Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
503 AM EDT Sun Mar 15 2020

Valid 12Z Sun Mar 15 2020 - 12Z Wed Mar 18 2020


...California...
Days 1-2...

...Prolonged Heavy Snow with Several Feet Expected for the
Northern and Central Sierra Nevada through Monday night...

A closed low stalls off far northern CA today before drifting
south to the central CA coast through Tuesday before weakening as
it moves to southern CA Tuesday night. Pacific moisture streams
into northern/central CA ahead of the low through Monday.
Prolonged synoptic ascent through upper divergence/low level
convergence, with aid from upslope flow in windward terrain will
continue produce heavy snow in a fairly narrow precipitation
stripe in the ranges of northern to central CA. Multiple
additional feet of snow are expected through Monday along the
entirety of the Sierra Nevada. The focus for heavy snow shifts
south a bit from Day 1 (when there are moderate probabilities for
12 or more inches in the Shasta Siskiyous) to Day 2 which is
mainly between Tahoe and Mammoth Mountain. The 48hr 95th
percentile (a 5% chance of exceeding) has a large area of over 48
inches across the northern and central Sierra Nevada.

The filling low elongates as it progresses south off the southern
CA coast Monday night, shifting the snow axis along the extent of
the Sierra Nevada and into the Transverse Ranges of southern CA,
though rates reduce with weaker inland moisture transport and
lift. The Day 2.5 probabilities for 6 or more inches are
moderately high extend from the Sierra Madre, up the entirety of
the Sierra Nevada.


...Northern Rockies...Northern Plains to Lake Superior...
Days 1-2...

The upper jet bringing the moisture plume from the closed low off
CA weakens through today with snows in ID to the Yellowstone
region down to the Wind River Range with Day 1 probabilities for 6
or more inches moderate to high for the highest terrain.

A 700 mb shortwave impulse moves east from MT across ND to
northern MN into Monday as a northern stream trough digs over the
Canadian Shield. This allows generally light snow to develop in
northern ND today and northern MN tonight. The steady progression
of the wave plus modest ascent should keep amounts light. Light
lake enhanced snow are indicated over the Keweenaw Peninsula, but
event potential is limited as the system moves east of the region
and drying aloft occurs Tuesday. 


...Great Basin and far northern California...
Day 3...

The elongating low off CA promotes low level low pressure
development over the central Nevada. Wrap around moisture and lift
is modest over northern Nevada into far northern CA. Day 3
probabilities for 6 or more inches are moderate to high over
terrain in these areas with snow levels around 3000ft.


...Northern New England Day 3...

The northern trough digging across the Canadian Shield shifts
across southern Quebec Tuesday. Mid-level warm advection changes
precip over to rain quickly in most of northern NY and VT, with
cold air lasting for a longer period when it is cold enough for
snow in interior Maine. Day 2.5 snow probabilities for 4 or more
inches are low over far northwest Maine.

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

Jackson