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

Valid 00Z Fri Dec 08 2023 - 00Z Mon Dec 11 2023

...Northwest ...
Days 1-3...

Modest IVT will persist across much of the West tonight into
Friday as pinched 700-500mb flow surges southeast into the coast
and spills across the Great Basin and into the Northern Rockies.
This will be accompanied by lobes of vorticity shearing out
through the flow, and an area of impressive upper level diffluence
as the 110kt Pacific jet streak pivots overhead and dives as far
southeast as the Four Corners by Saturday morning. The most
impressive snowfall is forecast at the leading edge of this upper
jet, limiting the potential for excessive snowfall due to the
progressive nature of this driving feature. However, snow levels
that will generally waver between 2000 and 3000 ft will allow for
impactful snow in the terrain, including many of the mountain
passes from the Cascades through the Northern Rockies and into the
NW WY ranges and Wasatch of UT. Accordingly, the latest WPC
probabilities denote a high (80-90%) chance of snowfall exceeding
6" in the peaks of the terrain through Friday night, where upwards
of a foot of snowfall is possible.

A brief respite from the snowfall is expected to start D2 as the
forcing ejects into the Plains, but secondary energy will quickly
approach the Pacific Northwest coast once again late Saturday as
yet another shortwave digs into the region. While this shortwave
and its accompanying height falls/PVA are not anticipated to reach
the immediate coast until late D3, confluent mid-level flow ahead
of this feature will again drive enhanced moisture onshore, with
both the GEFS and ECENS probabilities for IVT of 500 kg/ms
exceeding 80% on D3. With ascent intensifying within a Pacific jet
streak reaching 150kts in conjunction with shortwave energy,
increasing moisture, and upslope flow in the terrain most
orthogonal to the mean winds, heavy snow will once again spread
into the Cascades on D2 and then farther east into the Blue
Mountains, Salmon River, Sawtooth, and remaining Northern Rockies
on D3. Owing to the overlap of favorable Pacific flow and strong
dynamical forcing, WPC probabilities reflect a high (80-90%)
chance of snowfall exceeding 6-8" over the Washington Cascades by
Sunday morning, with an increasing signal for heavy snow also
noted over the Northern Rockies by the end of Day 3. As snow
levels lower to less than 2000 ft, especially in the WA Cascades
and into the Northern Rockies, impactful snowfall exceeding 4
inches will become likely at many of the area passes, including
Stevens, Snoqualmie, and Lookout Passes, where WPC probabilities
for more than 4 inches reach 50-80%.


...Northern Plains through the Upper Midwest...
Days 1-3...

A shortwave ejecting northeast from the Central Rockies will
amplify into a closed mid-level low over ND late tonight and then
slow considerably before diving southeast into MN Saturday morning
in response to a second vorticity impulse rotating around its
periphery. Weakly coupled jet streaks atop this mid-level
evolution will aid in the deepening, and although the zonal
downstream jet streak will weaken with time, a subtropical jet
streak arcing poleward Friday will maintain impressive upper
ventilation to help drive surface cyclogenesis across the Northern
Plains. 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. The
setup has become more favorable for an axis of heavy snow as it
appears a stripe of 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 noted by SREF
probabilities exceeding 70% for 100mb of depth, impressive snow
rates exceeding 1"/hr are possible. At the same time, SLRs may be
lower than soundings indicate due to intense winds reaching 50kts
beneath the DGZ, which could fracture dendrites to result in
effectively lower snow accumulations. Still, the threat for
impactful snow across ND has increased as this low pulls away,
with heavy snow and gusty winds resulting in areas of moderate
impacts in the latest WSSI. The last three cycles of the WPC
snowfall probabilities reflect the heightened ceiling for this
event, where now central ND has a 40-70% chance of snowfall
exceeding 4" through Saturday morning (up from 0-10% at this time
yesterday), with locally higher snowfall approaching 8 inches
possible (10-20%).

The mid-level closed low will become strung out 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 LFQ of the aforementioned jet streak will result in an
overall weakening of ascent, especially as the surface low pulls
away rapidly to the northeast and the region of greatest moisture
also shunts away to the north. However, the deformation axis
aligned SW to NE to the west of this surface low will likely
continue to track eastward with some additional mid-level fgen
advecting in tandem to maintain an axis of enhanced lift. This
could manifest as a stripe of moderate snow shifting east into MN,
northern WI, and the western U.P. of MI, with at least some modest
lake enhanced snowfall possible in the western U.P. Sunday.
Overall accumulations D2 are progged to be less than what occurs
D1, but WPC probabilities denote a 40-60% chance of greater than
2" snowfall through D2, with a local maxima of 20-30% for more
than 4 inches D2.5-D3 over the western U.P. on D3.


...Central Rockies...
Days 1-2...

Increasing moderate to locally mountain snowfall is expected to
begin later today across the Central Rockies as a potent 250 mb
jet streak digs into the base of a broad upper-trough and couples
with the entrance region of a weakening 250 mb jet over the
Northern Plains. The strong synoptic ascent and resultant
amplifying pattern will yield lee-cyclonegenesis Friday along a
strong front sweeping through the Southern Plains. Northeasterly
upslope flow in the post-frontal regime, combined with an area of
700-500 mb frontogenesis aloft (partially aligned in the DGZ) will
shift an axis of snowfall south and east of the Palmer
Divide/Raton Mesa, with perhaps some overlap over the Denver Metro
by Friday afternoon. Through Saturday evening, WPC probabilities
depict a high (>80%) chance of snowfall exceeding 4 inches in the
peaks of the Central Rockies, with a moderate (30-50%) chance
noted along the urban corridor of I-25 south of Denver.

Weiss/Asherman