Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
250 PM EDT Sat Mar 30 2024

Valid 00Z Sun Mar 31 2024 - 00Z Wed Apr 03 2024

...The West...
Days 1-2...

Anomalous closed mid-level low positioned west of the CA coast
Saturday night will gradually pivot southeast towards Baja
California by Sunday night while shedding additional vorticity
energy eastward into the Great Basin. This evolution will manifest
as a large scale longwave trough angled SW to NE from CA through
the Central Rockies, with additional spokes of energy rotating
around and through this trough. At the same time, the pronounced
and prolonged SW mid-level flow will draw Pacific moisture
northeast into the region, with PW anomalies reaching as high as +1
sigma according to NAEFS which will supply plentiful moisture for
widespread heavy precipitation from CA through the Great Basin and
into the Central Rockies.

During this evolution, a surface low is likely to strengthen over
the Great Basin, with the accompanying warm and cold front slowly
pushing east into UT and CO by Monday. This low will gradually
occlude as it becomes vertically stacked beneath secondary cutoff
500mb low development, but increasing moist isentropic ascent
wrapping into the system will provide additional enhancement to
precipitation, especially across NV and UT, where sloped fgen will
drive higher precipitation rates, and the guidance suggests a band
of heavy snow with rates 1-2"/hr will pivot across NV Sunday and
into WY Sunday night, with secondary band potential along the cold
front across AZ/NM. Snow levels will in general be 5000-7000 ft
ahead of the primary trough on D1, but will gradually lower behind
the front and beneath the cold core low to 4000-5000 ft, remaining
around those heights until precip winds down late D2. This
indicates the heaviest snowfall will occur in the higher terrain,
but beneath these bands, especially from NV to UT to WY, the
convective potential of the snowfall could result in significant
accumulations even into the valleys.

WPC probabilities on D1 for more than 6 inches of snow are high
across a large expanse of the western terrain. Probabilities
exceeding 70% extend across the Transverse and Peninsular ranges of
CA, across the Sierra, and eastward to include the Mogollon Rim,
Wasatch, Uintas, Wind Rivers/Tetons, and the western CO Rockies/San
Juans. Locally 1-2 feet is possible in the higher terrain.
Additionally, a more broad coverage of moderate to high
probabilities for 6+ inches exists across central/northern NV where
the aforementioned band may lower snow levels and cause
accumulations even into the valleys. By D2 the heavy snow continues
across the Mogollon Rim and White Mountains of AZ, but otherwise
become confined to the San Juans and NW WY ranges where WPC
probabilities for more than 6 inches reaches 40-70%.


...Central High Plains into the Northern/Central Plains...
Days 2-3...

Large scale positively tilted upper trough oriented SW to NE across
the Intermountain West will gradually pivot eastward and amplify,
with a closed low likely forming over the Central High Plains
Monday evening. This low may weaken slightly as it shifts east into
Tuesday, but will remain pronounced with impressive vorticity
swinging northeast even as the primary trough axis lags into the
Desert Southwest. During this evolution, a strengthening and
persistent subtropical jet streak will arc SW to NE downstream of
the longwave trough axis, providing both additional ascent through
its diffluent LFQ, and additional moisture on transport from the
Pacific, into the region. The combination of this jet streak and
the continued 700-500mb SW flow will surge PW to above climo
values, although NAEFS standardized anomalies are modest overall.
This overlap of moisture and forcing, aided by a stationary front
draped west to east across the region, will allow for lee
cyclogenesis beginning late Sunday, resulting in an expansion of
precipitation through D3.

The coverage, intensity, and timing of precipitation is still very
uncertainty due to model discrepancies and run-to-run
inconsistency. However, it appears there will be two primary waves
of precipitation, one with the lead shortwave and accompanying jet
streak moving into the Plains Monday, with secondary heavy
precipitation developing Monday night into Tuesday around the
deepening low and within increasing upslope flow into the Central
Rockies as the stationary front sags southward as a cold front
behind the eastward advancing low pressure. Mesoscale forcing both
due to fgen in the lead precip shield, and due to intensifying
upslope into the terrain, will likely result in at least periods of
heavy snow rates and dynamic cooling to cause p-type changeover
from rain to snow at times in otherwise marginal thermals. This
will causes even lower confidence than usual in snowfall amounts,
but WPC probabilities are 40-70% for 4+ inches from the Black Hills
eastward along the NE/SD border, and above 70% along the Front
Range due to upslope enhancement. For D3, WPC probabilities wane
across the Plains, but continue into the Southern Rockies/Sangre de
Cristos where they remain 30-40% for 4+ inches.


The probability of significant icing is less than 10 percent
across the CONUS.


Weiss