Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
305 PM EDT Fri Oct 25 2024

Valid 00Z Sat Oct 26 2024 - 00Z Tue Oct 29 2024


...Pacific Northwest...
Days 2-3...

An impressive closed low spinning off the British Columbia coast
will shed a shortwave and accompanying vorticity maxima into the
Pacific Northwest Sunday. This feature will spread height falls and
PVA to drive ascent eastward, with a collocated upper jet streak
also pivoting onshore to enhance lift. The best moisture advection
will occur downstream of this lead trough, but snow levels D2 will
be quite high, 7000-8000 ft, so despite impressive IVT exceeding
500 kg/m/s leading to widespread precipitation, snow will be
confined to only the highest peaks of the Cascades and Olympics.
However, during D3, the parent upper low shifts southward and opens
into a still sharp upper trough, lifting onto the WA/OR coast
Monday. While residual moisture will gradually decay through D3,
sufficient ascent downstream of this trough combined with upslope
ascent will drive continued moderate precipitation, with snow
levels falling to 4500-5000 ft. This will allow snow to accumulate
into lower elevations by the end of the forecast period, but
most snow is still expected only above pass level.

WPC probabilities for more than 6 inches of snow are only above
30% in the highest peaks of the WA Cascades D2, where locally 2
feet of snow or more is probably near the top of Mt Rainier. By D3
snowfall becomes more widespread, with WPC probabilities for more
than inches reaching 30-50% across much of the spine of the
Cascades from WA to OR, and several feet of event-total snow is
likely on the higher mountain tops.

The probability of significant ice across the CONUS is less than
10 percent.

Weiss