Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
406 PM EDT Sun Mar 24 2019

Valid 00Z Mon Mar 25 2019 - 00Z Thu Mar 28 2019

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

Days 1-3...

A series of mid-level impulses and upper level jet maxima will
bring periods of snow into the ranges of CA, with each disturbance
continuing inland to the Pacific northwest and northern Rockies,
although snow amounts taper going inland as each disturbance
weakens and becomes progressive. Days 1-3 snow totals may
accumulate to a couple of feet in most ranges of central to
northern CA. WPC probabilities show moderate to high risks for 8
inches and low risks for 12 inches days 1-2, increasing to
moderate to high on day 3.

The highest totals on day 1 are expected to be in the
Shasta/Siskiyous as an east Pacific system gradually approaches,
with the 300 mb jet streaming onshore Monday, accompanied by a 300
mb divergence bulls eye in northwest CA. The sustained lift does
not penetrate further inland, so none of the ranges outside of CA
have an 8 inch or higher risk.  

Further inland, a deamplfying lead 500 mb wave provides a period
of modest ascent in the Wyoming and Idaho/southwest MT Rockies. 
This system is weak and moves quickly, so total snowfall will be
light, with the low to moderate probabilities for 4 inches in the
terrain of the Sawtooth, Tetons, and Bighorn Mountains of WY.

On Day 2 Mon night-Tue, another round of heavy snow is expected in
the ranges of northwest CA.  Amounts might not be quite as heavy
as on day 1 as the day 1 jet maxima departs for the northwest,
with a late arriving second maxima Tue afternoon. The higher
amounts shift into the northern Sierra Nevada of CA.

Light to moderate snowfall will spread across the high terrain
from the OR Cascades and then the ranges of northeast OR/southern
ID/western WA as a jet rotates cyclonically around the trough,
with difluent flow aloft favoring ascent as a weak warm front
moves north across these areas. WPC probabilities are moderate for
4 inches in the Cascades/Olympics (low for 8 inches), with lesser
accumulations likely across the other ranges.

On Day 3 Tue night-Wed, an upper trough off the CA coast slowly
approaches, with the next in the series of upper jet maxima
streaming onshore into northwest CA and one max continuing across
OR into ID and another sliding south across CA. 

The greatest moisture advection and synoptic ascent due to jet
level diffluence will focus in the Sierra Nevada of California
during day 3, as well as the Shasta/Trinity/Siskiyous due to
closer proximity to the moisture source plus stronger low level
jet and 700 mb vertical velocity maxima.  The models have slight
timing differences regarding when the upper low arrives. 

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

Petersen