Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
317 AM EST Sat Dec 14 2024

Valid 12Z Sat Dec 14 2024 - 12Z Tue Dec 17 2024


...Western U.S...

An active period will bring widespread winter weather to much of
the West through early next week.

A large trough just offshore the Pacific Coast to start the period
will shed two distinct compact vort maxima/shortwaves onshore on
D1. One of these will pivot northward into British Columbia, while
the second one ejects into central CA. Although the northern
impulse is a bit deeper relative to the CFSR climatology (500mb
heights below the bottom 2.5 percentile according to NAEFS), the
greater moisture/IVT will pivot into CA associated with the
southern system as the northern moisture channel lifts into Canada.
Additionally, there will be a modestly coupled jet streak
downstream of the primary trough axis and in the vicinity of this
southern impulse, helping to enhance ascent to spread more
widespread moisture northeast. On D1, this will cause widespread
heavy snow from the Sierra and Shasta/Trinity region eastward into
the Salmon River/Sawtooth Ranges, Blue Mountains, and as far east
as the Tetons and Big Horns, WPC probabilities are high (>70%) for
above 12 inches in the CA ranges and into ID thanks to higher
moisture, strong synoptic lift, and impressive upslope flow driving
snowfall rates of 2+"/hr at times. 1-2 feet of snow is possible D1
across these areas.

More modest snowfall across the WA/OR Cascades and Olympics where
both ascent and moisture are more limited. Still, WPC probabilities
in the Cascades and Olympics are moderate to high (50-90%) for
more than 8 inches. Snow levels D1 will climb briefly to as high as
5000-6000 ft in the core of the max IVT/WAA/moisture plume, but
will fall gradually most of the day to as low as 2000 ft in the
Cascades and 3500 ft elsewhere. This will allow at least modest
accumulations below pass levels, but most of the accumulating snow
is likely during the period of higher snow levels.

On D2, the pattern evolves quickly as a narrow but amplified
longwave ridge builds over the Pacific Coast, forced between the
departing trough now moving into the northern High Plains and a
renewed closed low back over the Pacific Ocean. This will force a
respite in precipitation across the coast and into the Great Basin,
but wintry precipitation will continue into the Northern Rockies,
but in a slowly waning fashion. Snow levels will continue to fall
steadily D2, reaching below 1000 ft in the Northern Rockies, which
coincident with continuing (but easing) snow will result in heavy
snow accumulations across the NW WY ranges, with some light
accumulations down into most of the valleys. WPC probabilities D2
are moderate (50-70%) for an additional 6+ inches, highest in the
Tetons.

Late D2 and then more substantially into D3, the large closed low
west of WA/OR will spin two distinct vorticity maxima/shortwaves
onshore, leading to increasing waves of precipitation spilling
eastward once again. With this next impulse, snow levels will
initially be low, only around 1500 ft, with modest rising
occurring in the axis of greatest IVT. Despite IVT progged to
remain only around 250 kg/m/s, this will lift snow levels to around
3500 ft west of the Cascades in WA/OR, and then potentially as
high as 4500 ft later D3 with the second wave. With impressive
ascent increasing, and upslope flow amplifying into the terrain,
heavy snowfall will result, and WPC probabilities D3 are moderate
(50-70%) for 6+ inches from the Shasta/Trinity region northward
along the Cascades.

Moisture spilling east into the interior Northwest will encounter
some trapped cold air and modest easterly flow ahead of the
accompanying WAA/IVT, such that some light freezing rain could
result on the eastern side of the Cascades. Potentially more
impressively, the accompanying WAA and associated fgen could result
in heavier snow rates farther east which could reach the valley
floors of interior WA and OR. Confidence is low this far out, but
WPC probabilities do indicate at least a low chance for 1" of snow
across much of WA east of the Cascades, with heavier snow likely
moving back into the Salmon River/Sawtooth Ranges before the end of
the period.


...Central Plains into the Upper Midwest...
Day 1...

A mixed snow, sleet, and freezing rain event will be ongoing to
start the forecast period as a wave of low pressure moves steadily
from Kansas into the western Great Lakes. This low will be driven
by a compact but potent closed low moving over-top the surface
low, with downstream warm and moist advection expanding the
precipitation shield northeast through the region. As this
vertically stacked system moves east, it will run into some cold
and dry antecedent air as an expansive high pressure gradually
retreats towards New England. As WAA intensifies, it will push a
warm nose above 0C, while surface wet-bulb temperatures will remain
below freezing. The guidance has narrowed the corridor of
significant freezing rain, and despite what could be moderate
precip rates (supported by soundings) and a lack of dry advection
to offset latent heat release of freezing (and to prevent
substantial warming), there is high confidence in a stripe of
significant ice accretion, especially in eastern and central IA.
Here, WPC probabilities are moderate (50-70%) for an additional
0.1+ inches of ice, leading to local event totals exceeding 0.25".


...Central Appalachians...
Days 1-2...

Sprawling high pressure with impressive central pressure
approaching 1050mb will gradually pivot east through New England
during the weekend. While this feature will retreat, it will extend
down the coast east of the Appalachians as a cold-air damming
scenario, at least initially reinforced by mid-level confluence
ahead of an approaching shortwave. This impulse will lift from the
Corn Belt late Saturday to off the New England coast, with the
associated moisture and ascent producing a swath of precipitation
extending from the Southern Appalachians through southern New
England. The WAA overrunning the cold surface layer will likely
result in an axis of moderate to heavy freezing rain in the higher
elevations from southern WV through the MD Panhandle and into the
Laurel Highlands where WPC probabilities for more than 0.1 inches
of ice reach as high as 70%, and locally in excess of 0.25" is
possible near the MD Panhandle and into the southern Laurel
Highlands which is where the greatest duration of freezing rain is
expected. Surrounding this, light freezing rain could result in
accumulating ice exceeding 0.01 inches as far south as the northern
escarpment of SC, and as far north as some of the higher terrain of
Upstate NY.

While the most significant winter impacts are likely due to
freezing rain, many areas from PA through MA will experience
precipitation at least starting a snow with some light
accumulations likely in the higher elevations of the Poconos,
Catskills, and Upstate NY/western New England. At this time
however, WPC probabilities are only 50-70% for 2+ inches in these
areas. In places that receive mostly rain, a brief mix of snow and
sleet is possible during precipitation onset even as far southeast
as the I-95 corridor, but no accumulation is expected.


...Northern Plains...
Days 2-3...

Vorticity maxima leftover from a weakening shortwave will lift
northeast from the Great Basin, with secondary intensification of
this feature likely over eastern MT Sunday. The guidance has
continued to trend a bit deeper and farther south with this
secondary development, suggesting a higher potential for some
moderate to heavy precipitation from MT into ND.

As the lead shortwave weakens and pushes northeast, some leading
WAA will expand light precipitation into the northern High Plains.
Forecast soundings suggest considerable dry air to overcome, but
as this occurs it will cause some wet-bulb effects to keep temps
below freezing and an axis of light freezing rain is likely from
eastern MT into western ND Saturday night into Sunday morning.
Accumulations are likely to be modest, however, as reflected by WPC
probabilities for 0.1+" of ice peaking around 30% in eastern MT on
D2. Still, any freezing rain can cause hazardous travel
conditions.

Thereafter, the secondary impulse deepens into a closed low which
should cause some enhanced deformation to the north and west of the
system. This deformation acting upon some higher theta-e air
wrapping cyclonically around the low (emerging from intensifying
290K isentropic ascent downstream) will support some banded snow
rotating through ND and then into MN Sunday night and Monday. At
this time total snowfall is still expected to be light however,
with WPC probabilities for 2+ inches of snow reaching just around
50% from eastern ND into northern MN.


Weiss