Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Thu Nov 04 2021

Valid 12Z Thu Nov 04 2021 - 12Z Sun Nov 07 2021


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

Several shortwaves embedded within flat to broad cyclonic flow
will rotate onshore into the weekend bringing rounds of
precipitation to the area. These shortwaves will be accompanied by
prolonged Pacific jet streak pushing moisture onshore, and while
this feature will be generally zonal through the period, it will
begin to amplify and arc poleward D3, although the greatest PW
anomalies are expected on D1 /today and tonight/. Beneath each
shortwave a surface cold front is progged to lift onshore as well,
and the combination of low-level convergence, height falls, and
upper divergence will spread rounds of precipitation onshore each
day.

The most significant precipitation is likely D1, but this is also
the time of highest snow levels, which should fall slowly from
7000 ft early to around 4000 ft late. This will bring significant
snowfall above these levels to the WA and OR Cascades and WA
Olympics, but most of the snow should remain above pass level. WPC
probabilities for 4 inches are high in the Cascades, with several
feet of snow likely above 9,000 ft. The second impulse rotates
onshore with a cold front Friday evening with another round of
precipitation, along with snow levels falling to as low as 2500 ft
in the WA Cascades, and 3000-4000 ft in OR/ID/MT. This causes an
expansion of the WPC probabilities for 4 inches across more of the
Cascades and Olympics, with an additional 1-2 ft of snow possible
above 9000 ft. Some light accumulations are possible in
Snoqualmie, Stevens, and Washington Pass, especially if any
instability noted in the high-res guidance can be released to
intensify snow rates.

On D3, the most potent shortwave moves onshore accompanied by a
deepening of the longwave trough and most impressive upper
diffluence in the RRQ of the anticyclonic curving jet streak. Snow
levels fall even further as a third cold front moves to the east,
becoming as low as 2000 ft in the WA Cascades, and 3000-5000 ft in
CA/OR/MT/ID. WPC probabilities for moderate to heavy snow expand
in areal coverage again, with widespread probabilities for 4"
above 50% in the Cascades and Olympics, and lower end
probabilities as high as 30-40% in the Sawtooth Range and Northern
Rockies.


For Days 1-3, the probability of significant icing is less than 10
percent.

Weiss