Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
421 AM EST Thu Feb 27 2020

Valid 12Z Thu Feb 27 2020 - 12Z Sun Mar 01 2020


...New England...
Day 1...

Rapidly deepening low pressure will move northeast into Canada
today and then become vertically tilted/stall through Friday.
Robust warm and moist advection will spread precipitation into all
of New England early today, but marginal thermal profiles leave
primarily rain, with a rain/snow mix at elevation. Intense
synoptic ascent will accompany this feature in response to the
pivoting of the RRQ of a 180+kt jet streak lifting away into
Canada combined with height falls and PVA. Additionally, as the
primary low occludes over Canada, a secondary surface low will
develop from a triple point and move across far SE New England and
off the Maine coast. This will drag colder air back down into the
region, and transition p-type from rain to snow in Northern New
England, with heavy snow likely much of day 1. Intense mesoscale
forcing through deformation and a pivoting TROWAL into Maine
creates the heaviest snow across that state, and WPC probabilities
are high for 8 inches in northern Maine and the White Mountains of
NH. Isolated amounts over 12 inches are possible. Elsewhere, WPC
probabilities are high for 4 inches across most of Northern New
England except the Champlain Valley of VT.


...Great Lakes...
Days 1-3...

...Significant Lake Effect Snow event begins today...

The deepening low pressure in Canada will leave robust CAA in its
wake, with favorable wind trajectories for a long duration lake
effect event, especially downwind of Lake Erie and Ontario. The
Lakes have very little ice cover at this time, and sfc-850mb
delta-T values may reach 20C or more, especially over Lake
Ontario. The favorable fetch and strong instability/high inversion
will support intense snowfall, aided by what should be effective
fetch and enhanced moisture as streamlines indicate both a Lake
Superior and Lake Huron tap. Snowfall rates of 2-3"/hr are
possible, especially in the Tug Hill Plateau, with strong winds
pushing heavy snow as far east as the western Adirondacks off Lake
Ontario, and along much of the eastern shore of Lake Erie. WPC
probabilities are moderate to high for 8 inches on both day 1 an
day 2, with the heaviest snow, potentially reaching 4 feet,
expected downwind of Lake Ontario. Although not as significant,
WPC probabilities for 4 inches are moderate to high in the NW L.P.
of MI as well as the eastern U.P., primarily on day 1.

By day 3 the fetch gets shunted southward bringing an end to the
heaviest snow.


...Central and Southern Appalachians...
Days 1-2...

As the aforementioned low lifts out, CAA in its wake will produce
light upslope snow in WV today. More significant snowfall is
likely Friday as upslope flow persists, and ascent is aided by a
modest shortwave digging through the broad cyclonic flow
enveloping the eastern CONUS. This shortwave is progged to move
overhead the Southern Appalachians on Friday, and may be followed
quickly by a secondary impulse Friday night. While moisture won't
be significant, enough increase is expected to produce moderate to
heavy snow through PVA and upslope in the Appalachians from the
Smokey Mountains northward into WV. WPC probabilities for 4 inches
are highest in WV, but several inches of snow are possible along
much of the terrain through Friday.


...Pacific Northwest to the Northern Rockies...
Days 2-3...

A robust shortwave is progged to lift onshore Washington state
Friday night and push rapidly east into Alberta, Canada Saturday,
with a surface low advecting beneath it. Immediately in its wake,
a secondary shortwave will dive into the intensifying longwave
trough and approach the central CA coast Saturday night.
Increasing Pacific Jet energy will accompany these features into
the coast, bringing increased moisture to the region, which will
be wrung out as snow across the terrain from the Olympics and
Cascades D2, through the Rockies, Sierra, and other ranges of the
Great Basin on D3. The most robust 700mb forcing is likely across
the Cascades of WA on D2 where WPC probabilities are moderate for
8 inches. As snow becomes more widespread within the elongating
trough and intensifying jet, WPC probabilities for 4 inches spread
through much of the terrain from the Olympics east to the Northern
Rockies, and south through the Great Basin and Sierra. The
heaviest snow is again likely in the Washington Cascades, and
2-day totals there could exceed 2 feet in the highest terrain.


For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Weiss