Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
209 AM EST Fri Feb 02 2024

Valid 12Z Fri Feb 02 2024 - 12Z Mon Feb 05 2024

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

Widespread precipitation will continue to expand across the
Western CONUS as an elongated and amplified trough extends across
the region. A lead shortwave moving across southern CA and into
the Great Basin Friday will slowly advect east and amplify into a
closed low over the southern High Plains by Saturday morning. At
the same time, a second closed low will be dropping along the OR
Coast, resulting in a large inverted longwave trough with a
pronounced negative tilt draped from Texas through the Pacific
Northwest. This will produce lowered heights across nearly the
entirety of the western CONUS to support broad ascent.

As this trough evolves, moisture will spread across the West,
driven by both an atmospheric river (AR) advecting farther inland
today with IVT of 250 kg/ms or more, leading to PW anomalies that
will reach +2 to +3 sigma across a large part of the region
according to NAEFS. At the same time, a surface low is likely to
develop in the lee of the Rockies Saturday morning, which will
result in additional moisture advection as a theta-e ridge from
the Gulf of Mexico becomes enhanced and surges northward into the
Central and Northern Rockies. The convoluted upper level evolution
will result in a stationary boundary/surface trough developing
east of the terrain, into which this lower-level moisture will
isentropically ascend to expand precipitation northward. This will
manifest as upslope flow into the eastern terrain, which will be
enhanced across the Northern Rockies as a cold front drops
southward during D2. Most of the ascent during this period will be
modest, with some local enhancement likely across the Northern
Rockies due to the overlap of upslope flow/mid-level fgen, and
near the Front Range where easterly flow will become orthogonal to
the terrain, driving intense upslope ascent which will transition
to a pivoting deformation axis as the low pulls away late Saturday.

In general, with the airmass across the West being sourced from
the Pacific, snow levels will be generally high at 6000-7000 ft,
although the steepening lapse rates within the broad trough could
at times cause precipitation loading to lower the snow levels.
This should keep the heaviest snow accumulations confined to the
terrain, with more modest snowfall likely into portions of the
High Plains only during periods of more intense ascent.

On D1, WPC probabilities for more than 6 inches of snow are high
(80%+) along the Sierra, into the White Mountains of AZ, and
across most of the San Juans. Above 6000 ft in these ranges, local
snowfall exceeding 1 foot is likely on D1. Elsewhere across the
West, WPC probabilities 40-70% for more than 6 inches across the
northern CA ranges, The Uintas and Wasatch, as well as much of the
CO Rockies and Sangre de Cristos. During D2, the area of heavy
snowfall becomes more focused along the theta-e ridge north of the
surface low from the Sangre de Cristos through the Northern
Rockies. The heaviest snowfall D2 is expected in the Front Range
and Park Range of CO, with a secondary maxima near Glacier NP in
the Northern Rockies. WPC probabilities for more than 6 inches on
D2 are above 80% in these areas, with local totals above 1 foot
likely. Elsewhere in this swath, many of the ranges above 6000 ft
could see 5-10 inches of snow.


...California and Nevada...
Days 2-3...

The ongoing atmospheric river (AR) will shift inland today, but
will be followed almost immediately but a second, even stronger,
AR will approach the CA coast Saturday night and then shift
onshore Sunday. Max IVT within this second AR will likely exceed
500 kg/ms (60% chance from the ECENS, 80% from the GEFS) and shift
onshore southern/central CA with impressive IVT exceeding 250
kg/ms persisting well east of the Sierra Crest and spilling over
onto the Great Basin. This AR will be driven by impressively
confluent mid-level 700-500mb flow southeast of a closed low,
angling intensifying winds to drive the intense moisture advection
eastward. This will be overlapped by a robust and zonally oriented
170kt subtropical jet streak. Periodic spokes of vorticity
shedding inland combined with the LFQ of the upper jet will
provide strong deep layer ascent, resulting in a swath of heavy
precipitation beginning early Sunday and persisting into Monday
downstream of a potent surface low which will only gradually
weaken as it approaches the northern CA coast by Monday morning.

Snow levels during precip onset will be modestly low, around 3000
ft, but will rise steadily within the accompanying WAA, reaching
7000 ft in the Sierra by the end of D3, and 4000-5000 ft into the
Great Basin and northern CA. With warming snow levels, the snow
will likely become heavy and wet with low SLRs, so impacts are
expected to be substantial, especially in the Sierra as reflected
by high probabilities for major impacts in the WSSI-P due to snow
load and snow amount. Snow begins to accumulate late D2 in the
Sierra, but will become much more widespread and heavy during D3
when WPC probabilities for more than 6 inches rise above 90%, with
significant spill-over snow also probable. Snow levels climb
through the day, resulting in an increasingly wet snow, and with
snowfall likely eclipsing 36 inches above 7000 ft, this will
result in major impacts to some of the higher passes across the
region.


The probability of significant icing greater than 0.10" is less
than 10% for days 1-3.


Key Messages for next Atmospheric River into California are below:

Strong Atmospheric River
Another Atmospheric River (AR) event is expected to impact
California beginning Sunday and continue through Tuesday. This AR
will be of longer duration and potentially stronger than the one
that just moved through California.

Heavy Sierra Nevada Snow
A significant winter storm is anticipated for the Sierra Nevada,
with several feet likely in higher elevations. Disruptions to
daily life, including difficult to impossible travel conditions
are expected.

Risk of Excessive Rainfall and Flooding
Heavy rainfall is expected to impact lower elevation and coastal
areas, and some of the rainfall totals could be locally
significant from central to southern California including the
Transverse Ranges. Flash, urban, and river flooding, along with
debris flows, and mudslides, with locally considerable impacts are
likely.

Strong Winds and High Surf
Powerful, damaging onshore winds are expected over the central CA
coast and Coastal Ranges, with high and potentially damaging surf.


Weiss