Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
411 AM EST Fri Mar 08 2019

Valid 12Z Fri Mar 08 2019 - 12Z Mon Mar 11 2019

...Western United States...
Days 1-3...

A closed/cutoff low drifting southeast across WA today and bring
an onshore flow to the Pac NW coast with snow elevations
500-1000ft across the Portland metro. This low will persist over
the northern Rockies tonight into Sunday before eventually phasing
into a low currently on the west side of the eastern Pacific ridge
that shifts down the CA coast Saturday night into Monday. 

A positively tilted southern stream shortwave trough crossing the
central CA coast has a strong southwesterly jet that has pushed
Pacific moisture over the Great Basin where mountain snows are
ongoing. Moderate Day 1 probabilities for a foot are over the CO
Rockies with snow elevations decreasing through tonight.

The low pushing south along the northern CA coast Saturday with
the focus of snow in the Trinity Alps where snow elevations around
2000ft. Moderate Day 2 probabilities for 8 inches are over the
Trinity/Klamath/CA Cascades with some snow likely for the higher
coastal peaks south of the SF Bay area. A reinforcing shortwave
diverts energy and moisture off the southern CA coast for Day 3,
with mainly moderate probabilities for four inches over the Sierra
Nevada and Great Basin ranges.


...Central Plains to the Upper Midwest...
Days 1-2...

Surface low pressure will be developing to the lee of the Colorado
Rockies this evening as a southern stream trough becoming
neutrally tilted shifts east from the Great Basin today. Timing of
the trough crossing the central Plains Saturday and its turn
northeast across the Great Lakes Saturday night/Day 2.5 as
interaction with a northern stream trough over the Canadian
Prairies are in good agreement with the 00Z global deterministic
suite.

Overall QPF decreased with the 00Z package though intense rates
are still expected in the TROWAL developing on the north side of
the low tracking northeast from eastern KS Saturday afternoon. The
strong southerly flow across IA generally limits the southern end
of wintry precip to far northern NE/IA with enough dynamics to
have a period of sleet over eastern SD and southern MN with all
snow on the wrap around perhaps a little farther south than on the
front end.

The Moderately high Day 2 probabilities for 8 inches are across
central MN into northwestern WI. Of additional concern will be the
potential for strong surface winds to develop in the wake of the
low, resulting in snow/blowing snow capable of significantly
reducing visibility.

A stripe of light freezing rain accretion can be expected just
south of the snow transition line currently indicated by low
probabilities for a tenth inch accretion over eastern NE on Day 1
to northern IA and southern WI on Day 2.


...Central Appalachians and Mid-Atlantic...
Days 1-2...

A relatively weak shortwave trough shifting east into MO this
morning along 100kt westerly jet and a low level baroclinic zone
with leading snow now reaching the Allegheny Highlands of southern
WV/VA. The focus of this snow will be in the highlands of WV/VA
where orographics aide low to moderate Day 1 probabilities for
four inches. The trough axis will cross the central Mid-Atlantic
this evening with leading snow across the Baltimore/Washington
metro limited by super-freezing surface temperatures.


...Northeast...
Days 2-3...

A warm front off the Great Lakes low lifts over the northern
Mid-Atlantic Saturday night and New England Sunday. A strong low
level jet with 50kt+ 850mb flow allows a quick transition from
snow to sleet/freezing rain around NY state into New England with
only northern New England remaining snow in this fast moving
frontal zone. Day 2.5 ice probabilities are highest for a tenth
inch are from northeast PA into Maine with moderate probabilities
for four inches in higher elevations in that area.

Jackson