Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Mon Mar 25 2019

Valid 12Z Mon Mar 25 2019 - 12Z Thu Mar 28 2019

...Ranges of CA/Great Basin/Rockies...

Days 1-3...

An upper low dumb-belling off the Pacific Northwest coast will
eject waves of energy onshore through the forecast period
producing snowfall from the Northern Sierra of California north
through the Cascades and east into the Rockies. With the upper low
offshore, the trough axis will remain there as well until
potentially late on Day 3, producing high snow levels and thus
confining accumulations only to the mountain ranges.

Three distinct impulses are likely to produce snowfall across the
area. The first is a decaying shortwave moving across the Central
Rockies early today which will then dive southward downwind of the
amplifying mid-level ridge into the Plains this evening. Snow
levels will generally be around 5000 ft, but fall subtly lower to
4000 ft across WY directly beneath the shortwave itself. This
feature is transient and lacking significant moisture, so WPC
probabilities for 4 inches are low.

The second impulse will shed from the parent low offshore onto the
CA/OR coast this evening. This feature will be accompanied by a
more potent 300mb jet streak and enhanced PWAT on S/SW flow behind
a weak cold front. The nearly orthogonal flow into the
Shasta/Trinity/Siskiyous suggest the best upslope enhancement will
occur in those ranges, with significant snow also likely in the
far Northern Sierra which will eventually shift into the RRQ of
this jet streak. WPC probabilities are high for 8 inches, with
moisture spilling northward into the Cascades producing lighter
snows into OR and WA as well on Day 1.

A continuation of this moisture flux will produce a stripe of snow
on Day 2 from the Sierra northeast through the OR/WA Cascades and
into the ranges of ID/western MT. A secondary jet max rotating
onshore beneath the upper low will place the Sierra in the
favorable LFQ diffluent region for ascent, and snow is expected to
be more significant there on day 2 with WPC probabilities
indicating a moderate risk for 12 inches but with snow levels
staying above 5000 ft. Further north this flux of moisture will
produce snow into a colder airmass with snow levels as low as 2000
ft near the Canadian border, but a decrease in RH keeps WPC
probabilities
low for 6 inches, highest across Idaho.

The most robust impulse will rotate into CA on Wednesday, with a
stronger and more southerly trough and associated jet max
advecting high PWAT air as noted by anomalies reaching +3 standard
deviations. This suggests heavy snow in the Sierra which will be
beneath the best combination of upslope enhancement of moist
850-700mb flow and synoptic ascent. Snow levels begin to fall late
on day 3 as well as the trough axis shifts onshore, and WPC
probabilities are high for 12 inches across the Sierra. 3-day
totals will eclipse 2 feet in portions of the Sierra and other
ranges of northern CA.

Additionally, as this upper jet max shifts northeastward, it will
work in tandem with increasing frontogenesis along a southward
sinking cold front across MT/ID to enhance ascent and produce
heavy snow Wednesday/Wednesday night. WPC probabilities are
moderate for 8 inches from the Sawtooth northeast into the
Northern Rockies and Absarokas.
 

The probability of significant freezing rain is less than 10
percent.

Weiss