Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Tue Oct 15 2024

Valid 00Z Wed Oct 16 2024 - 00Z Sat Oct 19 2024

...Northeast...
Day 1...

An elongated and anomalous 500mb trough extending from a stacked
upper low will amplify and pivot southeast across New England. A
surface trough beneath this feature will additional move southeast,
providing a period of enhanced ascent through convergence and
height falls to wring out modest, and lessening, available
moisture. Forecast soundings suggest the DGZ will dry rapidly
tonight into early Wednesday, but forced upslope ascent should
offset this drying at least subtly, resulting in a period of light
to moderate snow in higher elevations above 1500-2000 ft,
especially the first half of D1. Snow should not be too
significant, but the nighttime event and periods of heavier snow
will cause some accumulation, and WPC probabilities for more then 4
inches reach 10-30% in the highest elevations of VT and NH, with up
to 6" possible at the top of Mt. Washington.


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

Two distinct shortwaves embedded within an amplifying trough across
the West will spread ascent and moisture into the Pacific Northwest
beginning later on Wednesday. The first of these shortwaves will
feature minimal amplitude as it digs across WA state Wednesday
evening, but will be accompanied by a surge in 700mb RH and modest
IVT lifting into the interior Pacific NW. A surface cold front
pushing east beneath this trough will help cool snow levels from
around 8000 ft to as low as 4500 ft by the end of D1, which when
combined with the enhanced ascent and moisture will result in
moderate snow which has a 30-50% chance of exceeding 4 inches in
some of the highest terrain of the Cascades.

Immediately in the wake of this first shortwave, a more pronounced
trough will dig along the Pacific Coast and amplify into the Great
Basin by Friday. A secondary surge in 7090-500mb moisture will
accompany this feature, but in general moisture will be waning as
IVT continues to push east. However, lower initial snow levels will
continue to fall, dropping to as low as 3000-4000 ft, resulting in
more widespread light to moderate snow, including some light
accumulations of 1-3 inches at Washington and Stevens Passes.
Otherwise, WPC probabilities D2 reach above 70% along the spine of
the Cascades as far south as central OR, with locally as much as
10" of snow in the higher terrain possible.


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

The first potentially significant winter storm of the season, at
least for the terrain, looks likey to develop late-week across the
Intermountain West.

A shortwave trough digging out of the Pacific Northwest Thursday
aftn will rapidly amplify into a closed low centered over the
southern Great Basin by Friday aftn before shifting into the Four
Corners by the end of the forecast period. Ascent associated with
this feature will expand and intensify downstream in response to
pronounced height falls, potent mid-level divergence, and an
increasingly coupled upper level jet structure to drive diffluence
atop the other ascent. This will result in widespread impressive
lift across the Central Rockies, especially D3 (and beyond). This
ascent will be aided additionally by a potent cold front surging
southeast across the area, leading to enhanced baroclinicity upon
which a wave of low pressure will develop, and subsequent upslope
ascent within the post-frontal flow.

This forcing will encounter increasing moisture across the Central
Rockies where modest IVT will surge PWs to as much as +2 sigma
according to NAEFS, resulting in an environment favorable for heavy
precipitation, including snow in the higher elevations.

The models have come into better agreement in the timing and
placement of the upper low, reflected by shrinking IQR in the 500mb
height probabilities across the Central Rockies according to DESI,
which leads to inreasing confidence in the overall evolution of
this event. However, the timing of the front is still uncertain,
which by D3 leads to challenges in timing of precipitation changing
from rain to snow due to lowering snow levels and cooling
temperaturres. At this time, the GEFS cluster appears to be the
fastest/coldest solution, while the ECENS/GEPS is more similar.
Regardless of timing, as snow levels fall from 8000-10000 ft early
Friday to as low as 5000-6000 ft Friday night, snow will overspread
the higher elevations, and likely become heavy at times as some
instability develops in conjunction with sharpening fgen to drive
rates potentially above 1"/hr.

On D2, heavy snow is focused across the Absarokas, and WPC
probabilities for 6+ inches are 30-50%. The more significant and
heavy snow is likely D3, with accumulating snow spreading across
much of the Central Rockies and eastern Great Basin ranges. WPC
probabilities D3 for more than 6 inches of snow are as high as
70-90% in the Uintas, Wind Rivers, and Big Horns, with small
corridors above 70% also extending into the Abasarokas and portions
of the Wasatch. At this time impacts appear confined to higher
terrain, but at least short-duration heavy snow rates combined with
the early-season event will cause hazardous conditions on Friday.


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


Weiss