Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 AM EDT Thu Sep 30 2021

Valid 12Z Thu Sep 30 2021 - 12Z Sun Oct 03 2021

...Southern Rockies...
Days 1-2...

Elongated trough embedded within an amplified mid-level flow
regime will transition into a closed low over the Four Corners
this aftn before slowly ejecting northeast and opening into the
westerlies on Saturday. As this trough develops, ascent will be
provided via increasing mid-level divergence and height falls, as
well as modest upper diffluence as subtle coupled jet streaks
develop both upwind and downwind of the primary trough axis,
producing an upper divergence maxima across NM/CO. This will occur
concurrently with a back door type front dropping through New
Mexico, and deep layer ascent within an environment characterized
by PWs of +1 to +2 standard deviations above the climo mean should
produce scattered to widespread precipitation Thursday into
Friday. The antecedent airmass is quite warm, so much of this
precipitation should fall as rain. However, snow levels are
progged to drop to below 11,000 ft overnight into Friday,
suggesting snow will accumulate in the higher peaks of the San
Juans and Sangre De Cristos. Some modest deformation invof the
mid-level low combined with upslope flow and at least subtle 700mb
fgen noted in guidance could lead to periods of heavy snow, and
WPC probabilities for 4 inches reach 40-50% in the San Juans
tonight, and above 60% in the southern Sangre De Cristos on
Friday. Total snowfall may approach 12" in the highest peaks.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.


Weiss