Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
237 PM EST Mon Nov 4 2024

Valid 00Z Tue Nov 05 2024 - 00Z Fri Nov 08 2024

...CO/NM border...
Day 1...

Upper low over NM this morning is lifting northeastward across the
TX Panhandle this afternoon and will become a more progressive open
wave as it enters the High Plains tonight. Snow on the NW side of
the low within a region of modest PVA and enhanced upslope into the
Raton Mesa will persist for a little while longer this afternoon
before tapering off overnight. Light snow will extend eastward
into the far western OK Panhandle as rain changes to snow but minor
accumulations are expected in these areas.

...Northwest and Northern/Central Rockies...
Days 1-2...

A vigorous shortwave trough racing through the Pacific Northwest
today is responsible for blanketing the Cascades and tallest peaks
of the Olympics in heavy snow today. Snow levels will fall to as
low as 3,000ft this evening and into tonight, allowing to snowfall
accumulations to occur across many of the lower passes across the
Cascades. As the 250-500mb trough moves inland today, the
diffluent left-exit region of the approaching jet streak will be
favorably placed of the Northern Rockies and, in turn, helps spawn
low pressure over the Canadian Prairies this evening. The upper
level divergence associate with the upper level jet will march east
into the Northern Rockies tonight, which combined with a surge in
700mb moisture will support periods of heavy snow along the
Bitterroots, Lewis Range, and Tetons. Lingering Pacific moisture on
the backside of the 700mb low near the Montana/Saskatchewan border
will stick around much of the day Tuesday with periods of heavy
snow unfolding along the Tetons, Little Belt, Big Snowy, and Big
Horn Mountains. Snowfall should gradually taper off in these areas
by Wednesday morning.

WPC PWPF shows high chances (>70%) for snowfall totals >8" over
parts of the Bitterroots, Lewis Range, Little Belt, and Big Snowy
mountains. These ranges, especially >6,000ft, have moderate-to-high
chances (50-70%) for >12" through Wednesday afternoon. Some of the
tallest peaks of these ranges could surpass 24" locally. The WSSI
highlights Moderate Impacts for many of these ranges (above 4,000ft
in the Cascades, above 6,000ft in the Northern Rockies ranges
listed) while the peaks of all ranges mentioned sport Major Impact
potential as a result of this early November snowstorm.

...Four Corners...
Day 3...

The same upper level feature responsible for heavy snow in the
northern Rockies will dive south on tuesday through the heart of
the Intermountain West and into the Four Corners region by early
Wednesday morning. By Wednesday, the upper level trough will cut
off into an anomalous upper level low thanks to a staunch anti-
cyclonic wave break event over the northwestern US and southwest
Canada. There does remain some uncertainty on whether this upper
low will feel the influence of a lingering disturbance over the
Central Plains (GEFS) or be fully cut off from the mean steering
flow pattern for multiple days (ECMWF EPS). While the exact
evolution is unclear beyond Day 3, this synoptic scale setup is
likely to produce the first significant winter storm of the season
in the Southern Rockies. The stage becomes set initially by a cold
front diving south to usher in a colder air-mass throughout the
region. Then, as the upper level low cuts off over the Four Corners
region Wednesday morning, strong upper level divergence aloft will
support excellent large scale ascent atop the atmosphere. Next, as
the upper low late Wednesday into Thursday deepens, the moisture
flow over the southern High Plains will accelerate, prompting
southerly IVT values above the 90th climatological percentile (on
both NAEFS and the ECMWF SATs) to be directed at the southern
Rockies.

Look for periods of snow to initially occur along and in wake of
the cold frontal passage from the Wasatch on east to not just the
Central Rockies but parts of the central High Plains as well. As
high pressure builds in to the north, sub-freezing air rushing
south along the Front Range of the Rockies will provide the
opportunity for periods of snow within the Denver/Boulder metro
area Tuesday night and into Wednesday morning. This upslope
component does provide a good setup for heavy snowfall in parts of
the San Juans, the Front Range, and the Sangre De Cristo through
the day Wednesday and into Thursday. The key on Thursday will be
where the upper low sets up and the extent of the moisture/upslope
flow into the Southern Rockies. Latest guidance trended a little
drier over parts of the south-central Colorado Rockies, but the
prolonged upslope into late Thursday-Friday may still result in
additional heavy snowfall by the end of the week.

WPC PWPF for the event through 00Z Fri shows high chances (>70%)
for snowfall >8" in the San Juans, the Sangre De Cristo, and along
the Front Range of the Rockies. For >12" probabilities, there are
high chances (>70%) along the Sangre De Cristo. The Denver metro
area sports low-to-moderate chances (30-50%) for snowfall >4" with
the southern suburbs on the higher end of that range. WSSI-P
moderate probabilities show moderate chances (40-60%) along the
Sangre De Cristo and along Raton Mesa Wednesday night and into
Thursday.


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


Mullinax