Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
344 AM EST Sat Nov 11 2023

Valid 12Z Sat Nov 11 2023 - 12Z Tue Nov 14 2023

...Pacific Northwest into the Northern Rockies...
Days 1-3...

A parade of weather systems will continue over the Pacific
Northwest/Northern Rockies over the next few days, bringing a few
rounds of precipitation. Early this morning, the analysis showed
an approaching cold front toward Washington State, associated with
a strong mid-level shortwave pushing onshore. This impulse arrives
by mid-day today and combined with a strong 130-150 kt jet streak,
favorable left exit region dynamics, and modest IVT values
(250-500 kg/m/s per CW3E), widespread precipitation is expected,
favored over the west-facing slopes of the Olympics and northern
Cascades. As the cold front sweeps eastward, snow levels are
expected to drop toward 3500-4000 ft later today, supporting
locally heavy snowfall. Higher elevations, including Washington
Pass, will likely receive more significant snowfall, and the
latest WPC snow probabilities are very high (>90%) for at least 12
inches in these areas, with localized 2-3 feet totals possible
through 12Z Sunday. As this forcing and moisture shifts eastward
later today/tonight, WPC probabilities for more than 8 inches
reach 40-60% near Glacier NP, with a slight chance (10-20%) of 12
inches.

The next system arrives late Sunday through Monday and compared to
the ongoing system, the moisture, large scale forcing, and
orientation/strength of the IVT is expected to be less, with the
S/SW IVT expected to only top out around 250-500 kg/m/s along the
coast. This system will draw warmer air in advance and snow levels
are expected to rise from 3500 ft early Sunday to above 4500 ft by
late Sunday/early Monday. The combination of the less favorable
ingredients and warmer air should suppress the snowfall
accumulations to generally under 6 inches Sunday-Monday for the
highest elevations of the northern Cascades as supported by the
WPC snow probabilities for 6 inches under 20-30 percent.


Taylor