Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
257 PM EST Wed Nov 15 2023

Valid 00Z Thu Nov 16 2023 - 00Z Sun Nov 19 2023

...Northern Rockies...
Day 1...

An upper level disturbance tracking through Washington State today
will generate PVA over the Northern Rockies at the same time that
a jet streak over Canada places its diffluent right-entrance
region overhead. Meanwhile, 700mb frontogenesis over Montana will
coincide with a narrow conveyor belt of 700mb moisture flux to
produce periods of heavy snow in mountain ranges such as the
Bitterroots and Lewis Range. Snow will pickup in intensity this
afternoon and persist into the evening hours as low pressure
develops over northern Montana. The snow will not last too long
but will pack a punch, thanks to not only the strong synoptic and
mesoscale forcing, but 700mb winds that are above the 90th
climatological percentile according to NAEFS will create a healthy
upslope component into these ranges.

Latest HREF shows high chances (80-90%) for snowfall rates >1"/hr
with the WPC Snowband Probability Tracker (SPT) suggesting 2"/hr
rates are possible between 21Z Wed - 06Z Thurs. WPC probabilities
for snowfall totals >4" are high (80-90%) for the Lewis Range,
including Glacier National Park and the Marias Pass. It is also
worth noting WPC probabilities show moderate chances (40-60%) for
snowfall totals >8", suggesting the potential for double digit
totals in the higher peaks of the Lewis Range. The WSSI shows
Minor to even some Moderate Impacts in the affected ranges of the
Northern Rockies with Snow Amount and Blowing Snow the primary
impacts that are likely to cause disruptions to daily life this
evening and into Thursday morning.

...Sierra Nevada...
Day 1 & 3...

As an impressive upper low swirls off the West Coast today, a lobe
of vorticity on the southern flank of the cyclonic circulation
will be directed at Southern California this afternoon. At 250mb,
the Sierra Nevada will be well located beneath the diffluent
left-exit region of a 250mb jet streak located to the south of the
upper low. In addition, a southerly IVT ranging between 200-400
kg/m/s over Southern California will bring about plenty of Pacific
moisture and forced topographic ascent along favorably positioned
terrain where upslope flow is maximized. However, the period of
heaviest snowfall is rather brief, starting late afternoon and
ending before sunrise Thursday. WPC probabilities show
low-moderate chances (30-50%) for snowfall totals >4". Heaviest
totals will occur in elevated terrain above 10,000ft.

By Day 3, the upper low off the West Coast moves east and makes
its way ashore on Saturday. The strongest IVT values will reside
well south of the Sierra Nevada, but modest PVA and diffluence
aloft working in tandem with SW upslope to generate periods of
heavy snow at elevations at/above 9,000ft. WPC probabilities show
low-moderate chances (30-50%) for snowfall totals >4" at those
higher elevations of the southern and central Sierra Nevada.

...Northern Maine...
Day 3...

While chances for >4" of snowfall remain <10% according to the
latest WPC probabilities, latest trends in deterministic guidance
are leaving the door open for a heavier snow event in northern
Maine on Saturday. The synoptic scale setup includes an upper low
off the Southeast coast on Friday merging wth a longwave trough
that will traverse the Great Lakes Friday afternoon. A lot comes
down to when these two troughs phase and how quickly an organized
area of low pressure can develop south of Nova Scotia. Should it
form sooner and track farther west, the deformation zone may flex
far enough west to generate heavy snowfall in northern Maine.
Given this setup is highly dependent on storm track and timing, a
large range of scenarios exist according to WPC's WSE Plumes. At
this time, WPC probabilities feature low chances (10-30%) for >2"
of snow in the North Woods, St. John Valley, and Central Aroostook
of northern Maine on Saturday. The WSSI-P also shows 30-40%
chances for Minor Impacts in northern Maine with Snow Amount and
Snow Load the primary drivers.

Mullinax