Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
251 PM EDT Tue Mar 26 2024

Valid 00Z Wed Mar 27 2024 - 00Z Sat Mar 30 2024

...Upper Midwest and Great Lakes...
Days 1-2...

The surface low which has been responsible for the major winter
storm and blizzard the last few days will begin to fill and pull
off to the northeast into Ontario by Wednesday morning. Residual
heavy snow is likely in the wake of this departing storm,
especially across northern MN, in response to a pivoting
deformation axis which will gradually lift northeast as well. With
strong CAA in the wake of this system helping to deepen the DGZ,
this will likely result in an axis of 1-2"/hr snowfall rates
persisting and translating northeast into northern MN, especially
across the Arrowhead, before departing. This results in WPC
probabilities for more than 4 inches of snow reaching as high as
30%.

Additionally, as the aforementioned CAA increases across Lake
Superior late D1 and D2, the setup should support W/NW lake effect
snow (LES) bands setting up across the U.P. of MI. At least modest
omega into the deepening DGZ driven by increasing instability as
850mb temps fall to around -15C atop lake temperatures that are
still +2 to +3 C. This will yield accumulating snow bands,
especially across the western U.P. and across the Keweenaw
Peninsula where WPC probabilities both D1 and D2 exceed 40%, which
could produce 2-day snowfall of more than 10 inches in a few areas.


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

A semi-persistent mid-level trough sitting across the West will
result in active winter weather through the end of the week for
much of the higher terrain.

The period begins with broad NW flow aloft upstream of a longwave
trough axis centered over the High Plains. This flow reverses into
brief westerly and then more substantial SW flow as the primary
trough shifts into the central part of the country leaving brief
shortwave ridging in its wake. However, by late Wednesday, a more
pronounced trough will again dig into the Pacific Coast as a closed
low drops out of the Gulf of Alaska, and a lobe of vorticity sheds
around that feature and onshore by Thursday night. This shedding
energy will drive a cold front and surface wave onshore by the end
of D1, with an accompanying modest AR with IVT reaching 300-500
kg/ms pushing PWs up above +1 sigma immediately ahead of it. This
will result in increasing precipitation spreading across the
Pacific Northwest and then expanding into the Northern Rockies,
Northern CA, and the northern Great Basin during D2 as the front
sags southeast. Snow levels ahead of this front will surge to
4000-6000 ft during the period of most intense warm/moist
advection, but then fall quickly behind the front to around 3000 ft
starting D2. This results in the highest WPC probabilities for more
than 6 inches reaching above 70% in the Shasta/Trinity region, as
well as along the higher terrain of the WA and OR Cascades, and
into the Olympics.

The front will continue to drop southeast through D2 as it
dissipates, but moisture return into the West will persist as the
aforementioned large closed low approaches the WA coast Thursday
before interacting with a secondary shortwave into D3 off the CA
coast. While there remains a lot of spread in how this interaction
will occur, the net result is likely to be persisting SW moist flow
into the West, spreading from CA through the Northern High Plains,
resulting in waves of precipitation through the forecast period. At
the same time, a northern stream trough moving near the Canada/US
border will drop a cold front into the Northern Rockies and
northern High Plains leaving additional ascent in that region due
to fgen and upslope flow.

The net result of this complex evolution will be multiple days of
moderate precipitation, with axes of heavier precip focused in more
favored upslope terrain. Snow levels ahead of the front will be as
high as 7000 ft in the Great Basin, but then fall to around 3000
ft, before leveling off much of D2 and D3 at 3500-500 ft. The
highest WPC probabilities for more than 6 inches D2 will be across
the Sierra where they exceed 90% and could reach nearly 2 feet in
the highest elevations, with additional heavy snow spreading across
the Cascades, Shasta/Trinity/Siskiyous, across to the Salmon
River/Sawtooth Ranges, and into much of NW WY near Yellowstone. By
D3, the focus shifts southeast and weakens, and while WPC
probabilities remain across the Sierra and northern CA, they fall
to less than 40% for an additional 6+ inches. Additional WPC
probabilities for more than 6 inches on D3 reach 20-40% across the
Uintas and into parts of the CO Rockies.


...New England...
Day 3...

A southern stream impulse lifting along the Atlantic Coast from the
Carolinas late Thursday will get captured by a lobe of vorticity in
the northern stream shedding from a gyre over Huron Bay. The result
of this will be a negatively tilting trough pivoting over New
England, and the resultant ascent through height falls/PVA and
weakly coupled jet streaks will drive surface low intensification
off the Northeast coast. This low will be accompanied by impressive
moisture advection on 290-295K isentropic ascent surging PWs to as
much as +2 to +3 sigma, highest over eastern New England. Although
the environment will be marginal for snowfall with this event owing
to modest antecedent thermals and the intense WAA, the higher
elevations of VT, NH, and ME will likely receive heavy wet snow D3,
especially later in the period as the low departs and some cold
advection commences. There is a lot of spread in the placement of
this low and associated moisture transport, but current WPC
probabilities for more than 4 inches of snow are 10-20% in the
higher terrain of NH and ME.

Weiss