Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
445 PM EDT Tue Apr 07 2020

Valid 00Z Wed Apr 08 2020 - 00Z Sat Apr 11 2020

...Northeast...
Days 2-3...

A low pressure system shifting through Ontario Wednesday will
drape a warm front into the Mid-Atlantic and off the New England
coast. Along this baroclinic zone, secondary low pressure
development will occur Thursday night, and then shift northeast
along the coast of Maine or just offshore in the Gulf of Maine. As
this low lifts northeastward, it will likely deepen rapidly,
potentially bombing out, as strong LFQ jet level diffluence from
an anomalously strong upper jet combines with a potent vorticity
impulse embedded within a negatively tilted mid-level trough
pivoting across the northeast. Although the low-level thermals
will be marginal for wintry precip outside of northern 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 east, 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"/hr are likely, and
WPC probabilities are high for 8 inches Thursday night and Friday.
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, 15 inches of snow is possible, despite the
relatively quick motion of this system. Across Downeast and
Coastal Maine, there is more uncertainty into how quickly cold air
will wrap southward as the low departs, but some light
accumulations are possible all the way to the coast.


...Southwest U.S....
Days 1-3...

A cutoff low off the central CA coast will pivot slowly southeast
to the southern CA coast through Wednesday and then likely linger
over southern CA/Desert Southwest through Friday. The new suite of
guidance has lessened the interaction between the northeast stream
impulses digging across the Northern Plains and this feature,
leaving the closed low over the Southwest to meander nearly in
place the next several days instead of being caught up and
shearing into the westerlies. With this in place, persistent
moderate moist advection will flow into southern CA, the Great
Basin, and into the Four Corners, with snow levels ranging from as
low as 4000 ft beneath the upper low, to above 6000 ft in the warm
conveyor of moisture. Above these levels, and in much of the
terrain, heavy snow is likely. WPC probabilities on D1 are high
for 8 inches in the Southern Sierra and transverse ranges of
Southern CA, and remain high on D2 across these areas while also
shifting east into Nevada and the Wasatch of Utah. By D3 as the
upper low begins to retrograde, moisture and forcing get cutoff
and shunted east, so only moderate snow is likely Thursday night
into Friday, and WPC probabilities for 6 inches are only
mentionable in the transverse ranges of CA, and east across the CO
Rockies.


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

Weiss