Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
455 AM EDT Mon Mar 16 2020

Valid 12Z Mon Mar 16 2020 - 12Z Thu Mar 19 2020


...California and Northwestern Nevada...
Days 1-2...

...Prolonged Heavy Snow in the Sierra Nevada Continues Today
Before Tapering Tonight...

A closed low stalled off northern CA drifts south to the central
CA coast tonight before elongating and becoming two separate
features through Tuesday. The plume of Pacific moisture stream
onshore ahead of the low shifts from central to southern CA today.
Prolonged synoptic ascent through upper divergence/low level
convergence, with aid from upslope flow in windward terrain will
continue produce heavy snow along the Sierra Nevada and extend
south to the Transverse Ranges through tonight. Snow level will
hold around 3000 to 4000ft on the western slopes of the Sierra
Nevada and increase to around 5000ft on the eastern slopes with
snow levels in the Transverse Ranges dropping from 5000ft to
around 3000ft tonight. One to two additional feet of snow are
expected through tonight in the highest terrain along the entirety
of the Sierra Nevada. The focus for heavy snow for the remainder
of this prolonged storm is farther south than the previous two
days and mainly south of Tahoe where there is a high risk of 18 or
more inches of snow in the highest terrain while there is a
moderately high risk for 6 or more inches in the highest
Transverse Ranges.

The filling low elongates as it progresses south off the central
CA coast tonight with the primary feature pushing inland to
western NV and the reinforcing feature swinging well offshore and
reaching a point off the southern CA coast Tuesday night. Surface
low pressure development is focused over the west-central Nevada
today that drifts northeast into southern ID by late Tuesday. Wrap
around moisture and lift is modest over northern Nevada into far
northern CA, but the slow movement of the low should lead to
moderate snow accumulations. Day 2 probabilities for 6 or more
inches are moderate to high over terrain in far northeast CA and
adjacent northwest NV and into terrain of south-central OR. 


...Four Corners States...
Day 3...

As the aforementioned reinforcing system crosses southern CA
Wednesday, the falling heights and increasing south to southwest
flow leads to moisture fluxes across southern CA to AZ/UT,
expanding to CO/NM and up the High Plains Wednesday night. Precip
breaks out Wednesday with snow expected in the higher elevations
of the AZ Mogollon Rim and Kaibab Plateau (north of Grand Canyon
North Rim) to the southern UT Wasatch, as well as Bryce/Zion
National Parks before expanding to the San Juan/Sangre De Christos
and central CO Ranges.  Lee side surface low pressure then
develops in eastern CO late Wednesday night which combined with
sprawling high pressure centered over the northern Canadian
Prairies aides east-northeasterly upslope flow over the High
Plains/eastern slopes of the WY and the Black Hills of SD where
snow breaks out late Wednesday night. Day 3 probabilities for 6 or
more inches are high for the highest terrain of AZ/southern
UT/northern NM and the San Juans of CO with moderate risks for the
rest of the high terrain of CO into southern WY.


...Northern New England...
Days 1/2...

A northern stream trough swings across the northern Great Lakes
tonight, crossing Maine by late 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 1.5 snow probabilities for 4 or
more inches are low to moderate over northwest Maine near the
Quebec border and for the highest peaks of Maine/NH and the
Adirondacks. The quick movement of the system and that precip is
from warm air advection limits the duration of the event and keeps
potential for heavy snow low.


...Great Lakes to Northeast...
Day 3...

An impulse shifting northeast from New Mexico late Tuesday reaches
the Great Lakes by late Wednesday and New England late Wednesday
night. Strong southerly flow and ample Gulf of Mexico moisture
with this quick moving system will limit snow fall, but the
overrunning flow and cold surface temperatures raises a freezing
rain risk over northern WI/MI on Wednesday and potentially
interior Valleys of the Northeast Wednesday night with 20 to 30
percent risks for a tenth inch of ice in these locations, as well
as a 10 percent risk for a quarter inch near the central WI/U.P.
of MI border.


For Days 1/2, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson