Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 AM EST Wed Nov 29 2023

Valid 12Z Wed Nov 29 2023 - 12Z Sat Dec 02 2023

...Great Lakes...
Day 1...

The amplitude of the eastern CONUS trough will weaken today, but
one last shortwave and associated vorticity lobe swinging through
its base will result in a final day of additional lake effect snow
(LES). This shortwave is progged to move from northern lower
Michigan this morning into New England tonight, and some moderate
synoptic snow will accompany the weak PVA and height falls.
However, this feature is transient, so synoptic snowfall
accumulations are expected to be minor. More significant snowfall
is likely downwind of Lakes Erie and Ontario, but as low-level
flow backs ahead of the shortwave trough axis, the bands should
shift to a more WSW to ENE direction in response to subtle WAA,
which will result in weaker snowfall rates and an axis positioned
more to the NE of the lakes. As the trough axis pivots eastward
this evening, a brief surge of renewed CAA may occur, but with
more marginal thermal structure supporting a mix of rain and snow
before secondary shortwave ridging approaches rapidly from the
west bringing an end to the heavy LES. WPC probabilities for more
than 4 inches of snow are as high as 50%, but focused east of Lake
Ontario near Watertown.


...Southwest and Southern and Central Rockies...
Days 1-3...

A 500mb trough along the CA coast today will dive southeast
towards the Four Corners while amplifying into a closed low near
NM on Thursday. This low re-open and begin to fill as it advects
quickly into the Plains during Friday, but will be immediately
followed by a secondary shortwave trough Thursday night into
Friday, and potentially a third trough before Saturday morning as
spokes of energy rotate through an amplifying longwave trough
across the Western CONUS. Aloft, the subtropical jet streak will
pivot across northern Mexico and the Desert Southwest, becoming
more favorably aligned for diffluence beginning Thursday night.
This jet stream evolution will provide additional ascent, while
also supplying more significant Pacific moisture into the region.
PW anomalies according to the NAEFS ensemble tables are progged to
remain modest, however, peaking at +1 to +2 sigma Thursday night
into Friday, but this will be wrung out efficiently by the
synoptic lift and additional low-level ascent through upslope flow
and convergence as a back door cold front sags to the south. This
evolution will result in multiple waves of precipitation, with
light snow occurring D1, generally above 7000 ft, and more
significant accumulations likely D2-3 as a surface low develops in
the lee of the Sangre de Cristos to enhance moisture and ascent in
an already favorable synoptic pattern. WPC probabilities on D1 for
more than 4 inches of snow are modest around 20-30%, highest atop
the Kaibab Plateau and into the White Mountains of Arizona. WPC
probabilities D2-3 expand northeast into New Mexico and Colorado,
favoring heavy snowfall exceeding 6 inches in the Jemez Mountains
and San Juans.


...Cascades through the Northern Rockies...
Days 2-3...

An increasingly wet period is likely beginning Thursday night and
continuing through the end of this forecast period. A sharpening
shortwave will move onshore WA/OR Thursday night with PVA and
height falls to drive ascent, followed immediately in its wake by
confluent mid-level flow beneath an intensifying zonally oriented
jet streak aloft. Within this confluent flow, additional spokes of
shortwave energy and accompanying vorticity maxima will shed
onshore, driving multiple weakening cold fronts on shore in
tandem. This will manifest as a surge of PW and IVT, approaching
+2 sigma at times, which will fuel expanding precipitation, some
of which could be heavy, especially late Friday into Saturday.
There may be multiple rounds of moderate to heavy snow: the first
Thursday into Friday, and the second, likely more impressive, late
Friday and Saturday. Snow levels will fluctuate through the period
between cold fronts and in-between WAA, but should generally be
around 2000 ft north, 5000 ft south. This will likely result in
heavy pass-level snow producing significant travel impacts by D3.
WPC probabilities for more than 6 inches peak above 60% D2,
highest in the Oregon Cascades and above 4000 ft, and then expand
eastward to the Northern Rockies, while rising above 90% in the WA
and OR Cascades by D3. 2-day snowfall in the Cascades could exceed
3 feet in the higher elevations.


...Central Plains into the Midwest...
Day 3...

An area of low pressure emerging from the Southern Plains Thursday
night will shift northeast through Friday as a potent shortwave
opens from the Four Corners and ejects into the westerlies. This
shortwave will likely shear out and leave the low behind during
Friday as a secondary vorticity lobe swings through the region,
and it is this secondary feature that will finally cause the
precipitation to exit to the northeast. This evolution will result
in two waves of precipitation, one late Thursday night into
Friday, with a second area developing Friday night. The first is
expected to be more impressive due to more intense synoptic lift
and better theta-e advection northward, but the column will be
marginally supportive for any wintry precipitation. The guidance
has trended a bit wetter and farther north tonight, and with cold
high pressure centered over Canada, wet-bulb cooling could result
in a stripe of freezing rain from eastern Kansas through southern
Iowa. There is a lot of uncertainty with how this will develop,
but the setup favors at least the threat for minor icing, and WPC
probabilities for 0.1" of ice are as high as 5-10%. The second
area of precipitation may be a bit colder and more strongly
mesoscale forced as an axis of deformation develops upstream of
the second shortwave. This could result in a stripe of light to
moderate snow from northeast KS through eastern IA, but at this
time WPC probabilities for more than 1 inch of snow are less than
10%.



Weiss