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

Valid 00Z Mon Mar 16 2020 - 00Z Thu Mar 19 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 off northern CA drifts south to the central CA coast
through Tuesday and possibly to southern CA Tuesday
night/Wednesday.  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
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 on Day 1 is mainly between Tahoe
and Mammoth Mountain, with a high risk of a foot of snow .

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 probabilities for 6 or more inches are moderate to
high along the Sierra Nevada, with secondary maxima in the
Transverse range peaks.  The probabilities wane in southern CA on
Day 3 as the moisture plume extends further inland into the
southwest.


...Great Basin, Southwest, and northeast California...
Day 2/3...

The models forecast low pressure development over the central
Nevada Monday that drifts northeast into southern ID by Wed. Wrap
around moisture and lift is modest over northern Nevada into far
northern CA, but the slow movement of the low could lead to
moderate sow accumulations, with several inches expected each day.
Day 2 probabilities for 4 or more inches are moderate to high over
terrain in northeast CA to adjacent northwest NV and also in west
central NV.  On Day 3, probabilities lower in central NV as the
low departs, but increase in the ranges of southern ID and south
central MT as the low approaches, with a continuing threat in
northeast CA to adjacent northwest NV. 

On Day 3 in the southwest, on day 3 a triple point low may develop
from the sfc-700 mb and move from off the CA coast to the
coastline.  As the system approaches, the falling heights and
increasing south to southwest flow leads to moisture fluxes across
southern CA into AZ and UT.  Precip breaks out with snow expected
in the higher elevations of the AZ Mogollon Rim and mountains of
northwest AZ to the southern UT Wasatch, as well as Bryce/Zion
National Parks.  Several inches of snow are expected in these
areas.  Uncertainty relates to the timing/strength of the low and
associated QPF.

...Northern Plains to Lake Superior...
Day 1...

A 700 mb shortwave impulse moves east across ND to northern MN
into Monday. This allows light snow in north central ND to 
develop move across eastern northern ND 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.

...Northern New England Day 2...

The northern stream trough continues to move steadily east across
Quebec Tuesday and then across Maine, departing Maine Tue night.
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
snow probabilities for 4 or more inches are low to moderate over
northwest Maine near the Quebec border.  The quick movement of the
system limits the duration of the event and keeps potential for
heavy snow low.

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

Petersen