Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Sat Nov 07 2020

Valid 12Z Sat Nov 07 2020 - 12Z Tue Nov 10 2020


...Northern Rockies and Montana...
Days 1-3...

...Major winter storm to impact Montana this weekend...

A mid-level impulse ejecting out of the Southwest will amplify as
it approaches the Northern Rockies late tonight into Sunday,
closing off over Montana before lifting into Manitoba by Monday.
Rapid height falls in advance of this feature combined with
intense mid-level divergence and upper diffluence within the LFQ
of a potent jet streak will lead to pressure falls and surface low
developing in WY Saturday. This low will then deepen as it advects
northeast into Montana/ND and eventually Canada by Monday.

Moisture being drawn northward ahead of this system will become
anomalous as noted by PWs of +2 standard deviations above the
climo mean and mean mixing ratios along the 300K isentropic
surface as high as 6g/kg. This moisture will be streamed from both
the Pacific and Gulf, and will be drawn northwestward within the
WCB and into a robust TROWAL developing on Sunday. Deep layer
ascent through the aforementioned synoptics will combine with
intense mesoscale forcing through upslope flow, isentropic
upglide, and strong deformation to cause heavy precipitation to
develop across the Northern Rockies and then gradually spread
eastward. Initially, precipitation outside of the terrain will
likely be rain as the column is too warm to support snow. However,
rapid dynamic cooling of the column, especially as the low
strengthens and isallobaric acceleration occurs from the north,
will cause a p-type transition to snow, with a brief period of
freezing rain possible in far NE MT and NW ND. Freezing rain
should only feature minimal accretion, and WPC probabilities for
0.1" are less than 10%. Snowfall rates are likely to be intense as
strong omega intersects a saturated DGZ. The latest HREF
probabilities suggest at least 1"/hr, but it is likely that 2"/hr
rates could develop on Sunday, beyond the current HREF forecast
time range. It is likely that most of the precipitation will fall
as snow, and with heavy rates continuing within the slowly
advecting system, accumulations are forecast to be significant.
WPC probabilities are greater than 70% for 12 inches across much
of the Northern Rockies eastward into the High Plains of MT,
including Great Falls and Glasgow. Accumulations in excess of 20"
are possible in the favored upslope terrain as well, or where any
banding lingers for a long duration, and this event could
challenge November record snowfall across parts of Montana. In
addition to this heavy snow, strong winds will likely produce
blizzard or near blizzard conditions at times.


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

An anomalous and large upper low will develop across the West
through Monday as two separate but potent upper lows moves across
the area. As the latter ejects into the Plains late Monday, yet
another shortwave will begin to approach the Pacific Northwest by
Tuesday morning.

For Days 1-2, the two potent upper lows will cause height
anomalies that reach as low as -3 standard deviations from the
climo mean. Each of these upper lows will be accompanied by
Pacific moisture within associated jet streaks, through which the
combination of moisture and ascent will produce widespread
moderate to heavy snow across much of the western terrain. The
anomalously low heights will produce falling snow levels all the
way to the Mexican border, so heavy snow accumulations are
possible as far south as the San Gabriels and San Bernadinos, as
well as the Mogollon Rim, and most other ranges of the Great
Basin. The heaviest snow through Sunday is likely in the San Juans
where orthogonal mid-level flow will produce upslope enhancement,
and WPC probabilities are high for 18", with more than 3 ft
possible through the first 2 days. Other ranges that will likely
receive significant snowfall include the Sierra and San
Gabriels/San Bernadinos of CA, the Uintas and southern Wasatch in
Utah, and parts of the Mogollon Rim. Here, WPC probabilities are
high for 8 inches, with local amounts greater than 12 inches
likely above 4000-5000 ft.

By D3, the complex upper low begins to fill and shift east cutting
off most of the forcing across the West, leaving lingering light
to moderate snow across the Four Corners. However, quickly behind
this exiting system yet a third shortwave will begin to dig into
the PacNW with an accompanying Pacific jet streak to provide
moisture. Snowfall Monday will increase across the Olympics and
Washington Cascades, where WPC probabilities feature a high risk
for 4 inches.

 

Weiss