FOUS11 KWBC 271922
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
322 PM EDT Sun Oct 27 2024

Valid 00Z Mon Oct 28 2024 - 00Z Thu Oct 31 2024


...Pacific Northwest to the central Rockies...
Days 1-3...

A much more active pattern is set to develop this week as a large
trough begins to expand across the western CONUS.

A Pacific trough will encroach on the coast D1, with the primary
trough axis pivoting onshore by Tuesday morning. The most
impressive vorticity impulse embedded within this trough is progged
to surge into central CA, which will then help to amplify this
feature as it advects east on Tuesday into the Great Basin and then
Four Corners. Initially, this trough will remain progressive and
positively tilted, but should deepen, and some of the guidance
suggests a negative tilt or even closing off of the upper height
fields late in the forecast period. Although there is, as expected,
some uncertainty into the placement and speed of this evolution,
500mb heights across the West feature minimal spread in the IQR
according to DESI, so confidence is pretty high in the progression.

As this trough surges eastward, it will drive increasingly intense
ascent through height falls and downstream divergence, especially
during the period of best amplification D2, and combine with a
strengthening 250mb jet streak pivoting from CA into the central
Rockies to force impressive deep layer ascent, especially in the
Rockies. At the same time, moisture will become increasingly robust
due to onshore flow of Pacific moisture streaming ahead of the
trough axis, and pushing PWs to +1 to +2 sigma according to NAEFS
ensemble tables. This will result in widespread and expanding
precipitation, first in the WA/OR Cascades, but then becoming more
robust across the central Rockies beginning Tuesday aftn.

The challenge with this event will be how much snow will spread out
of the higher terrain and into the High Plains Tuesday and
Wednesday. While forcing will remain impressive, a cold front
digging southward will begin to cool the column, while also
providing a region of enhanced baroclinicity leading to stronger
frontogenesis. This should result in a surface wave developing near
UT/CO and then lifting E/NE into the Plains by Wednesday. The
combination of fgen, this modestly strengthening surface low, and
the southward push of the cold front should help cause snow levels
to crash from around 8000ft early to 4500-5000 ft late. This,
combined with periods of dynamic cooling, will result in
significant snow accumulations in many areas.

The other challenge with this system will occur D3 as the low
ejects into the Plains and leaves what could be a potent comma
head/deformation axis with precipitation to its NW. There is
considerable model spread in the placement/timing of this feature,
so confidence is lower, but some of the more aggressive guidance
indicates the band will be strong enough to cause dynamical
cooling to support of swath of snow into the Dakotas and maybe even
MN before the end of the forecast period. Despite favorable omega
into a subjectively deep DGZ, the weaker/less snowfall solutions
are preferred based on consensus and ensemble means, but it will be
worth monitoring the next few model runs to see if this stripe of
heavier snow could materialize.

At this time, while uncertainty still exists in both the elevations
at which significant snow can accumulate, and the eastern footprint
of the heavy snow axis, heavy snow is likely in a lot of the higher
elevations. On D1, WPC probabilities for more than 6 inches are as
high as 70-90% in the WA Cascades as a weak low pressure approaches
the coast beneath the primary mid-level trough. By D2, more
expansive precipitation begins in the central Rockies, and WPC
probabilities for more than 6 inches are above 50% in the San
Juans, Uintas, Big Horns, Wind Rivers, and around Yellowstone NP.
By D3 precip begins to wane, but periods of moderate to heavy snow
will persist across the higher terrain of CO and WY, and as far
east as the Black Hills of SD where WPC probabilities are 30-70%
for an additional 6+ inches. Storm total snow of 12-18" is likely
in some of the highest elevations by Wednesday night.


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

Weiss



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