Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 AM EST Mon Feb 25 2019

Valid 12Z Mon Feb 25 2019 - 12Z Thu Feb 28 2019


...Pacific Northwest, California, and the Northern Rockies...

Days 1-3...

The long duration multi-day heavy snow event has started for the
ranges of western OR, northwest CA, southern ID and western MT,
and is forecast to continue. Multi-day total snows of 3 to 5 feet
are forecast in the ranges of central to northern CA, southern ID,
and northwest WY.

On Day 1 Monday, the amplifying trough off the Pacific northwest
coast allows the upper jet maxima to sink south, starting in WA
this morning and then moving into OR by this evening. The 00z
ECMWF forecasts a 110-120 kt jet maxima impinging on OR and then
continuing into southern ID/MT. Enhanced moisture and and synoptic
ascent, combined with upslope lift and low-mid-level frontogenesis
will produce rounds of heavy snow from the Oregon Cascades through
the ranges of southern to central ID/western MT, and NW Wyoming. 
WPC probabilities are high for 12 inches over multiple ranges,
with 2 feet or more possible in the highest terrain of the
Boise/Salmon River Mountains.
As the jet sinks south, snow extends across the Shasta/Trinity
Ranges of northwest CA and into the northern CA Sierra.

On day 2, Tuesday, jet energy will move slowly, allowing long
duration snows to occur where prolonged ascent occurs underneath
upper divergence maxima and the nose of the 700 mb jet of 50-60 kt
coming into the CA Sierra Nevada, where an additional couple of
feet of snow are expected. With 700 mb convergence and theta-e
advection maxima crossing northern NV through southern ID and
western WY, secondary maxima of heavy snow are expected in the
ranges of southern ID and northwest WY.

On day 3, Wednesday, the target areas for heavy snow are the
ranges of northern to central CA inland through southern ID and
northwest WY owing to favorable upper divergence maxima.  The
strength of the upper jet is forecast to taper, with the weakening
upper jet and weakening 700 mb moisture fluxes forecast in the GFS
output to weaken, leading to slightly lower QPF amounts and lower
snow totals.  While heavy snow is expected, there are no areas of
2 feet of snow forecast on day 3.

...Upper MS Valley/Great Lakes/Northeast...

Days 1 through 3...

The day 1 heaviest snow is likely downwind of the southeast corner
of Lake Ontario as the initial burst of snow responded to a
westerly boundary layer wind, and boundary layer winds are
forecast to veer today to more like 290 degrees, allowing bands to
sink south from the Tug Hill area to the southeast corner of lake
and even into the adjacent Mohawk Valley.
Snow coverage and intensity tapers tonight as drier air advects in
from the west.   Elsewhere, snow showers are focused on west
facing aspects of the Adirondack and Green Mountains, with low
level ascent lasting a bit more over the northern Greens, allowing
several inches to accumulate.

On Tuesday night into Wednesday, a strung out vorticity lobe
ejecting from a west coast trough will race across the Plains and
upper MS Valley towards the Great Lakes, with a low amplitude 700
mb wave leading to a band of confluent flow and light snows within
the 700 mb jet axis and 850 mb warm advection zone.  Light snow
are expected with current models only forecasting a  couple of
tenths liquid equivalent 12z Tue to 12z Wed, with a slight uptick
in the lower Lakes to northern PA/southern NY/southern New England
on Wed.

The uncertainty is the precip type transition zone in northern
PA/southeast NY, including New York City and Long Island, where
low level warm advection may change snow over to mixed precip
types or rain.

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

Petersen