Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
321 AM EDT Thu Oct 27 2022

Valid 12Z Thu Oct 27 2022 - 12Z Sun Oct 30 2022

...Central and Southern Rockies...
Day 1...

A rapidly amplifying shortwave will deepen into a robust closed
500mb low as it drops across the Four Corners and progresses into
the TX Panhandle on Friday. NAEFS ensemble tables suggest this low
will feature 500mb heights that are more than 3 sigma below the
climatological mean, indicative of the strength of this feature.
At the same time, an intensifying jet streak will develop
downstream of the primary trough axis, with 300mb winds reaching
130 kts in the core of the jet. The favorable LFQ for diffluent
ascent is progged to move across CO/NM this aftn and evening,
combining with impressive height falls and strong PVA to drive
deep layer ascent. A surface low will slowly deepen in the lee of
the Rockies and drop southeast with this upper low, and the
guidance is suggesting this will evolve a bit more to the south
than previously anticipated. This will allow cooler and subtly
more intense 700mb winds to rotate around the low and upslope into
the terrain, especially the Sangre de Cristos and out onto the
Raton Mesa. At the same time, strong low-level fgen will help
drive omega into the DGZ to increase dendrite growth, and combine
with periods of theta-e lapse rates less than 0C/km to produce
convective snow rates. The WPC prototype snow band tool indicates
that snowfall rates may reach 2"/hr at times, which when combined
with the cooler temperatures as the low shits south, will likely
result in rapid accumulation, and the pWSSI shows a high chance
for moderate impacts, with even some low probabilities for major
impacts due to snow rate. WPC probabilities for 6 inches or more
have increased, and are now 60-70% in the Sangre de Cristos, and
30-40% along the Raton Mesa, and locally more than 10 inches is
possible.


...Pacific Northwest...
Day 1...

Confluent and moist mid-level flow will continue to angle onshore
WA state, with an embedded shortwave progged to rotate onshore
tonight. A brief surge of WAA ahead of this shortwave trough will
produce moderate ascent into a moistening column, but also with
snow levels rising to 6000 ft. Above this level, heavy snow will
accumulate, with WPC probabilities for more than 4 inches reaching
20-30%, generally just for the far northern WA Cascades, and above
pass level.


The probability of significant icing is less than 10 percent
nationwide.

Weiss