Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
331 PM EDT Mon Mar 11 2024

Valid 00Z Tue Mar 12 2024 - 00Z Fri Mar 15 2024

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

A strong upper level trough taking on a negative tilt as it
approaches the British Columbia coast will accompany a strong
130kt 250mb jet streak that positions its divergent left exit
region over the Pacific Northwest and northern California this
evening. The upper trough will direct a healthy fetch of 850-700mb
moisture at the Pacific Northwest and northern California while at
the same time, a surface frontal system heads east for the WA/OR
coast at 00Z this afternoon, helping to provide additional lift
along and out ahead of the front. Add in a strong upslope
component with WSW winds within the mean 850-300mb flow pattern
and this is a classic recipe for heavy mountain snow along the
Cascades, the Olympics, and as far south as the Salmon/Trinity
mountains of northern California. The WPC Snowband Probability
Tracker shows snowfall rates gradually picking up in intensity
beyond 00Z with rates generally hovering between 1-2"/hr, although
some members of the HREF suggest up to 3"/hr rates are possible.
WPC PWPF sports high chances (>70%) for >12" of snow in the WA/OR
Cascades with elevations above 4,000ft seeing totals that could
event approach two feet. Later tonight, the steady surge in 700mb
moisture will push inland into the northern Great Basin and
northern Rockies with the same upper level divergent flow also
present. This will support periods of heavy snow in mountain
ranges such as the Blue Mountains, the Boise/Sawtooth, and
Bitterroots tonight, then snow will pick up in the Tetons, Bear
River Range, northern peaks of Nevada, and the Wasatch on Tuesday.
WPC PWPF shows high chances (>70%) for >8" of snowfall in the
Blue, Boise/Sawtooth, Bitterroots, and Tetons through Tuesday.

As the 500mb trough works its way into the interior Northwest on
Tuesday, 500mb height falls will ensue over southern Montana and
Wyoming while residual Pacific moisture streams over the
Intermountain West. While snow rates will gradually taper off in
the Northwest and northern California, periods of heavy mountain
snow will transpire over the Wasatch, Absaroka, and Big Horns late
Tuesday and into Wednesday. WPC PWPF shows moderate chances
(40-60%) for snowfall totals >6" Tuesday night into Wednesday in
these mountain ranges. The WSSI shows mostly Minor Impacts in
these mountain ranges, although localized Moderate Impacts in the
higher terrain cannot be ruled out.

...Central Rockies, Southern Rockies, & Central High Plains...
Days 2-3...

**A long duration significant snowstorm is becoming more likely
from the Southern and Central Rockies to the Central High Plains
beginning Wednesday and lasting through Thursday.

Tuesday night features a shortwave trough in the Great Basin
tracking towards the Central Rockies, which is responsible for
some modest 500mb PVA aloft as well as a strengthening 110kt 250mb
jet streak over northern AZ whose divergent left-exit region will
be placed over Colorado. Periods of moderate-to-heavy snow will
ensue over western Colorado, while at the same time, low pressure
in lee of the Colorado Front Range deepens Wednesday AM. In fact,
the expectation is for a 700mb low to form over eastern Colorado
which will wrap 700mb moisture around the northern and western
flanks of the low. Stronger vertical velocities aloft and heavier
precipitation will allow for snow to transition to snow in the
foothills of the Front Range from southeast Wyoming on south along
I-25 to the Denver/Boulder metro and Palmer Divide during the
daytime hours. Farther north, high pressure building over southern
Canada and the Northern Rockies will also support northeasterly
flow into the mountain ranges of southern Wyoming on south along
the Front Range of the Colorado Rockies through Wednesday night.
The lead surface low Wednesday night will track into central
Kansas with a robust deformation axis setting up from far
northeast Colorado and far northwest Kansas to central Nebraska.
Boundary layer temperatures will be above freezing to start, but
should the column cool enough late Wednesday night and into
Thursday morning to dip below freezing, a narrow band of heavy
snow could occur within this corridor. This does bear watching in
future forecast cycles.

Back to the Intermountain West, an upper level low is expected to
deepen over the Southwest thanks to impressive anti-cyclonic wave
breaking over southwest Canada, forcing the trough to steadily
plunge south through the Lower Colorado River Valley on Thursday.
This setup will continue to maintain healthy diffluent flow over
Colorado, eastern Utah, northern New Mexico, and northern Arizona,
fostering periods of heavy mountain snow Wednesday night and into
Thursday. A big question mark is whether or not 700mb moisture
flux will persist into the Central Rockies, which is heavily
dependent upon the strength/speed of the initial shortwave trough
that produced snow on Wednesday, and the strength of high pressure
to the north. Farther southwest, 700mb moisture will becomes
wrapped around the northern flank of the upper low in the
Southwest, giving risee to heavy mountain snow in the Mogollon Rim
and San Juans by ate Thursday. Ensemble guidance over the past 24
hours has trended towards the upper low being positioned over the
Lower Colorado River Valley and less so over central Arizona.
There is also uncertainty in the positive tilt/strength of the
ridge over the northwestern CONUS, which is also playing a role in
the depth and forward speed of the upper low. Regardless, a steady
diet of 700mb moisture associated with healthy diffluent flow at
upper levels and upslope flow into the San Juans, Front Range, and
event as far south as the Sangre De Cristo is a recipe for heavy
snowfall that sticks around into Thursday night.

Overall, this setup is likely to be one that produces snowfall for
multiple days in the central and southern Rockies starting
Wednesday and lasting through the end of the work-week. WPC's
Probabilistic WSSI (WSSI-P) shows the Front Range, Palmer Divide,
and on south to the Sangre De Cristo with moderate probabilities
(40-60%) for Moderate Impacts Wednesday evening and into Thursday
evening. WPC PWPF shows high probabilities (>70%) for snowfall
totals in these aforementioned ranges, as well as in parts of the
Wasatch, the various ridge lines of western Colorado, and
Wyoming's Medicine Bow and Laramie Ranges. This remains a fluid
forecast heavily reliant upon the depth/track of the upper low in
the Southwest, so residents in these areas should monitor the
forecast closely over the next 24-48 hours.

The probability of receiving at least 0.1" ice is less than 10%
across the CONUS through Day 3.


Mullinax