Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
220 PM EST Tue Nov 22 2022

Valid 00Z Wed Nov 23 2022 - 00Z Sat Nov 26 2022

...Northern to Central Rockies...
Days 1-2...

A shortwave embedded within otherwise fast flow will move onshore
the Pacific Northwest this evening and then dive rapidly southeast
to be positioned over Wyoming on Wednesday. This feature is then
likely to begin to deepen as it closes off across the Central
Rockies and splits from the faster northern stream on Thursday.
The residual closed low will then dig into the Southern High
Plains on Friday, leaving lingering strung out vorticity across
the region. As this evolution occurs, an accompanying Pacific jet
streak will arc southward as an impressively amplified
ridge-trough pattern develops across the western half of the
country. Combined jet level diffluence with height falls and PVA
at the mid-levels will result in widespread deep layer ascent,
although omega is progged to be modest overall. Additionally, a
surface wave of low pressure will move eastward near the Canadian
border, dragging a cold front eastward. Some enhanced fgen along
and just behind this front combined with increasing E/NE upslope
flow will help to produce enhanced ascent from the Northern
Rockies southward through the CO Front Range. Snow levels will
generally be 3000-4000 ft during the period of greatest ascent,
and while forcing is likely to be most intense across ID/MT,
enough ascent is likely that periods of moderate to heavy snow are
expected through much of the region. For D1, WPC probabilities are
high for more than 4 inches from the Northern Rockies through the
Absarokas and Tetons, with locally 10-12 inches of snow likely in
the highest terrain. As the forcing sinks southward, WPC
probabilities for more than 4 inches climb to 10-30% for the
higher terrain from the Cheyenne Ridge through the CO Front Range
and onto the Raton Mesa. Locally as much as 6 inches of snow is
possible where upslope flow can maximize wringing out the QPF.



...Southern High Plains...
Day 3...

A mid-level shortwave will dig into the Southern Plains while
amplifying into a full latitude positively tilted trough, within
which a closed low will rotate near the Texas Panhandle. Coupled
jet streaks are likely to intensify both upstream and downstream
of this trough axis, resulting in a period of impressive deep
layer ascent. The models feature considerable spread in the
placement of this upper low which will have significant impacts on
the placement of heaviest precipitation, but it is becoming more
likely that intense UVVs driven by fgen and deformation N/NW of
the low, and aided by steepening lapse rates near the cold core
aloft, should produce enough dynamic cooling that moderate to
heavy snow accumulations are possible from the High Plains of NM
through the Panhandle of TX and into western OK. There remains
uncertainty into how significant the snowfall could be as the
low-level thermals are modest, and the most intense UVVs appear to
overlap with the warmest aftn hours. However, impressive moist
isentropic upglide at 295K-300K is characterized by mixing ratios
of 4-6 g/kg, which will help moisten the column, while also
lifting within the WCB into a modest TROWAL to potentially enhance
instability aloft noted by theta-e lapse rates falling to <0C/km
late Friday. Together this suggests that at least a period of
heavy snow rates are likely which should overcome marginal thermal
structure. The latest WPC probabilities for snowfall have
increased, and now feature a 30-50% chance for more than 4 inches
from the Sacramento mountains of NM eastward through the TX
panhandle near Amarillo and Lubbock, with probabilities for 8+"
reaching 10-20% in this same area.

The potential exists for much more snowfall in some areas as well
as reflected by ECMWF EFI tables that depict only a modest
probability of this event, but feature a high shit of tails
indicating the potential for substation snow if it does occur.
Additionally, a bimodal distribution within the WSE plumes
indicate a "boom or bust" potential in the vicinity of this band,
with also an impressive spread in both WSE and NBM percentiles.
PWSSI values for moderate impacts have also increased, driven
primarily by snow amount and snow load, suggesting impactful snow
is possible.

Weiss