Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 AM EST Thu Dec 07 2023

Valid 12Z Thu Dec 07 2023 - 12Z Sun Dec 10 2023

...Northwest ...
Days 1 and 3...

Onshore flow into the Pacific Northwest under multi-impulse
troughing will continue through tonight with a reinforcing
shortwave trough shifting southeast (on an intensifying NWly jet)
over Oregon late tonight brings about a gradual ends that lasts
into Friday. Slow height falls under this troughing allows snow
levels to drop from around 3500ft now to around 1500ft as snow
tapers off early Friday. Divergent left-exit region of the NWly
jet streak combined with steady 700mb moisture flux and
topographically-favored ascent leads to heavy snow from the
Cascades through the Northern Rockies. Day 1 snow probs for >8"
are high (>80%)  over the OR/WA Cascades, the Salmon
River/Clearwater/Bitterroots and Lewis range in ID/MT and through
the Absarokas around Yellowstone.

Ridging quickly builds east Friday with the ridge axis crossing
the OR/WA coast Friday evening. The next atmospheric river arrives
into WA/northern OR late Saturday morning with snow levels quickly
rising to 5000-7000ft. Day 3 snow probs are high for >8" in the WA
Cascades.


...Northern Plains...
Days 1/2...

A shortwave trough currently over the MT/WY border will allow
further development of a surface low over the Dakotas tonight,
drifting into Ontario Friday as the upper wave closes into a low.
Aloft, a modest SWly jet streak will shift across Neb/IA into
Friday with the favorable left exit diffluent region overlapping
the best height falls to help strengthen a surface low through
Friday. Modest upstream theta-e advection to produce a swath of
precipitation on the north side of this low. This precipitation
will be wrung out by additional ascent as a modest deformation
axis pivots west of the low as it pulls away, with the developing
bands becoming snow across mainly North Dakota. The DGZ deepens
considerably during this period and will support some higher SLR,
but total forcing is still expected to be modest and there is some
uncertainty as to how far south into the CONUS the heavy snow will
fall. Day 2 WPC probabilities for >4" are moderate (30-60%) over
northern ND.


...Central Rockies & High Plains...
Days 1/2...

By Friday morning, lee-cyclonegenesis is well underway over the
southern Great Plains as the digging NWly jet-streak reaches the
Four Corners. The resulting enhanced region of deformation and
frontogenesis in the 700-550 mb layers (possibly maximized in the
DGZ), with 850-700mb moisture flux streaming in from the south and
west, will result in a burst of moderate to locally heavy snowfall
late tonight through Friday over the Colorado Rockies and High
Plains. This activity is expected to spread southeastward with
time toward the Sangre De Cristos as east-northeasterly surface
flow ascends the terrain behind a strong cold-frontal passage as
snow levels quickly drop below 4000ft. Day 2 snow probs for >6"
are low to moderate (30-60%) over the central CO Rockies and a
swath along the Palmer Divide into eastern CO.

For the Denver metro area is for bands to quickly change to snow
Friday morning with another round Friday evening.


...Upper Midwest...
Day 3...

The amplifying long wave trough over the Rockies Friday night
allows rapid development of the surface low tracking northeast
from the southern Plains late Friday through the Great Lakes on
Saturday. Marginal thermals should be overcome in deformation
banding around the back side of the low. Day 3 snow probs for >4"
are lower moderate (20-40%) over far northern WI and the western
half of the U.P. of MI.


Jackson