Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
451 PM EDT Sat Apr 25 2020

Valid 00Z Sun Apr 26 2020 - 00Z Wed Apr 29 2020

...Northeast...
Days 1-3...

A band of accumulating snow is possible over inland parts of NY
State across central/northern New England (along with heavier snow
in terrain) Sunday night through Monday.

A fairly compact occluded low over the lower OH valley this
afternoon will be reinforced by a shortwave currently over the
south-central High Plains. The reinforcing wave will round the
trough and promote coastal low development Sunday over the central
Mid-Atlantic which will become the dominant (but not rapidly
intensifying) surface low as it slowly shifts northeast past Long
Island Sunday night and Cape Cod Monday. While the central surface
pressure is not expected to intensify (remain just above 1000mb),
the upper levels will encounter a favorable development
environment with a coupled jet streak structure downstream
(northern stream jet over Quebec and the one south of the low over
the Southeast. This will amplify and enlarge the upper portions of
the system, particularly in the preferred ECMWF solutions, and
cause it to slowly swing negatively tilted over the Northeast,
making for only slow, steady movement northeast through Monday
night.

Warm air advection ahead of this low will shift the northern edge
of precipitation northeastward from PA into northern New England
Sunday. Despite a mesoscale band developing north of the low level
trough as energy is translated to the coastal low Sunday, the
strong diurnal effects of late April are expected to overcome
dynamical cooling and keep precipitation rainfall during day light
hours Sunday except for the highest terrain of the Alleghenies of
northern PA/western NY. Frontogenesis in the mesoscale band
intensifies Sunday night especially north from the Catskills and
along a swath to the E/NE across interior New England to coastal
ME. This frontogenesis will occur concurrently with strengthening
mid-level deformation pivoting across the region and mid-level
omega becomes maximized in a region of high DGZ RH. This, on top
of ageostrophic cooling from a favorably placed Canadian high
pressure, suggests p-type will transition from rain to snow Sunday
night in all but the lowest terrain of Upstate NY and southern New
England. While the heaviest accumulations should be confined to
the terrain above about 1500 ft, where the most intense ascent
occurs, dynamic cooling should bring light accums all the way to
the valleys, and even to the coast of Maine. Day 2 WPC
probabilities for 2 or more inches are low to moderate for the
Catskills, Adirondacks, Greens, Berkshires, Whites, and much of
north-central Maine with moderate probabilities for 4 or more
inches limited to the highest Adirondacks and the White Mtns.
There are moderate Day 2 probabilities for 8 or more inches for
the Presidential Range of the Whites in NH.

As the negatively tilted low re-closes east from the Gulf of Maine
Monday night, wrap around snow due to continued deformation,
continued cold air drainage, and modest upslope ascent should
produce additional light to possibly moderate snow. The 12Z ECMWF
and UKMET both feature a persistent interior New England (or even
Hudson Valley with the UKMET) snow band Monday night and even
would feature ocean enhanced precip/possibly snow over Cape Cod.
Day 3 snow probabilities for 1 or more inches are moderate across
the Adirondacks, greens, Monadnocks of southwest NH and the
Worcester Hills of north-central Mass. Will need to monitor the
placement of the mid-level forcing for this feature in future
forecasts.


...Pacific Northwest...
Day 2...

An active pattern for BC and WA will continue with the next
trough/frontal pattern from a deep low over the Gulf of Alaska
will arrive Sunday night and push inland Monday. A moisture plume
of subtropical Pacific origin will keep snow levels high,
generally 5000 to 6000ft over the Olympics and northern Cascades.
Southwesterly flow should limit most interior precip to southwest
Canada with only moderate precip for the far northern US Rockies
(where snow levels will also be above 5000ft later Monday). Day 2
WPC snow probabilities for 6 or more inches are limited to the
higher WA Cascades and Mt. Olympus. There are moderate Day 2.5
snow probabilities for 4 inches for the highest parts of Glacier
NP in MT.


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

Jackson