Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
420 AM EDT Thu Mar 21 2019

Valid 12Z Thu Mar 21 2019 - 12Z Sun Mar 24 2019


...The West...
Days 1-3...

An active pattern returns to The West as two mid-level impulses
bring widespread mountain snow from the Sierra, to the Cascades,
the Northern Rockies, and southward through Colorado and into New
Mexico.

The first of these impulses is a potent negatively tilted
shortwave lifting onto the California coast this morning. This
will be accompanied by a modest Pacific Jet and moist advection,
with height falls and jet level diffluence producing heavy
snowfall from the Great Basin eastward into the Southern Rockies.
The heaviest snow Day 1-2 is expected in the Sierra, as well as
the San Juans of Colorado where upslope flow is enhanced as 700mb
flow becomes due south around the elongated trough. WPC
probabilities show a high risk for 12 inches in the San Juans,
with 1-2 feet possible. Probabilities across the other ranges
south of 40N latitude are moderate for 8 inches, with snow levels
primarily remaining above 5000 ft.

Saturday into Sunday, the next shortwave and accompanying surge of
700 mb warm/moist advection and ascent through jet level
diffluence and height falls leads to likely accumulating snows
from the Sierra Nevada, extending across the Siskiyous and north
into the southern Cascades.  As the wave moves further inland, a
period of snow is expected in the ranges of southern ID and then
into northwest WY, as well as the mountains of central NV.
Although widespread heavy accumulations are not expected, WPC
probabilities indicate a slight risk for 8 inches from the Blue
Mountains of OR into the Sawtooth range and eastward into NW WY.


...Central Appalachians into New England...
Days 1-2...

Complex upper pattern featuring the interaction of no less than 3
shortwaves will spawn a deepening surface low near the Delmarva
today which will move northeast into Maine on Friday. Further
interaction with the final shortwave phasing into a closed low
over the northeast will help to occlude and vertically stack the
system, producing very slow movement to the northeast through
Saturday.

Despite very good model agreement in the interaction of the three
shortwaves and that effect on the surface low evolution, this
remains a challenging snowfall forecast. As the first two
shortwaves phase and cause a negative tilt to the upper pattern
over the Mid-Atlantic, this will spread anomalously high moisture
into New PA/NY/New England on a strong LLJ/WAA. Column RH rises
quickly today across PA/NY and western New England which will be
wrung out as precipitation due to intense height falls and
increasing upper diffluence. 850mb temps climb above 0C all the
way to the Canadian border by tonight as the low tracks inland,
suggesting terrain-dependent precipitation type, with snow
expected above 2-3 kft and a mix or rain below that level. Snow
levels will remain near the surface across Maine, but that area
will be under the influence of the anticyclone and lower RH today
so little snowfall is expected. In this setup, the highest
probabilities for 4 inches of snow are just above 50% across the
Adirondacks and Catskills. There is likely to be significant
snowfall gradient in the terrain however, especially in the
Champlain Valley of VT.

As the low moves into Maine Friday, a tertiary shot of northern
stream energy will dig into the negatively tilted trough to
enhance diffluence and height falls, close off the mid-level
circulation, and cause the surface feature to stall. At the same
time, cold air will wrap quickly southward as NW winds through the
column increase, which also enhances upslope snowfall. Falling
snow levels combined with more robust forcing including increasing
850-700mb fgen and mid-level deformation W of the 700mb low will
produce heavy snowfall from the Adirondacks and points east into
the Green and White Mountains, as well as into northern ME. WPC
probabilities show a moderate to high risk for 8 inches of
accumulation in the highest terrain, with light accumulations
expected down into the Valleys as snow levels crash.

An additional caveat is the potential for a heavy band of snow to
develop from central PA into the Poconos and Catskills and
potentially into the Berkshires as shown by much of this mornings
suite. This band is due to a collocation of pivoting low-level
fgen and enhanced deformation near the low center. Despite good
model agreement and HREF mean accumulations of over 6" in many
areas, the strongest forcing remaining below the DGZ, a warm nose
above 0C, and rapid northward advance of the TROWAL suggests this
snowfall may be overdone. Have trended upward to show moderate
probabilities for 4 inches in these areas, but the potential
exists for much more if the column cools more quickly during the
period of heaviest precipitation.

As the low pulls away slowly, NW flow will be favorable for
upslope snow as far south as the highest NC/TN mountains where
light accumulations are possible, and in the high terrain in WV
where WPC probabilities are up to 30% for 4 inches.


The probability of significant freezing rain is less than 10
percent.

Weiss