Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
234 AM EST Fri Mar 08 2024

Valid 12Z Fri Mar 08 2024 - 12Z Mon Mar 11 2024

...Southern Rockies and High Plains...
Day 1...

A surface cold front dropping south combined with a broad longwave
trough pivoting eastward across the Intermountain West will result
in broad scale ascent across the region, with focused lift
occurring primarily within post-frontal NE flow. A weak wave of
low pressure moving along the front may additionally enhance
ascent, but the primary mechanism for lift on D1 will be upslope
flow into the southern Rockies including the Sangre de Cristos,
San Juans, and even as far south as the Sacramento Mountains as
the front sags southward before weakening into D2. This lift will
wring out the available moisture which will be near normal
according to NAEFS PW anomalies, but some enhanced fgen noted in
cross-sections will favorable drive ascent into the DGZ, which
when combined with the upslope flow will result in an axis of
heavy snow in the terrain. Snow levels will start around 5000-6000
ft, but fall steadily behind the front to as low as 2500-3500 ft
by the end of D1. The heaviest snow accumulations will remain
above these levels, and WPC probabilities for more than 6 inches
are above 80% in the eastern San Juans, southern Sangre de
Cristos, and even into the Jemez Mountains of NM. Additional high
probabilities for more than 6 inches reaching 50-70% extend north
into the Front Range and south as far as the Sacramento Mountains.


...Great Lakes and Northeast...
Days 2-3...

Phasing shortwaves over the Great Lakes will help rapidly deepen a
surface low moving northeast from the TN VLY into Ontario
Saturday/Sunday, with secondary re-development occurring along the
occluded front to the east later Sunday. This secondary low will
then track across New England before exiting to the Canadian
Maritimes Monday. Increasingly robust mid-level ascent through
height falls and downstream divergence will overlap with
intensifying upper level diffluence as the downstream jet streaks
gets pulled poleward and arcs to place its favorable diffluent LFQ
atop the best mid-level forcing. This will be the root cause of
the rapid surface low intensification, and low-level southerly
flow downstream of this wave will promote impressive moisture
advection into the Northeast. PW anomalies according to NAEFS are
progged to reach +2 to +3 sigma as isentropic ascent surges the
theta-e ridge northward, which results in expanding precipitation
as it gets acted upon by the strong synoptic lift.

There are some timing and longitudinal differences amongst the
various global deterministic and ensemble members, but in general
this appears to be a warm storm overall, suggesting primarily rain
for much of the Great Lakes and Northeast. However, there will
likely be two regions of heavy snow during this evolution.

The first will be from the U.P. of MI beginning early Saturday,
and then expanding southeast into the eastern Great Lakes by
Sunday. This will occur as the low departs to the northeast,
leaving strong CAA in its wake on increasing N/NW flow atop the
warm lake temperatures. This should result in expanding lake
effect snow (LES), with the heaviest snow occurring southeast of
Lake Erie due to a prolonged duration of favorable wind direction
combined with an effective fetch tapping moisture all the way from
Lake Superior. Additionally, there is the potential for some
synoptic snow before the LES as a deformation axis results in
dynamic cooling on the back side of this system, but the column
appears to remain marginally favorable for heavy snow even during
this time, so most of the accumulations appear to be related to
LES. WPC probabilities for LES D2 are confined generally to the
U.P. where they reach 20-30% for more than 6 inches, but expand
rapidly and impressively D3, especially east of Lakes Erie and
Ontario and into the higher terrain of the Tug Hill Plateau and
Chautauqua Ridge where they exceed 50% for 6+ inches.

The other region of heavy snow is likely to be the higher terrain
of Upstate New York and New England. Here, pronounced WAA on the
southerly flow ahead of the primary low and then enhanced by the
secondary development will spread precipitation northward. This
strong WAA will result in primarily rain in the lower elevations,
but with snow levels starting around 500-1000 ft and then rising
to 2500-3000 ft during the heaviest precipitation, this will
likely result in heavy accumulations in the Adirondacks and Whites
(NH/ME), with low SLR producing a heavy wet snow and substantial
impacts due to snow load (as reflected by WSSI-P). As the
secondary low ejects eastward D3, rapid cooling on strong NW flow
will cause a rapid lowering of snow levels and deepening of the
DGZ which will support increased upslope snow on the upwind (NW)
terrain despite waning column moisture. In the WAA regime, WPC
probabilities D2-D3 for more than 6 inches of snowfall reach
30-40% in the Adirondacks, and 50-80% in the Whites and much of
northern ME where locally 1-2 feet of snow is likely. During the
period of greatest NW flow/upslope ascent, additional snowfall has
a 50-70% chance of exceeding 6 inches in the northern Adirondacks,
and 20-40% along the Green Mountains.


..Northwest...
Days 2-3...

A series of shortwaves will move onshore the Pacific Northwest
beginning this weekend resulting in waves of precipitation
spreading onshore from the Pacific Northwest through the Northern
Rockies, and extending as far south as California and the Great
Basin.

The first of these shortwaves will advect eastward to lift onshore
OR Saturday afternoon as a potent and negatively tilting shortwave
with impressive PVA, but this feature will remain transient and
weaken quickly as it shears out to the north and ejects into
Saskatchewan by late Saturday night. IVT ahead of this feature is
progged to be modest, characterized by 70-90% probabilities of 250
kg/ms IVT moving east, so despite the impressive ascent, the rapid
motion and some what limited moisture will result in only modest
snowfall accumulations D2. Snow levels during this time will be
around 3500-4000 ft in the strongest WAA immediately ahead of the
accompanying cold front, falling to 2000-2500 ft coincident with
the drying column. This results in WPC probabilities rising to
50-80% for more than  inches in the WA Cascades, and above 80% in
the higher Olympics, with more modest probabilities reaching along
the OR Cascades and into the Shasta/Siskiyou region of CA.

A much more impressive mid-level impulse will follow immediately
in the wake of the first, evolving from a closed low off the
British Columbia coast Sunday morning before weakening as it too
shears off to the northeast and dives inland Sunday into Monday
/D3/. Height falls and PVA are progged to be more impressive with
this second impulse than the first, leading to more expansive
coverage of moderate to heavy snowfall despite once again modest
IVT probabilities. Snow levels will again rise to 3000-4000 ft
ahead within the best warm advection ahead of the cold front,
falling back to 2500-3000 ft in its wake by the end of the period.
The more pronounced ascent and widespread coverage of at least
subtly anomalous moisture will cause WPC probabilities exceeding
70% for 6+ inches of snowfall to extend along most of the
Cascades, into northern CA, and the northern half of the Sierra,
while also reaching east into the Northern Rockies, Blue
Mountains, and Salmon River/Sawtooth region of ID.



Weiss