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

Valid 00Z Thu Apr 09 2020 - 00Z Sun Apr 12 2020

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

...Significant snow event likely for parts of Northern New
England...

A northern stream low pressure system shifting through Ontario
will weaken as energy shifts to a surface low developing off the
Massachusetts coast. This secondary low will deep extremely
rapidly Thursday night and Friday as a potent shortwave rotating
through a negatively tilting trough across the Great Lakes acts in
tandem with the LFQ of an anomalously strong jet streak and
increasing coupled jet structure to drive intense synoptic ascent.
Initially, precipitation will spread across New England on strong
WAA ahead of the first low, but as secondary development occurs,
mesoscale forcing in the form of modest frontogenesis and intense
deformation will drive precipitation. Although the column will
initially be marginal for snow outside of Maine and the high
terrain, there is a high likelihood that isallobaric cooling and
dynamic cooling will occur as the surface high wedges weakly into
Maine, and the surface low experiences explosive deepening. During
this time, precipitation through much of the Northeast Kingdom of
VT, the terrain of N/Central NH, and much of northern/central
Maine wile turn to heavy snow. Instability will be enhanced by a
robust theta-e ridge rotating within the TROWAL into Maine, and
where this occurs in conjunction with -EPV should lead to
thunder-snow. WPC experimental snowband tool and HREF
probabilities suggest snow rates will easily eclipse 1"/hr, and
may reach 3"/hr at times in the region of strongest forcing
collocated with the pivoting deformation band. While banding is
likely and will cause a large range in snowfall amounts, WPC
probabilities have increased and show a high risk for 6 inches
across this region, with a narrow corridor of greater than 20%
probability for 18 inches, most likely where the longest banded
structure pivots through the first part of D2. While pinpointing
the axis of heaviest snow is still difficult at this time range
and may shift depending on the exact position of features, there
is high confidence that the terrain from the northern Adirondacks,
through parts of the Greens, and points eastward into Maine, will
experience a significant snow event, with major accumulations most
likely across northern and central Maine.

On Friday, NW flow behind the departing system combined with
spokes of PVA will drive periodic snow showers, some of which may
be of moderate intensity due to upslope enhancement. WPC
probabilities for 4 inches are moderate to high on D2 in the
favored upslope regions of the Adirondacks and northern VT, as
well as downwind of Lake Erie. Some light accumulations are also
possible into the upslope region of WV on Friday.


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

A cutoff upper low will drift onshore Southern CA Thursday morning
and then pivot around the Great Basin, completing a cyclonic loop
through Saturday morning in response to spokes of shortwave energy
rotating around it. This will persist 2 more days of ascent and
moist advection on mid-level confluent flow, but with subtle
weakening as the low begins to fill, especially by Day 2 /Friday/.
Snow levels through Friday will range from 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, Thursday
night and Friday, the forcing becomes more diffuse through PVA and
mid-level divergence, spreading the heaviest snow into the CO
Rockies, while persisting in the transverse ranges of Southern CA.
Day 2 probabilities for 6 inches are high in these areas only.


...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. Just ahead of this feature a strong cold front
will drag southward into MT and the Northern High Plains. This
baroclinic gradient will be acted upon by synoptic forcing
approaching from the NW as height falls and PVA associated with
the shortwave, as well as LFQ diffluence ahead of a jet streak
provide regional ascent. Additionally, flow behind the cold front
will become upslope into the terrain, and snowfall will likely
develop int he Northern Rockies and spread eastward with time into
the High Plains and as far south as the Big Horn range in WY. WPC
probabilities for 4 inches are high in the NE foothills east of
the Rockies where upslope becomes enhanced, as well as in the
higher terrain from Glacier NP through the Absarokas and Big
Horns. Much higher amounts are possible, and some low end
probabilities for 12 inches already exist for D3.


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

Weiss