Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
245 PM EST Sat Mar 06 2021

Valid 00Z Sun Mar 07 2021 - 00Z Wed Mar 10 2021


...Pacific Northwest/California to the Northern Rockies...
Day 1-3...
Shortwave energy moving through the base of an upper trough
associated with a closed low centered west of British Columbia is
expected to drive another another frontal band into the Northwest
as it lifts toward Vancouver Island Sunday morning.  This will
bring another round of locally heavy mountain snow into the
Olympics and northern Cascades Saturday night and Sunday. 
Shortwave ridging will then likely bring a brief period of drier
weather across the Northwest Sunday evening.  Then Sunday night
into Monday, the latest guidance disagrees on the timing and
extent to which wet weather will return to portions of the
Northwest and northern Rockies.  Overall, models are in generally
good agreement with the synoptic setup -- showing a well-defined
shortwave lifting northeast from Northern California to the
northern Rockies, ahead of the previously noted low as it drops
farther to the south off of the Washington and Oregon coasts. 
Left-exit region upper jet forcing along with a low-to-mid level
baroclinic zone dropping southeast is expected to interact with a
modest increase in moisture to support increasing precipitation
coverage from Northern California to western Montana.  However,
models differ on the extent of the more organized heavier
precipitation, with the NCEP guidance generally wetter than the
non-NCEP consensus.  While forecast confidence is generally
limited, probabilities for heavier snow accumulations have
increased some, especially over northeastern Oregon into northern
Idaho during the Day 2 period.  As this leading shortwave lifts
northeast across the northern Rockies and High Plains into central
Canada on Tuesday, upstream energy moving through the base of the
trough will draw it farther south across California, with
increasing potential for heavier mountain snow developing across
the Klamath Mountains into the southern Cascades and northern
Sierra.

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

Pereira