Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
401 AM EST Fri Dec 08 2023

Valid 12Z Fri Dec 08 2023 - 12Z Mon Dec 11 2023

...Pacific Northwest through the Northern Rockies...
Day 1...

Onshore flow over the Northwest continues today until a ridge axis
that reaches the WA/OR coast this evening. Snow levels dip below
2000ft today with Day 1 snow probs for >2" moderate high (40-70%)
across the WA/OR Cascades with ridges around Glacier NP in MT
getting the most snow today with moderately high (40-70%) probs
for >6".

Days 2/3...

The next atmospheric river arrives into western WA/northern OR
Saturday before drifting south over OR through Sunday. PWs of
1.25" south of a 110kt Pacific jet streak bring a quick thump of
snow to the WA Cascades (including Stevens, Snoqualmie, and
Lookout Passes) with levels around 3000ft Saturday afternoon
before levels rise quickly above 6000ft Saturday night before
shifting east over the Blue Mountains, Salmon River, Sawtooth, and
remaining Northern Rockies Saturday night into Sunday. Owing to
the overlap of favorable Pacific flow and strong dynamical
forcing, Day 2 WPC probabilities reflect a high (80-90%) chance of
snowfall exceeding 6" over the Washington Cascades with Day 2.5
probs moderately high (40-70%) over the northern ID ranges.


...Northern Plains through the Upper Midwest...
Days 1/2...

Shortwave troughs currently over southeast MT and northeast ND
phase into an upper low today over eastern ND with a developing
sfc low and a slow pivoting deformation band wrapping around and
over much of ND through tonight before slowly weakening over MN
Saturday. As this surface low develops, moist isentropic ascent
will intensify downstream, lifting into a modest TROWAL as the WCB
surges N/NW into Canada and wraps cyclonically back in ND. An axis
of heavy snow is expected as mid-level fgen will overlap
efficiently with a deformation axis beneath some of this higher
theta-e air. Mixing ratios within the best isentropic ascent are
quite high around 4g/kg, which will support moderate snow rates,
but with mesoscale ascent intensifying and the DGZ depth
increasing, snow rates should reach 1"/hr. Strong winds reaching
kt beneath the DGZ, should fracture dendrites to result in lower
SLRs/snow accumulations. Still, the threat for moderately
impactful snow and blowing snow across most of ND is maintained in
the WSSI. Day 1 snow probs are moderately high (40-80%) over
central and northern ND.

The mid-level closed low will become strung out Saturday in
response to increasing shear to the northeast occurring
concurrently with secondary shortwave energy rotating southward
out of Saskatchewan. This combined with the progressive nature of
the upper diffluence in the left exit region of the SWly jet
streak will result in an overall weakening of ascent, especially
as the surface low ejects northeast Saturday. However, the next
impulse from the Plains tracks across northern MO tonight before
lifting across Lake Michigan Saturday. A stripe of moderate snow
looks to develop over northern WI west of the surface low with at
least modest lake enhanced snowfall off Lake Superior Saturday
night in the western U.P. Snow probs on Day 2 for >4" are low over
northern WI and moderate (40-60%) in more typical lake effect
zones such as the Porcupine Mtns in the western U.P.


...Utah, Colorado and South-Central High Plains...
Day 1...

A digging upper trough trailed by a strengthening NWly jet streak
shifts from Utah through Colorado today which will provide lift
over the terrain, then enhance post-cold frontal NNEly flow over
the High Plains and focus snow bands over the eastern CO
Rockies/Palmer Divide with perhaps some overlap over the Denver
Metro by this afternoon. WPC probabilities depict moderately high
(40-70%) chance of >6" for the Wasatch, most CO Rockies, and along
I-25 south from the Palmer Divide. Snow bands will extend east to
at least the KS border and generally shift south in time.


...Central Appalachians through the Interior Northeast...
Day 3...

A positively-tilted trough over the Great Plains Saturday sharpens
as it digs to the central Gulf Coast Sunday morning. The
downstream southerly jet ahead of this meridional trough then
rapidly intensifies Sunday night as the northern stream portion
develops the dominant upper low over the eastern Great Lakes.
Rapid development of the surface low along the strong baroclinic
zone over the central Appalachians/Mid-Atlantic Sunday is expected
as the low lifts to New England Sunday night. Quick cooling behind
the cold front with an easterly component from the developing low
sends moisture into the cold sector which is aided by Appalachian
terrain to bring bands of moderate to locally heavy snow to
interior sections of the Northeast. There is decent agreement
among 00Z global guidance with this development pattern with a
stronger northern stream wave in the CMC leading to a farther west
solution and fairly agreement among the 00Z ECMWF/GFS. Day 3 snow
probs are moderate for >6" for the central Appalachians of WV,
western NY and northwest PA and low values for the Adirondacks
(though heavy snow will continue there Monday morning). Given
today is Friday and winter impacts begin Sunday night, winter
watches should be raised today.


Jackson