Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
434 AM EDT Wed Apr 14 2021

Valid 12Z Wed Apr 14 2021 - 12Z Sat Apr 17 2021

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

A late season nor'easter will bring rain changing to heavy snow
across primarily interior portions of the Northeast and New
England through Friday.

A 500mb closed low progged to reach -2 standard deviations below
the climo mean and within the bottom 10th percentile for mid-level
temperatures according to NAEFS ensemble tables will drop
southeast from Michigan Thursday morning to near Long Island
Friday morning to finally east of Cape Cod Saturday morning. This
slow moving feature will maintain intensity through the period as
it is reinforced by waves of vorticity rotating around it. At the
same time, a zonally oriented jet streak south of the primary
trough axis will intensify, and the resultant LFQ diffluence
aligned with significant height falls will promote surface
cyclogenesis off the VA coast Thursday morning. This low will then
deepen as it lifts slowly northeast to a position near Cape Cod
Friday morning and then stalls briefly as it becomes stacked
beneath the upper low.

Robust moist advection will precede this mid-level trough as
moisture from both the Gulf of Mexico and the Atlantic are drawn
northward into the system. Rich theta-e advection wrapping around
the deepening low as a WCB/TROWAL will help spread heavy
precipitation northward into New England, and heavy QPF is likely.
Initially, all of the precipitation should be rain. However, as
cold advection commences behind the system and within the upper
low, rain will begin to change to snow. This cooling will likely
be intensified as well by dynamic effects through a pivoting
deformation axis overlapped with mid-level fgen, and some elevated
instability on the periphery of a westward advancing dry slot into
New England. As rain changes to snow, it will likely be heavy at
times where any banding can occur, and also where low-level
upslope flow can enhance the already intense dynamics. Heavy snow
is most likely in the terrain of the Catskills, Adirondacks, and
Whites, but most impressively in the Berkshires and Greens where
the longest duration of heavy precip and cold temperatures are
predicted.

SLRs during this event are likely to be quite low both due to
marginal thermal profiles and the fact that it is April, with
daytime snow across eastern new England struggling to accumulate.
In fact, many of the SLR ensemble plumes depict ratios as low as
4:1, suggesting outside of the terrain snowfall will struggle to
accumulate. Still, periods of moderate to heavy snow are likely,
and WPC probabilities for 4 inches are high on D2 in the
Adirondacks, and especially Greens and Berkshires generally above
1500 ft. Locally more than 8 inches is possible. On D3, the
heaviest snow should shift northeast to include the Whites of NH
and ME, where WPC probabilities are again high for 4 inches with
locally more than 8 possible. Additionally, some light snow is
possible as far as the NH/ME coast as cold air funnels into the
systems despite mid-April sun angle.


...Great Basin to Central High Plains...
Days 1-3...

Long duration snowfall begins today across the Rockies as a deep
closed low moves slowly form the Great Basin to the Central Plains
while gradually weakening. This closed low is positions over
Nevada this morning and will maintain intensity through Thursday
evening will drifting eastward in an amplified flow pattern across
the CONUS. Height falls, PVA, and mid-level divergence all
combining across the Great Basin and Rockies will drive deep layer
ascent, aided by persistent diffluence within the LFQ of a
subtropical jet streak south of the trough axis. While guidance
still features some discrepancy into the placement of both the
mid-level wave and resultant surface low, there is better
agreement this morning overall.

Embedded within this jet streak and due to long duration fetch
from the Pacific south of the 500mb low, moisture will begin to
increase across the region. However, the most robust moist
advection will stream out of the Gulf of Mexico Thursday into
Friday as 850-700mb winds back to the S/SW driving PWs to +1.5
standard deviations above the climo mean. The prolonged synoptic
ascent within this moist airmass will spread precipitation across
the region, and by Thursday night there is likely to be an
expansive plume of snowfall covering much of the Central Rockies
and High Plains.

The heaviest snowfall is likely to be in the upslope favored
terrain of the Uintas, Wind Rivers, southern Absarokas, Tetons,
and Front Range, where WPC probabilities on D1 and D2 are moderate
to high for 6 inches. While widespread intense snowfall (rates
>1"/hr) are not expected, the enhancement due to upslope in the
colder high elevation areas could produce locally well in excess
of 12", highest in the Uintas and Wind Rivers. Additional heavy
snow is likely in the Front Range. Late D2 and into D3, the
heaviest snowfall is progged to spread eastward into the High
Plains before the primary wave opens and shunts southward. While
prolonged moderate snow is likely during this time frame, marginal
thermal structure outside of the terrain (generally below 4000 ft)
will limit total accumulations. Still, WPC probabilities are
moderate for 6 inches in the Black Hills, Pine Ridge, and NE CO
Plains. As the forcing shunts southward D3, heavy snow should
become confined to the Front Range once again.


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

Weiss