Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
431 PM EDT Fri Oct 27 2023

Valid 00Z Sat Oct 28 2023 - 00Z Tue Oct 31 2023

...Central & Southern Rockies to the Central High Plains...
Days 1-3...

An upper level trough with embedded shortwave energy will move
through the Western U.S. over the next couple of days. A strong
upper level jet streak forming over the Midwest along with the
height falls will bring a rather extended period of forcing for
ascent across portions of the Rockies and High Plains later
tonight through Sunday. While this is happening, a very
strong/early season dome of high pressure will settle southward
through the northern Rockies, bringing sufficiently cold air well
southward through the central Rockies and High Plains. At lower
levels, a steady diet of 850-700mb moisture flux that runs
directly into a secondary cold front over the Central Rockies. By
midday Saturday, the cold front is forecast to race south through
the central High Plains and force surface temperatures to plunge
below freezing while southwesterly flow at 700mb fosters a classic
overrunning event in place. An enlarging DGZ aloft Saturday
evening and into the overnight will also result in rising SLRs
that will be supportive of heavier snowfall rates across CO,
southern WY, and central NE. Across portions of western KS and the
OK/TX Panhandles, a narrow warm nose aloft overrunning the
sub-freezing temperatures will support the threat of freezing
rain/drizzle late Saturday through Sunday morning.

WPC snow probabilities through the weekend are highest across the
CO Rockies (70-80%) but also are notable across the southern
ranges of WY (40-60%) and Sangre de Cristo mountains in southern
CO and northern NM (40-50%). Within that, the CO Rockies have the
greatest probability for localized 12"+ totals (30-50%). For the
Denver/Boulder metro area, the WPC snow probabilities for 8 inches
have increased over the last forecast cycle to between 30-50% and
the latest WSE snow plumes for Denver/Boulder show a few scenarios
for 10-12" (PWPF at 5-10%). Should heavier snow bands (1-2"/hr
rates) materialize, the higher end scenarios may come to fruition.
The Probabilistic Winter Storm Severity Index (WSSI-P) also has
trended higher for moderate impacts for the urban I-25 corridor
from Fort Collins to Denver, now highlighting high probabilities
(70-90%). Further east, WPC snow probabilities for 4" remain
moderate (30-50%) across portions of Nebraska, particularly
north-central/northeast Nebraska where some stronger mid-level
frontogenesis may aid in localized/narrow heavier snow bands.
Behind the southward racing cold front, a narrow transition area
of freezing rain/drizzle may develop across portions of western
Kansas through the OK/TX Panhandles. While the latest WPC ice
probabilities are high (above 80%) for 0.01" of ice across western
KS, the probabilities drop off considerably (less than 30%) for
OK/TX Panhandles and are very low (less than 10 percent) for 0.1".
However, the light glaze may pose travel issues late Saturday
through Sunday morning.

...Northern New England...
Day 3...
An area of low pressure moving northeast from the Ohio Valley to
the Gulf of Maine will spread precipitation across much of New
England late Sunday through Monday. A sufficiently cold airmass in
place thanks to high pressure anchored to the north will result in
a mix of rain and snow in the area, predominately snow for the far
northern/interior New England areas, particularly upstate/northern
Maine where the latest WPC snow probabilities for at least 4
inches peak at 20-30 percent.

Taylor