Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
257 PM EDT Fri Apr 5 2024

Valid 00Z Sat Apr 06 2024 - 00Z Tue Apr 09 2024

...Northeast...
Day 1...

The nearly stationary and vertically stacked nor'easter will
gradually begin to pivot away from New England tonight and Saturday
as it drops southward and then ejects east and weakens considerably
late D1. This anomalous track is due to the persistent and
anomalous upper low sitting over the Northeast, which will
continue to send lobes of vorticity cyclonically around it to
impact the track of the surface low. Despite the slow movement,
total forcing will be waning tonight as the upper jet remains well
east of the region and height falls are no longer driving ascent.
Moisture will also begin to wane, especially the second half of D1
as the flow becomes more unidirectional from the north as the low
pulls away, which when combined with marginal thermals and early
April sun should limit accumulations across New England despite
several rounds of snowfall expected through the day. The exception
will likely be confined to the highest terrain where colder
temperatures could still support additional moderate accumulations,
reflected by WPC probabilities for more than 4 inches reaching as
high as 50-80% in the highest terrain of the Adirondacks of NY.
Whites of NH, and ranges of northern ME.


...Western U.S....
Days 1-3...

Extremely amplified pattern across the CONUS will manifest as an
anomalously deep 500mb trough closing off and advecting slowly
across the Intermountain West with anomalies reaching as low as -4
sigma at 700-500mb according to NAEFS. The slow eastward advance of
this strong trough will drive widespread large scale ascent from
the Pacific Coast through the High Plains, with additional lift
provided via the diffluent LFQ of a downstream upper jet streak
pivoting through the base of the trough. The combination of this
jet streak with the associated height falls and waves of PVA will
result in widespread precipitation falling as snow, especially in
the terrain, with several surface lows also contributing to the
precipitation.

The most significant snowfall and associated impacts are likely
D1-D2 as the closed low pivots northeast from the Great Basin into
the Northern Rockies while weakening, becoming displaced by a
secondary strong lobe of vorticity amplifying into a closed low
over the central High Plains by Saturday night. This will help
drive dual surface lows, one lifting north from the Great Basin
towards the Northern Rockies, with a second more intense low
developing in the lee of the Rockies Saturday morning and then
rapidly deepening as it moves east into the Central Plains on
Sunday.

The first surface low and accompanying broad upper level ascent
will spread precipitation northward through the Great Basin and
into the Northern Rockies, with heavy snow likely above 3000-4000
ft. Heavy snow rates of 1"/hr are possible, especially where
upslope flow can contribute to greater ascent in parts of the
Northern Rockies. WPC probabilities for more than 6 inches are
above 70% in a widespread area from the Blue Mountains of OR
through the Sawtooth/Salmon River ranges, into the Absarokas and
Northern Rockies, with additional high probabilities stretching
into the Ruby Mountains and Northern Wasatch. Locally 1-2 feet is
possible in the higher peaks of these ranges, with light
accumulations also possible into the valleys.

The more substantial snow event will begin Saturday morning in the
CO Rockies/San Juans and then pivot into the Front Range as the lee
low develops. As this low then strengthens, impressive moisture
advection on broad isentropic ascent out of the Gulf of Mexico will
intensify and surge a theta-e ridge northward into the central High
Plains. This additional moisture and instability will help expand
and intensify the precipitation shield, with a focus of moisture
wrapping cyclonically into MT/WY. This moisture will impinge into a
strengthening deformation axis which will sharpen NW of the low and
into the secondary upper impulse, providing additional ascent,
resulting in a NW to SE oriented axis of heavy snowfall. While the
low-level thermal structure will remain marginal, this intense
ascent could result in 1-2"/hr snowfall even outside of terrain
features, which will help dynamically cool the column and result in
heavy snow accumulations across a significant swath of the High
Plains, with the most significant accumulations likely in the Black
Hills, Pine Ridge, Laramies, and Big Horns. In addition to the
heavy snow rates, strong winds gusting over 50mph will likely
produce significant blowing snow impacts, with blizzard and near
blizzard conditions possible. WPC probabilities for this portion of
the event D1 are high for more than 6 inches from the San Juans
into the CO Rockies, and then expand northward D2 where
probabilities reach as high as 80-90% in the Laramie Range, Pine
Ridge, and Black Hills. Heavy accumulations are also expected all 3
days in the Big Horns, where locally 2+ feet is possible.


Weiss