inland southern CA through Friday before
ejecting east ahead of a northern stream trough crossing the
northwest CONUS Saturday. Pacific moist advection around this low
will flow into the Desert SW, into the Great Basin and back toward
the typically lee-side of the White Mtns/Sierra Nevada with snow
levels ranging around 4000 ft in southern CA under the upper low,
to around 6000 ft in the warm conveyor of moisture wrapping around
the system. Heavy snow will persist along the southern Sierra
Nevada and San Bernardinos into Thursday. Day 1 WPC snow
probabilities on are high for 8 inches in the Southern Sierra
Nevada, San Gabriel and San Bernardino Mtns of Southern CA and the
highest central NV ranges. By Day 2 the occluding low means precip
focus is more washed out and only moderate risks for 6 or more
inches for the same mountains, with an expansion east into central
CO.


...Northern Rockies...

Day 3...
A potent northern stream shortwave trough digs down the Canadian
Rockies Friday night, reaching the border with WA/ID/western MT
Saturday morning. Surface ridging east of this mid/upper trough
pushes into north-central MT Friday night setting up a combination
of upper level lift and low level upslope flow that will bring
heavy late-season snow to the eastern slopes of the MT Rockies.
Moderately high Day 3 snow probabilities for 6 or more inches are
across the eastern slopes of the Rockies in MT including Glacier
NP.


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

Jackson