Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
257 AM EDT Sun Apr 09 2023

Valid 12Z Sun Apr 09 2023 - 12Z Wed Apr 12 2023

...Pacific Northwest...
Days 1-3...

An upper trough that deepens as it shifts southeast from the Gulf
of Alaska today will direct an atmospheric river into the Pacific
Northwest today through Monday the before the trough moves onshore
Monday night and settles over the Northwest into the middle of the
week.  Through this evolution, mid-level flow will back from zonal
today to SWly Monday, and then become more intense from the SW on
Tuesday. Within this flow, spokes of vorticity will shed away from
the aforementioned approaching trough to spread rounds of enhanced
ascent through PVA into the Pacific Northwest, with more robust
PVA and height falls spreading onshore Tuesday. This evolution
will be accompanied by increasing Pacific jet energy, and while
the core of this jet is progged to angle northeast into British
Columbia through Monday, it should arc back into the Pacific
Northwest Tuesday, bringing increased diffluence within its
favorable right entrance region. The overlap of PVA, height falls,
and this upper diffluence will increase synoptic ascent, while
dual cold fronts moving eastward will drive low-level convergence
and falling snow levels. This deep layer lift will act upon a
moistening column as the confluent mid-level flow and overlapping
jet streak drive the atmospheric river with high probabilities for
IVT exceeding 500 kg/m-s onshore. Days 1 and 2 snowfall
probabilities for 8"+ are generally moderate for only high western
Washington terrain with due to lesser moisture and snow levels of
5000-7000 ft. However, the approaching trough and accompanying
cold front will shift the precip shield inland with snow levels
falling to 2000 ft Tuesday and below 1000ft in places Tuesday
night when some impactful accumulations are possible at the
Cascade road passes. Day 3 WPC probabilities for 8"+ extends along
the Cascades north from central OR, the Olympics, to the Salmon
River range of ID. Additionally, with snow levels falling below
pass levels, there is an increasing chance for at least 6 inches
of snow at Snoqualmie and Stevens Passes Tuesday/Tuesday night.


The probability for icing over 0.1" is less than 5% Days 1-3.


Jackson