Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
322 AM EST Fri Nov 06 2020

Valid 12Z Fri Nov 06 2020 - 12Z Mon Nov 09 2020


...Western U.S....
Days 1-3...

...A major winter storm is likely for parts of Montana this
weekend with heavy snow and strong winds...

A closed 5H low dropping across Oregon this morning will move
quickly southward into the southern Great Basin Saturday night
while filling, and being followed closely by a secondary closed
low moving south out of British Columbia. The interaction of these
two features will lead to a larger mid-level gyre across the West,
with trailing shortwave digging into the Great Basin Sunday,
causing the lead impulse to lift into Montana while closing off
once again. At the same time, a potent 130kt jet streak will
blossom downstream of the sharpening trough, placing the favorable
LFQ and a divergence maxima lifting across the Rockies and into
Montana Saturday/Sunday. While heavy snow is likely across most of
the western terrain during the next few days, the most impactful
snow is likely across Montana this weekend.

For Montana, the combination of sharp height falls and intense
upper divergence will drive pressure falls, and surface
cyclogenesis is likely Saturday in Wyoming and then lifting
northeast across Montana and into Manitoba by late Sunday. Strong
WAA ahead of this system will transport anomalous moisture (PWs of
+2 standard deviations) as Pacific and Gulf moisture lift
northward. A cold front dropping across the Plains early in the
forecast period will chill low-levels of the column, with S/SW
flow leading to increasing 300K isentropic ascent to spread
precipitation across much of MT. Initially, the column outside of
the terrain will be too warm for snow, but intense forcing through
synoptic and mesoscale ascent, including a developing and intense
TROWAL and isallobaric acceleration into the deepening low will
dynamically cool the column to allow for a changeover from rain to
snow. A brief period of freezing rain is also possible in eastern
MT early on Sunday. However, the majority of the precipitation
from the Northern Rockies and points east will be snow, and this
should be extremely heavy at times. Strong ascent coupled with
anomalous moisture should produce snowfall rates that will exceed
1"/hr as the TROWAL pivots overhead, and WPC probabilities have
increased tonight. There is now a high risk for 8 inches from the
Northern Rockies near Glacier N.P. and points east to Great Falls,
Havre, and Glasgow and widespread 10-15" is becoming more likely.
Locally 20" is possible in a few locations, and this event could
challenge November record snowfall across parts of Montana.
Additionally, strong winds will accompany this snowfall to produce
significant blowing and drifting.

Elsewhere across the West, heavy snow is likely across most of the
terrain from the OR Cascades southward through the San Gabriels
and San Bernadinos of CA, and east through the Mogollon Rim, the
San Juans, and much of ID/WY. This broad and widespread snowfall
will be due to crashing snow levels beneath the aforementioned
train of shortwaves and upper lows, with 5H heights dropping to -3
standard deviations from the climo mean. Each shortwave will bring
additional forcing and moisture to the West, with snow levels
gradually lowering. Heavy snows Days 1 and 2 (Friday/Saturday)
will accompany the lead shortwave and intensifying jet streak,
with moderate/heavy snow spreading on D1 across the OR Cascades,
northern CA ranges, and the Sierra, and lighter snows extending
into the higher terrain of the Great Basin. WPC probabilities for
snowfall greater than 4" on D1 are confined to the terrain in OR
and northern California. By D2 /Saturday/ the upper low drops
further south towards the Great Basin, with more significant moist
advection occurring into parts of the West. Lowering snow levels
with ample moisture and considerable ascent will spread heavy snow
as far south as the Transverse ranges of Southern California and
the Mogollon Rim of AZ. Repeated surges of moisture and 700mb
ascent into the San Juans will produce heavy snow there as well
during Saturday, and the heaviest D2 snow is likely in the San
Juans where WPC probabilities are high for 12 inches of snow, and
locally more than 2 ft is possible. Elsewhere, WPC probabilities
for 4 inches are moderate to high, with the highest amounts
outside of the San Juans likely in Uintas of Utah and San
Bernadinos of CA.

By D3 /Sunday/, the secondary low drops from WA into the Great
Basin producing the larger mid-level low while the lead shortwave
lifts into Montana. Additional heavy snow is likely in the Sierra,
Southern CA ranges, Mogollon Rim, and into the Wasatch and San
Juans. Some lighter snows may also accumulate down into the
valleys as snow levels fall. However, WPC probabilities for 4
inches or more are confined to the terrain, and highest in
AZ/UT/CO where a moderate risk for more than 8 inches exists.
 

For Days 1-3, the probability of significant icing is less than 10
percent.

Weiss