Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 AM EST Sat Mar 09 2024

Valid 12Z Sat Mar 09 2024 - 12Z Tue Mar 12 2024

...Great Lakes, Northeast, and Mid-Atlantic...
Days 1-3...

A primary surface low moving north out of Michigan and into
Ontario Saturday morning will yield to secondary cyclone
development over the Mid-Atlantic late Saturday into Sunday
morning. This secondary surface low development will occur along
an occluded front extending east from the primary low, and in
response to an impressive phased closed low digging out of the
Great Lakes towards New England. The guidance has continued to
trend south and east with this closed low while maintaining
amplitude, which is resulting in a stronger but farther displaced
southeast surface low. At the same time, the downstream
subtropical jet energy is progged to arc impressively poleward to
place the most intense LFQ diffluence directly atop the greatest
height falls to help rapidly deepen this surface low as it lifts
across New England. The exact placement and intensity of this low
will determine both type and amount of precipitation, but model
trends are for more wintry precip, especially in northern New
England, both due to the more intense low and also the farther
south track now being progged by much of the guidance.

As the surface low deepens and shifts E/NE, downstream moisture
advection surging from the Gulf of Mexico will intensify
northward. This will manifest as potent 290-295K isentropic ascent
with impressive mixing ratios of 4-6 g/kg, resulting in PW
anomalies reaching as high as +2-+3 sigma into southern New
England, slightly lower farther north. This will additionally lift
the pronounced theta-e ridge northward into what is now progged to
be a strong TROWAL wrapping cyclonically into the system over New
England, likely in response to the rapid intensification now
forecast thanks to the coupled synoptic ascent, which will further
enhance precipitation intensity. While all this is occurring, the
moisture will wrap into an impressive deformation axis noted by
intense Fn-vector divergence across northern New England by Sunday
morning, which should be more than sufficient to cause dynamic
cooling in an otherwise marginal thermal structure. In fact, a
deep isothermal layer noted in regional forecast soundings beneath
this TROWAL despite modest DGZ depth indicates that any dendritic
growth should maintain aggregates as they fall to cause higher
SLRs despite the moist column. This is reflected by high snow load
probabilities in the WSSI-P, especially in the higher terrain
above the NBM 75th percentile snow level of 1500-3000 ft on Sunday
morning. While the heaviest snow within this leading WAA snow is
likely at higher elevations, there is an increasing potential for
some lower elevation accumulation as well due to intense ascent
noted in area cross-sections suggestive of CSI/CI supporting
1-3"/hr snowfall rates within a translating band across VT/NH/ME.
WPC probabilities D1 within this WAA are high (>70%) for more than
6 inches in the higher Adirondacks and Whites, extending into the
higher elevations of ME D2, with moderate probabilities in the
Greens. Some light accumulations of snow are possible into the
valleys as well, especially where snow rates can be most intense.

As the strong low begins to depart into the Canadian Maritimes
late Sunday into Monday, strong CAA in its wake accompanied by
potentially two vorticity streamers rotating through the flow will
combine to re-invigorate ascent within a rapidly cooling column.
The rapidly cooling column will have the two-pronged effect of
steepening the low-level lapse rates while also yielding a
deepening DGZ noted by SREF probabilities reaching a (albeit still
modest) 30% for 100mb of depth, highest across Upstate NY into VT.
Still residual column moisture noted in regional soundings within
the low-levels will then be wrung out through the PVA, with
additional moisture likely increasing as the strong CAA moves atop
the still warm lakes, driving impressive late-season LES
downstream of Lakes Erie and Ontario. Additionally, the intense
flow will favorably upslope into the Adirondacks and Greens, with
Froude numbers indicating subcritical flow to result in the
heaviest snow axes along or upstream of the terrain crests. The
SLRs will likely be much higher with this secondary snowfall,
especially in LES regimes, and as the column cools more
substantially later in the event. WPC probabilities are high,
above 80% downstream of Lakes Erie and Ontario, especially into
the terrain features of the Tug Hill Plateau and Chautauqua Ridge,
with high probabilities also existing in the favorable upwind
terrain in the NW Adirondacks and northern Greens. Moderate
probabilities for more than 6 inches extend as far south as the
Appalachians of WV/MD/PA, with some linger potential continuing
into D3 in NH/VT. Locally 1-2 feet of snow is likely from the
multi-day and multi-phased event in the higher terrain of New
England.

Additionally, as the low pulls away Sunday, dual cold fronts will
follow in its wake, with the reinforcing front Sunday likely
producing some cellular/linear clusters of snow bursts reflected
by simulated reflectivity amongst the various high-res models. The
environment appears favorable for at least isolated snow squalls
from southern Upstate NY southward through PA and into MD/WV, with
the greatest risk likely across the western/central parts of these
states where 0-2km RH is highest. The snow squall parameter is
high here as well, driven by the favorable overlap of instability
and fgen, so it appears some snow squalls are likely late morning
through the aftn on Sunday. Temperatures will be somewhat modest
so the flash freezing of roadways may be somewhat inhibited, but
still, difficult travel is possible due to gusty winds and heavy
snow rates causing rapid changes in visibility to travelers.


...Northwest...
Days 1-3...

A series of shortwaves will lift onshore through the weekend and
into early next week embedded within otherwise onshore zonal flow,
to produce an extended period of unsettled weather with heavy snow
each day across much of the Northwest terrain.

The first shortwave will pivot onshore and advect rapidly
northeast while weakening this afternoon. This impulse will move
from near the OR/CA coast northeast into Saskatchewan D1, but the
amplitude of this wave will decay rapidly, leaving the most
intense ascent focused along the Olympics and Cascades, with
minimal additional lift extending into the Northern Rockies.
However, immediately following this first impulse, a more
substantial wave will drop along the British Columbia coast and
then surge onshore WA/OR Sunday afternoon. This second wave is
more intense, and is accompanied by stronger height falls and
downstream divergence, embedded within impressive moisture
advection on downstream WAA as reflected by a narrow corridor of
IVT exceeding +1 sigma according to NAEFS ensemble tables due to
confluent and onshore mid-level flow overlapping a zonally
oriented Pacific jet streak. The combination of this intensifying
synoptic ascent and increasing moisture will result in an
expanding area of precipitation from the Olympics eastward through
the Northern Rockies, and diving as far south as the
Shasta/Trinity/Siskiyou region of CA, and even reaching the
northern Sierra. This second wave moves quickly eastward,
following in the path of the previous impulse in still generally
progressive flow, only to be replaced by yet a third shortwave
with an additional wave of moisture and ascent leading to another
round of snowfall in the terrain. Snow levels vary through the
period with a lot of rises/falls within periods of WAA/CAA, but
will generally hover around 2500-3000 ft each day.

WPC probabilities for more than 6 inches of snow D1 reach above
50% in the higher terrain of northern CA and the OR Cascades, and
above 80% beneath the strongest forcing into the Olympics and WA
Cascades. D2 appears the most active and intense snowfall day,
which is reflected by widespread WPC probabilities for 6+ inches
reaching above 70% along much of the Cascades, the Olympics, the
Shasta/Siskiyou region, and the northern Sierra. Additional
moderate to high (50-70%) probabilities exist in the Northern
Rockies, Blues, and Salmon River/Sawtooth ranges of ID. The
coverage of high probabilities D3 is similar although somewhat
less widespread, than on D2. Storm total snowfall, especially
above 3000 ft, could reach 2-4 ft in the Cascades and northern CA
ranges, with locally more than 6 ft possible on the highest peaks.


Weiss