Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 PM EDT Thu Oct 17 2024

Valid 00Z Fri Oct 18 2024 - 00Z Mon Oct 21 2024


...Central Rockies...
Days 1-3...

**Significant early season winter to bring periods of heavy snow
 beginning tonight***

Mid-level shortwave trough digging south from Oregon will amplify
into a potent closed low across the Great Basin early Friday, and
then continue to deepen as it ejects into the Four Corners Friday
night. NAEFS ensemble tables suggest mid-level heights will fall to
-3 sigma, reflecting the impressive intensity of this feature.

As is typical with lows of this magnitude, it will be very slow to
move and begin to fill, not exiting from Colorado until beyond
this forecast period. This will result in a prolonged period of
intense synoptic ascent focused across the Four Corners and Central
Rockies, where height falls, downstream divergence, and upper level
diffluence in response to modestly coupled jet streaks overlap. At
the same time, a surface cold front will traverse west to east,
moving across the Central Rockies Thursday night through Saturday
morning. This will have a two-pronged effect of enhancing ascent,
as a wave of low pressure develops along it, while impressive
baroclinicity results in sharpening fgen to drive locally more
intense lift. Where this most intense ascent occurs, especially in
areas of upslope flow, precipitation rates could become
intense.

Downstream of this upper low, moisture advection will ramp up on an
impressive surge of 700-500mb RH leading to PWs that are progged to
reach as high as +2 to +3 sigma, additionally supporting an
environment favorable for widespread heavy precipitation. Snow
levels initially will be quite high, varying between 8000-11000 ft,
but will fall quickly behind the front and in response to the
rapidly lowering heights beneath the anomalous low. Snow levels may
fall as low as 4000-5000 ft in the Great Basin and into WY, but
will struggle to only around 6000-8000 ft downstream of the low,
and even higher out into the High Plains. This will limit
significant snow accumulations to the higher terrain, although
convective snow rates could drag these snow levels at least subtly
further down during the event.

Heavy snow is likely D1 in the higher terrain spreading from the
Absarokas southward through the Big Horns, Wind Rivers, Uintas,
Wasatch, and into the San Juans. Snow exceeding 12 inches is
possible in the Uintas and San Juans, but otherwise WPC
probabilities for exceeding 8 inches are generally 50-70% across
these ranges. For D2, The heavy snow shifts primarily into the San
Juans, where WPC probabilities for more than 8 inches are again
above 70%, and locally an additional 12+ inches of snow is likely
in the San Juans, however some moderate snow will also expand into
the Sangre de Cristos. By D3 things start to ramp down quickly, but
an additional few inches of snow is possible in the San Juans, with
event total snow exceeding 3 feet in some places likely.

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

Weiss