Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 AM EST Tue Feb 26 2019

Valid 12Z Tue Feb 26 2019 - 12Z Fri Mar 01 2019


...Pacific Northwest/California to the Northern Rockies and Great
Basin...

Several additional feet of snow is forecast for portions of
Sierra, with an additional couple of feet into the ranges of
southern ID and northwest WY/south central MT.

On Day 1, Tue, confluent flow south of the circulations off the
OR/WA coasts will result in deep layer jet maxima moving onshore
into northern and central CA, resulting in strong coupled upper
divergence/lower convergence maxima and 700 mb moisture fluxes
that extend inland across northern CA into southern OR, southern
ID, southwest MT, and northwest WY.
The continued long duration lift, along with orographic effects,
is expected to support additional heavy snows from the southern OR
Cascades and northern CA Cascades/Shasta/Trinity ranges into the
northern and central Sierra, as well as across the Boise Mountains
of ID  to the Tetons of northwest WY.  WPC Day 1 probabilities
(ending 12Z Wednesday) indicate a High Risk for additional amounts
of a foot or more for portions of the these areas.

On day 2, Wed, with an upper low remaining anchored off of the
Washington-Oregon coasts, models show a well-defined shortwave
moving inland south of the low into Northern California Wednesday
morning.  Accompanied by a fresh surge of Pacific moisture, this
system is expected to support additional heavy snows, with WPC Day
2 probabilities (ending 12Z Thursday)
indicating a Moderate to High Risk for accumulations of a foot or
more across many of the same Day 1 areas.  Amounts are not
forecast to be as high as on day 1 as the strength of the mid to
upper jet gradually decreases, along with a building upper ridge
in the northwest.

On Day 3, Thu, a transitional period commences as a piece of the
circulation off the Wa/Or coast stream onshore and inland,
bringing another round of snow to the ranges of northeast WA and
northern Rockies.  Further south in CA, the strength of the 700 mb
jet and resultant moisture advection and convergence continues to
wane, with the 700 mb convergence maxima moving steadily east
across the Great Basin into the ranges of northern UT and CO.
This gives the ranges of CA a respite, with snow amounts much less
than days 1-2.

For the three day period ending 00Z Fri, WPC probabilities are 50
percent or greater for amounts exceeding 30 inches across a large
portion of the Sierra, as well as portions of the Sawtooth in
central Idaho and the Tetons in western Wyoming.

...Upper MS Valley, Great Lakes and Northeast...

Shortwave energy ejects east from the northern Rockies, Plains,
and upper mid west on day 1.  The 850-700 mb waves are fairly low
amplitude, with a jet streak the ECMWF is showing as 130 kt at 300
mb providing the impetus for lift, producing snow in the Upper MS
valley to upper Great Lakes Day 1.  A moderate risk for 4 inches
is sown over north central WI.

On day 2, the jet streak continues out of the lakes and across New
York and New England.  The right entrance region crosses western
and central NY, focusing upper divergence and snow in this area. 
The low level circulation crosses from Ohio into Pa before
weakening and then redeveloping off the coast of southern New
England.  The low moving east off the coast leads to the event
ending by early Thu.

The probability of significant icing is less than 10 percent days
1-3.

Petersen