Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
441 PM EDT Tue Mar 26 2019

Valid 00Z Wed Mar 27 2019 - 00Z Sat Mar 30 2019


...Pacific Northwest and Northern California to the
Northern/Central Rockies and High Plains...

A vertically-stacked low will gradually weaken as it drifts slowly
east-northeastward toward the Pacific Northwest coast this period.
 Shortwave energy moving south of the low will send another
occluded frontal band into northern California on Wednesday. 
Strong moisture transport, supported by deep southwesterly flow,
interacting with divergent flow aloft is expected to generate
widespread precipitation, with orographic enhancement producing
some locally heavy totals.  Snow levels which are forecast to
begin above 5000 ft, are expected to lower to around 4000 as the
front pushes through late Wednesday.  Along the western slopes of
the northern Sierra, WPC Day 1 probabilities (ending 00Z Thursday)
show a Moderate Risk for accumulations of a 8-inches or more for
areas above 6000 ft, with a Moderate Risk for a foot or more
across some of the higher peaks.

Post-frontal showers are expected to continue, possibly producing
additional locally heavy totals along the Sierra.  However with
onshore flow weakening, the potential for widespread additional
heavy amounts is expected to decrease on Day 2. 

Meanwhile farther to the northeast, increasing divergence along
the left-exit region of an upper jet extending out ahead of the
low, combined with strengthening low-mid level frontogenesis and
upslope flow is expected to  support some locally heavy snow
totals over portions of the northern Rockies - particularly the
ranges of southwestern Montana.

As upstream energy continues to move across the region, mountain
snows will continue from central Idaho into western Montana and
Wyoming into Day 3.  As the leading shortwave moves across the
northern and central Rockies, snows are expected to spread
southeast across of the Divide.  There is a fairly good model
signal for moderate to heavy snows developing over the central
High Plains from southwestern South Dakota and the Nebraska
Panhandle back into eastern Wyoming.  WPC Day 3 probabilities
(ending 00Z Saturday) show a Slight Risk for accumulations of
8-inches or more centered across this region.  However, models are
showing a fair amount of spread with respect to the details, so
subsequent issuances may reflect significant changes.

Pereira