Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
345 PM EDT Wed Mar 11 2020

Valid 00Z Thu Mar 12 2020 - 00Z Sun Mar 15 2020



...Pacific Northwest/Northern Rockies...
Days 2-3...

A potent shortwave trough will dig southward along the British
Columbia and Washington State coast Friday before closing off and
sinking further south off the Oregon coast Saturday. To the east
of this feature. mid-level divergence will intensify, combining
with increasing diffluence within the RRQ of a departing jet
streak to produce large scale ascent across the Pacific Northwest
and into the Northern Rockies. In this pattern, 700-500 RH will
increase quickly on moist SW flow, which will begin to overrun the
low-levels as a surface cold front digs southward from Canada. The
cold air sinking southward interacting with warm advection aloft
will lead to intensifying 850-700mb frontogenesis which is likely
to persist and become intense across parts of Montana. The
combination of synoptic ascent with this intense fgen and
low-level upslope flow will produce an extended period of moderate
to heavy snow. The heaviest snow is likely from the Northern
Rockies near Glacier NP and points east to the ND line along the
northern half of MT. Here, WPC probabilities are high for 8 inches
D2 and D3, and isolated snowfall amounts over 18" are possible.
Further south across SE MT, a dry slot is forecast to wrap into
the system creating much lower snowfall amounts.

To the west of this heaviest snow axis, prolonged synoptic ascent
through mid-level divergence, height falls, and jet streak
diffluence will produce widespread precipitation across the
terrain from the Olympics, southward through the Northern
California mountains, and east into Idaho. Snow levels will drop
as well as the upper low sinks southward, and light snow is likely
even in the valleys as far south as northern Oregon. The highest
snowfall is likely in the Olympics where 2-day totals may exceed
12 inches. Otherwise, WPC probabilities are moderate for 6 inches
in the Washington Cascades on D2, spreading southward through the
Oregon Cascades, the Siskiyous, and eastward to the ranges of
Idaho and western MT D3.


...Southwest through the Central Plains...
Days 2-3...

A large closed low will be positioned just west of Southern
California Friday morning as it begins to fill and eject northeast
to become embedded in the merging flow across the Central Plains
Saturday. As this feature lifts northeast, it will spread
precipitation northeast from the southern Sierra through the Four
Corners and into the Central Plains. Precipitation will expand in
coverage through warm/moist mid-level advection and height
falls/PVA as the upper feature shifts northeast. Additionally, a
secondary jet streak is likely to develop and shift towards the
Great Lakes placing favorable diffluence for ascent across the
region. This will all be ongoing atop a surface cold front
dropping southward creating a band of enhanced frontogenesis as
well. While guidance does feature a wide variation in latitudinal
placement of the heaviest snow, a blend of the favored models
(ECMWF/ECENS/GEFS) suggests the highest accumulations will be in
the terrain of the San Juans and Wasatch on D2 where WPC
probabilities are high for 6 inches, and then into central
Nebraska D3 where WPC probabilities for 6 inches are moderate.


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

Weiss