Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
152 PM EST Fri Feb 10 2023

Valid 00Z Sat Feb 11 2023 - 00Z Tue Feb 14 2023


...Southern Appalachians...
Days 2-3...

Difficult forecast Sunday into Monday as an intense closed and
vertically stacked mid-level low ejects slowly across the
Southeast before lifting off the North Carolina coast Monday
morning /D3/. As this trough advects to the east, a trailing
subtropical jet streak will strengthen along the Gulf Coast
providing favorable LFQ diffluence over the Southeast, resulting
in impressive synoptic layer ascent across much of the area. As
this evolution occurs, moisture advection will intensify out of
the Gulf of Mexico and the Atlantic noted by 300K isentropic
upglide surging into the Carolinas with 6g/kg mixing ratios,
lifting into within the WCB into an impressive TROWAL by Sunday
morning, with PW anomalies rising to +1 to +2 standard deviations
above the climo mean. This should result in heavy precipitation
amounts, with rates becoming impressive especially within what
should be a strong pivoting deformation axis overlapping negative
theta-e lapse rates NW of the mid-level low. Additionally, upslope
flow into the terrain will likely drive additionally enhanced UVVs.

The concern for this event will be how much cold air can be
dynamically produced by the system itself, as the antecedent
airmass is modest for early February. This could result in a
situation where snowfall accumulates during periods of more
intense ascent (within the deformation, in the terrain due to
upslope) but changes back to rain when rates lighten. The guidance
is fairly well clustered overall, but there are clear camp
differences noted in DESI clusters with the ECENS a slow/strong
solution, the GEFS a faster weaker model, and the CMCE somewhere
in between. With such an impressive closed low, the slower
solution seems more reasonable, and while this would limit the
snowfall potential towards the Mid-Atlantic, it could enhance the
snowfall potential in the Appalachians from northern GA through
WV, with additional bursts of snow possible anywhere from MS
through central VA during periods of more intense lift.

The other challenge with this event will be the snow-liquid ratio
(SLR). Even during periods when precipitation changes to all snow,
the soundings suggest the near-surface layer will be right around
freezing with a similar near-0C isothermal layer above. The median
February SLR for this area is only 9:1 according to the Baxter
climatology, and this is maybe a ceiling for the SLR forecast for
this event. With rain and snow mixing at times, SLR will be highly
variable and probably quite low, which is also reflected by PWSSI
for moderate impacts being driven primarily by snow load. A lot of
variability results in a lower than usual confidence forecast, and
CIPS analogs suggest accumulations will be confined to the higher
terrain, which in this case would be above around 3000 ft
according to NBM snow levels. However, as noted above, dynamic
cooling under intense ascent could result in light accumulations
almost anywhere in the vicinity of the upper low. Still, the
greatest risk for any significant accumulations will be in the
terrain, and WPC probabilities for 4 or more inches of snow reach
50-80% across the NC mountains and into southwest VA where locally
8-10 inches of snow is possible. Probabilities for more than 2
inches extend northward almost to the MD Panhandle.

Additionally, as the WAA spreads northward, especially into the VA
terrain, a warm nose above 0C will move over sub-freezing surface
temperatures. This could result in some modest to significant
accretions of ice as far north as Shenandoah NP, although with
surface temps just around freezing, a lack of significant dry
advection to maintain lower wet bulb temperatures, and what could
be heavy precipitation rates, the guidance appears overdone in its
ice forecasts. Still, both the NBM probabilities and WSE plumes
have increased their potential for 0.1 to 0.2 inches of freezing
rain, and the current WPC probabilities for more than 0.1" of
freezing rain have increased to as high as 30-50% in far NW NC
into SW VA, with a broad area of 10-30% encompassing much of the
southern and central Appalachians.


...California through the Southern Rockies...
Day 1...

Anomalous closed low with heights falling to -2 standard
deviations below the climo mean according to the NAEFS ensemble
tables will drop along the CA coast D1 and then move into Baja
California Sunday aftn. Southeast of this feature, a subtropical
jet streak will overlap with the mid-level confluent flow and
associated height falls to produce deep layer ascent over an area
of modestly enhanced PWs. This will produce a weak surface low
advecting southeast over the western Great Basin, with the
associated cold front trailing across CA. Snow levels within the
pre-frontal airmass will be 4000-6000 ft, but fall quickly to
2000-3000 ft behind it. This will result in areas of moderate
snow, generally along the Sierra where upslope will drive more
intense omega to produce heavier snow, and WPC probabilities are
20-30% for more than 6 inches of snow along the Sierra and
portions of the Shasta/Trinity/Siskiyou region.

This same upper low will pinwheel across Baja California and then
into the Desert Southwest on Monday. While a break in snowfall is
expected Sunday due to warmer temperatures, slightly weaker
forcing, and less sufficient moisture, redevelopment of moderate
to heavy snow is likely downstream across the Southern Rockies on
D3. Divergence ahead of the closed low will provide ample deep
layer lift, which will manifest as snow in the higher terrain from
the White Mountains of AZ northeast into the southern Sangre de
Cristos. WPC probabilities for more than 6 inches reach as high as
50% in the highest elevations.


...Pacific Northwest...
Day 3...

Shortwave ejecting out of the Pacific will drop onshore the
Pacific Northwest coast Monday morning with modest height falls
but impressive PVA for ascent. This will be accompanied by the LFQ
of a 130kt Pacific jet streak dropping southeast, leading to
robust deep layer ascent shifting into the region on D3. As the
most intense lift moves eastward, it will encounter an
increasingly moist column driven by WAA, confluent mid-level flow,
and the upper jet to drive a modest AR with high probabilities of
250 kg/m/s IVT shifting into British Columbia and Washington
state. Initially, snow levels will rise as high as 5000-6000 ft
within the WAA along the eastward advancing warm front, but strong
CAA with the cold front and height falls the latter half of D3
will drop snow levels down to around 1500 ft while precipitation
persists. The heaviest snowfall will likely occur during the
period of higher snow levels, but impactful snow is likely by the
end of the forecast period even down below pass levels, and the
guidance has become more aggressive with accumulations this aftn.
WPC probabilities for more than 6 inches are 50-80% across the OR
and WA Cascades, and 10-30% over towards the northern Rockies.


Weiss