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

Valid 12Z Mon Mar 11 2024 - 12Z Thu Mar 14 2024

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

Amplified closed low will continue to pivot into the Canadian
Maritimes Monday, with the accompanying surface low lifting
northeast out of Maine. Behind this low, strong CAA will persist
into Monday with gusty NW winds cooling 850mb temps to below -10C
which will steepen low-level lapse rates across New England to
support some increased instability. At the same time, this NW flow
will favorably upslope into N/NW facing terrain, generally upwind
of the Greens and Whites early Monday, resulting in continued
heavy snow with rates exceeding 1"/hr at times through the first
half of D1 before precip wanes due to the drying column. WPC
probabilities for more than 4 inches of snow are confined to the
higher terrain of the northern Adirondacks, Greens, and Whites,
where they exceeding 70%, and locally more than 8 inches of
additional snow is possible.


...The West...
Days 1-3...

The active pattern across the West continues into mid-week as
multiple shortwaves embedded within confluent mid-level flow
eventually result in an amplifying trough across the West. The
first of these shortwaves will push east across WA/OR this
morning, but then weaken quickly as it approaches the Northern
Rockies by tonight. This weakening will be due in part to shearing
of the energy in response to upstream activity dropping along the
British Columbia coast. However, enough ascent overlapped with
modest moisture downstream of the impulse will result in moderate
to heavy snow from the Cascades through the Northern Rockies, and
WPC probabilities for more than 6 inches reach above 80% for the
Olympics, along the crest of the WA and OR Cascades, into the
Shasta/Trinity region of northern CA, and across some of the
higher terrain of the Blue Mountains.

Immediately following the lead shortwave, the second, more
intense, closed low dropping along the British Columbia coast will
shed spokes of vorticity onshore the Pacific Northwest as early as
late Monday night, with more impressive height falls and PVA
advecting onshore during the day on Tuesday. This feature will
then rapidly deepen as embedded shortwaves and vorticity maxima
rotate southward along the upstream side of this trough, causing a
full latitude trough to develop across the Intermountain West,
with a potent closed low progged to setup over the eastern Great
Basin and then continue to amplify as it digs almost due south by
the end of the forecast period leaving an amplified ridge along
the West Coast. This will bring an end to snowfall by Wednesday,
but not before another round of heavy accumulation occurs on
Tuesday from the Cascades through the Northern Rockies and into
the Wasatch, with the heaviest snow likely above 3000 ft. WPC
probabilities exceeding 70% for more than 6 inches of snow on D2
continue across the Cascades and Olympics, but extend much farther
east than on D1 to include the Northern Rockies, The Tetons and
other NW WY ranges, and even parts of the Great Basin around the
Ruby Mountains and into the Wasatch Front. Lower probabilities
extend into parts of the Sierra and Big Horn range as well.


...Central Rockies...
Day 3...

A long duration significant snowstorm is becoming more likely for
the Central Rockies beginning Wednesday.

The mid-level pattern will become increasingly amplified beginning
Tuesday as a closed mid-level low forms over the Northern Great
Basin, and then amplifies while dropping due south towards the
Desert Southwest on Wednesday. This mid-level evolution will
result in an intensifying downstream jet streak rotating around
the base of the trough, and the overlap of impressive divergence
and upper diffluence will help spawn a surface low over the
Southern High Plains. This low will strengthen along a cold front
and the accompanying baroclinic gradient, resulting in
additionally enhanced ascent. At the same time, moisture advection
will increase on SW flow pivoting out of the Pacific and around
the base of the trough, driving increasing 300-305K moist
isentropic upglide. The result of this deep layer ascent into the
moistening column will be an expanding area of precipitation
beginning Wednesday evening, with heavy snow likely developing on
the north side of a southward advancing cold front. This front
will additionally produce northeast post-frontal winds to upslope
into the terrain, with the cooling also resulting in a deepening
DGZ as reflected in SREF probabilities.

Snow levels will fall gradually through the end of D3, starting
around 6000 ft to start D3 then falling to around 4000 ft by the
end of the forecast period, which suggests the heaviest snow will
likely occur in the Front Range, but additional heavy snow is
likely in the Cheyenne Ridge, Palmer Divide, and down into the
Sangre de Cristos as well. The models still feature some spread in
low placement and intensity, and whether the best forcing can
align appropriately into the DGZ to produce the most intense snow
is still uncertain, which is reflected in model camp separation in
the WSE plumes, but overall there is increasing confidence in a
long duration heavy snow event for much of the region beginning
Wednesday and possibly persisting into the weekend. Impacts are
likely to become significant by Thursday morning as reflected by
WSSI-P probabilities. WPC probabilities D3 for more than 6 inches
are already above 70% from the Laramie Mountains southward along
the Front Range and into the Sangre de Cristos, including the
Palmer Divide and much of the CO Rockies. Before this event ends,
multiple feet of snow are possible in the higher terrain, with
impacts to the I-25 urban corridor also probable.


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




Weiss