FOUS11 KWBC 160752
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Wed Oct 16 2024

Valid 12Z Wed Oct 16 2024 - 12Z Sat Oct 19 2024

...Pacific Northwest...
Days 1-2...

An upper level shortwave trough ushering in a surge of 850-700mb
moisture and resulting in upper level height falls will cause snow
levels to gradually drop from 6,000ft early this morning to around
4,000ft by Wednesday evening over Washington's Olympic and Cascade
Ranges. As this initial 500mb vort max exits to the east Wednesday
night, a second trough will dive south through the region on
Thursday with snow levels bottoming out just below 4,000ft in some
cases. The heaviest snowfall is likely to occur along the
Washington Cascades Wednesday evening with moderate-to-heavy
snowfall possible along the tallest peaks of the Oregon Cascades
Thursday morning. WPC probabilities depict high chances (>70%) for
>4" of snowfall in the Washington Cascades and peaks above 5,000ft.
The taller volcanic peaks of the Washington Cascades sport
moderate-high chances (50-70%) for snowfall >8" through Thursday.


...Northern & Central Rockies...
Days 2-3...

**First Significant Winter Storm of the Season to Impact the
 Central Rockies**

The evolution of the first significant winter storm of the season
across portions of the Intermountain West begins as an upper level
trough enters the Pacific Northwest on Thursday. As Thursday
progresses, the upper trough will deepen as it tracks south through
the Great Basin Thursday night, then becoming a closed low along
the UT/AZ border by midday Friday. The low continues to slowly
track across northern Arizona Friday night but remain stalled over
the Four Corners region early Saturday morning.

NAEFS shows upper level evolution that is quite anomalous from a
variety of parameters. At 250mb, a jet streak over the northern
High Plains will be coupled with a jet streak at the base of the
amplifying upper trough, thus maximizing upper level divergence
over the Intermountain West on Thursday. As the upper trough cuts
off into an upper low by Friday, strong divergence beneath the left
exit region of the 250mb jet streak will be placed over Utah and
the Central Rockies. NAEFS shows 500-700mb heights and temperatures
that are routinely below the 10th climatological percentile,
particularly over the Great Basin and Utah late Thursday into
Friday. Perhaps most impressive are the IVTs which will eclipse
300 kg/m/s (above the 99th climatological percentile) in southern
UT Thursday night. Mean flow out of the SW will result in strong
upslope ascent into the Uinta and San Juans in particular.

In the Absaroka, Big Horns, and Wind River Ranges, WPC PWPF shows
high chances (>70%) for storm event snowfall totals >8" with at
least moderate chances (>50%) for snowfall >12" in the tallest
peaks. Farther south, the Uinta and San Juan sport high chances
(>70%) for snowfall totals surpassing 12". In fact, the Uinta sport
moderate chances (40-60%) for >18" of snowfall through Saturday
morning, with high chances (>70%) in the San Juan for >18" of
snow. The WSSI-P Moderate probabilities are keying in on the San
Juans as seeing the most impactful snow from this event, primarily
due to a combination of the algorithm's focus on snow totals,
snowfall rates, and snow load. Probabilities are showing moderate
chances (40-60%) for snowfall that would result in hazardous
driving conditions and potential closures and disruptions to
infrastructure in the San Juans, Wasatch, and Uinta. While Moderate
Impacts are deemed lower chance (10-30%) in the Absaroka, Wind
River, and Big Horn Ranges, potential impacts are elevated compared
to what the algorithm suggests given it is the first significant
winter storm of the season in these Ranges above 7,000ft. There are
moderate-high chances (50-70%) for Minor Impacts in these Ranges,
as well as far south as parts of the Sangre De Cristo and far
northern mountains of Arizona.


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

Mullinax



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