Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 PM EST Tue Nov 5 2024

Valid 00Z Wed Nov 06 2024 - 00Z Sat Nov 09 2024


...Montana and Wyoming Mountain Ranges...
Day 1...

Vigorous upper trough moving through the northern Rockies today is
starting its transition to split into two separate systems, with
the northern entity becoming an upper low that will wobble eastward
into through the northern Plains overnight. Broad divergence will
favor light snow over much of Montana but the eastward then
southward movement of the forming upper low will turn the flow
northerly over central portions of the state, which will enhance
upslope flow into the Little Belt, Big Snowy, and Big Horn
Mountains. Snow will fall at a modest to heavy rates per the 12Z
HREF which shows >1"/hr probs of 50-90%. Lighter snow is expected
farther south through WY as the southern portion of the trough
begins to split off and sink toward the Four Corners, with some
higher totals in the high terrain. Snowfall should gradually taper
off in most areas of MT and WY by Wednesday morning. WPC PWPF shows
high chances (>70%) for snowfall totals >8" over many of the MT
ranges, especially >6,000ft, through Wednesday afternoon. Some of
the tallest peaks of these ranges could surpass an additional 15"
locally.

...Central/Southern Rockies and Adjacent High Plains...
Days 1-3...

The southern part of the upper level feature responsible for the
heavy snow in the northern Rockies will sink southward through
eastern UT on Wednesday into AZ by early Thursday. After that, it
should slowly turn eastward into NM as a closed upper low and set
the stage for a potentially significant and long-duration heavy
snowfall event for parts of the southern Rockies and nearby High
Plains. The guidance still remains somewhat uncertain on the
evolution of this system, but has narrowed in on a slower/amplified
solution with today's 12z runs. A cold front will bring in colder
air to the southern Rockies and adjacent High Plains as strong
upper level divergence aloft will support broad ascent over the
region. An initial area of mesoscale banding associated with this
cold front, a leeside cyclone, and left exit region of a
strengthening upper jet over the central High Plains on Wednesday
may produce a swath of heavy snow from the Palmer Divide to the
KS/CO border. Hires guidance, including the 12z HREF snowband tool,
highlight this region where 1-2" snowfall rates could occur.
Medium probabilities (30-50%) of at least 8" of snow exist across
parts of east-central Colorado through early Thursday according to
WPC's PWPF, which is an increase from previous runs.

More directly related to the upper low forecast to impact NM and
southern parts of CO between Wednesday night and Friday, moisture
levels will be modest and IVT on southwest to southerly flow is
forecast to near the 90th percentile. This should act in concert
with upslope enhancement to promote moderate to heavy snow over the
southern Rockies. Trend has been for greater QPF over the High
Plains of southeast CO and northeast NM, as well as into the
terrain of northern NM and south-central CO. Some uncertainty
remains in the ensemble guidance and cluster analysis regarding the
western extent of heaviest QPF, with a notable westward trend in
global guidance today in response to higher heights forecast over
the central United States. However, even more uncertainty exists
across the High Plains regarding low-level thermals associated
with this westward trend as the event carries on and warmer air
advects into the High Plains. Nevertheless, significant snow is
still expected in at least portions of southern CO into northern NM
with this dynamic system.

The upslope component should favor both the SW/S-facing slopes of
the San Juans and Sangre de Cristos as well as farther north into
the Front Range via the cold front. The key on Thursday will be
where the upper low sets up and the extent of the moisture/upslope
flow into the Southern Rockies. By Friday morning, the upper low is
forecast to drift into NM with the brunt of the snowfall over NM.

WPC PWPF for the event through 12Z Fri shows high chances (>70%)
for snowfall >8" in the San Juans, the Sangre De Cristo, and into
parts of the far western High Plains (including the Raton Mesa).
Highest totals are likely over the Sangre De Cristo where 2ft+ is
possible (>50-70% chance). Around the Denver metro area, WPC PWPF
for >4" of snow are around 30% but increase to the south (i.e.,
Palmer Divide) and then increase again near/south of Colorado
Springs and Pueblo to reach 50-70%.


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


Snell