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

Valid 00Z Mon Apr 10 2023 - 00Z Thu Apr 13 2023

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

Confluent mid-level flow downstream of a shortwave digging out of
the Gulf of Alaska will gradually back to the S/SW, especially
Tuesday, as the shortwave amplifies into a closed low and pivots
onshore near the WA/OR border Wednesday morning. While the
approach and passage of this closed low will bring the most
intense ascent through the period, lobes of vorticity shedding
from this feature D1-2 will also result in enhanced ascent
stretching from the Pacific NW coast through the Northern Rockies.
This ascent will be aided as well by a shifting Pacific jet
streak, which will exit to the east late Tuesday leaving the
favorable RRQ for diffluence overhead, before a secondary jet
streak and its LFQ approach late in the forecast period. This
overlap of synoptic lift will result in a surface low developing
over the Pacific and then moving onshore Wednesday, leading to
locally even more pronounced ascent in a column that will remain
moistened by an atmospheric river with IVT reaching above 500
kg/m-s along the coast, and even approaching 500 kg/m-s across the
Northern Rockies despite inland decay.

Initially, WAA on the increasingly backed flow and ahead of a
leading cold front will drive snow levels to 5000-8000 ft D1,
falling to around 3000 ft in the Olympics to start D2. These snow
levels will continue to fall behind the front, becoming just
1500-2500 ft during D2, lowest to the west of the Cascades.
Additional significant lowering is possible into D3, potentially
falling to as low as just 500-700 ft near Seattle and Portland,
with 1000-1500 ft widespread elsewhere. These lowering snow levels
will allow precipitation to become increasingly snow-dominated
after primarily rain on D1, with pass level snow likely becoming
significant. On D1, high WPC probabilities for more than 6 inches
are confined to the higher terrain of the Olympics and WA
Cascades. However, by D2, precipitation expands south and east
concurrently with lowering snow levels. This extends WPC
probabilities of at least 30% along the length of the Cascade
crest into OR, with some 10-30% probabilities reaching the
Bitterroot range of ID. During this time, light to moderate
snowfall accumulations are also likely at the Cascade Passes
including Snoqualmie and Stevens.

While the placement of the surface low D3 and its associated
forcing is still uncertain, this occurring during the period of
lowest snow levels despite slowly reducing IVT could result in
event some lowland snow, especially where precip is heaviest as
steep lapse rates noted in regional forecast soundings support
convective snow rates which could allow precip loading to drag
snow levels down to sea level.  WPC probabilities for more than 6
inches on D3 continue to be moderate to high along the Cascades of
WA and OR, with modest probabilities continuing into the Salmon
River range. Additionally, with the lower snow levels, some low
probabilities for 0.1" and 1" reach the lowlands around Portland,
OR, which is also reflected by a WSE mean of around 0.5" for the
Portland metro area.

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


Weiss