Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
431 AM EST Tue Jan 26 2021

Valid 12Z Tue Jan 26 2021 - 12Z Fri Jan 29 2021


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

Pacific Northwest/California/Ranges of ID/WY...

A major winter storm is expected to produce multiple feet of snow
across the CA Sierra Nevada Mountains and
Shasta/Siskiyou/Trinity/Klamath mountains of Northern California. 
Heavy snows are expected to develop across the mountains of
Northern California as a deep low dropping south from the Gulf of
Alaska approaches the coast late Tuesday. 
The 850 mb jet of 50 kt coming onshore into northwest CA provides
both strong moisture fluxes and then also strong upslope flow into
windward terrain.
In OR, the heaviest amounts are expected to fall across the higher
elevations of the southern Cascades and coastal ranges following
the stronger 700 mb vertical velocity maxima rotating onshore and
inland underneath 300 mb divergence maxima aloft. 

By Wed morning, the 850 mb jet max moves south to the central CA
coast, so on Day 2/3 the max shifts south out of northern CA into
central CA.
Wed might feature the heaviest snow of the three days because of
both the strength, persistence, and slow movement of the low level
jet.
On Thu., the trend is for the strength of the 850 mb jet to weaken
across central to southern CA, so the snow amounts are not as
heavy as Wed as a result.
Heavy snows will likely continue along the central and southern
Sierra through Thursday as the upper trough slowly pivots east.   
 

The long duration of the event leads to prodigious snow totals as
3 day gridded forecast totals in the central to southern CA Sierra
Nevada mountains are 100-130 inches in the 00z Canadian regional,
Canadian global, and UKMET forecasts.

Heavy snow extends inland on Day 2 into the ranges of northeast OR
and southern ID, such as the Boise Mountains, as the lead 700 mb
shortwave crosses the region with a surge of moisture fluxes and
ascent.  Several more inches are expected on day 3 as the moisture
influxes continue inland with high relative humidity persisting in
southwest ID in confluent flow. Lift is provided by a 300 mb jet
max crossing ID that places the ranges of southern ID to northwest
WY in a coupled jet region, favoring upper divergence maxima and
ascent in the terrain.

...Great Lakes to the Northeast and Mid-Atlantic...
Days 1-2...

As the the low-to-mid level warm front crosses lower Michigan into
New York and New England, low-mid level warm/moisture advection in
advance of the front combines with
upper divergence maxima in the left exist jet region to produce
lift across much of central to eastern New York and New England.
Pre-frontal convergence combines with orographically enhanced
totals to produce maxima from the southern Adirondacks into the
southern Green and Berkshire mountains today.
Easterly to northeasterly flow early across Lake Michigan and
continuing through early today is expected to support
lake-enhanced snow in eastern WI and northeast IL, with a few
additional inches before drying aloft occurs and low level winds
back to the north, reducing snow coverage and rates. 

The snow amounts taper on Wednesday across New England the deep
layer flow weakens and the front departs. Further south, a
progressive 700 mb wave crosses the OH/TN Valleys across the
central to southern Appalachians, and then the wave progresses
across the Piedmont and coastal areas of the Carolinas and
southeast VA.  Several inches of snow are possible where
topography enhances snow in the central to southern Appalachians,
but the models in North Carolina and Virginia are showing
temperature forecasts that are initially too warm for snow.  The
the GFS shows a significant duration of temperature cold enough
for snow while precipitation is occurring so the probability for
heavy snow in southeast VA and eastern NC is low.

The probability of a quarter inch of freezing rain is less 10
percent Days 1-3.

Petersen