Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
407 AM EST Thu Mar 07 2024

Valid 12Z Thu Mar 07 2024 - 12Z Sun Mar 10 2024

...Central/Southern Rockies and High Plains...
Days 1-2...

An expansive but broad mid-level trough across the middle part of
the country will shed two distinct lobes of vorticity northeast
through the base and into the Southern/Central Rockies through
Friday to produce waves of synoptic ascent across the region.
These shortwaves/vorticity impulses will interact with an
intensifying baroclinic gradient along a southward advancing cold
front to drive lee cyclogenesis beginning Thursday morning, and
this surface low will advect gradually eastward into the Central
Plains Friday. Downstream of this wave, moisture advection will
increase on isentropic ascent surging out of the Gulf of Mexico
and lifting northward into the system. This moisture will try to
wrap cyclonically northwestward, although theta-e plumes reflect
only a modest TROWAL overall. Despite that, moisture running
northwest and into the High Plains/Central Rockies is progged to
reach nearly +2 sigma according to NAEFS ensemble tables, which
will result in an expanding precipitation in response to the
synoptic lift, and likely more impressively, upslope life into the
Front Range on NE flow behind the southward advancing front.

With this evolution, there may be two relative maxima in snowfall
accumulations. The first, and most significant, is likely along
the Front Range from the Cheyenne Ridge through the Raton Mesa,
and into the Sangre de Cristos and San Juans. Here, upslope flow
will produce heavier precipitation rates through greater ascent,
which will manifest as heavy snow with higher SLRs and longer
duration of precipitation. WPC probabilities for more than 6
inches are above 70% along the Front Range and into the San Juans
D1, sinking south into the Sangre de Cristos on D2, and locally
1-2 feet of snow is possible. Lighter snows are likely along the
Palmer Divide and Raton Mesa, with minimal accumulations of
generally less than 2 inches forecast for the I-25 urban corridor.

Farther east into the High Plains, especially in western KS and
NE, a pivoting deformation axis will overlap with some increasing
fgen behind the front to potentially create a band of heavy
snowfall with rates above 2"/hr. The placement of this axis is
still very uncertain and features a lot of spread in the global
and high res models, but the setup seems to support a pivoting
band at some point on Thursday concurrent with a deepening DGZ and
weak symmetric stability to support slow moving and heavy snowfall
rates, with -EPV overlapping folded theta-e surfaces to result in
CI. These bands tend to verify a bit S/W of the model progs, and
the high-res trends have been a bit west today. Despite the low
confidence in placement, after coordination with the affected
offices, some areas have been upgraded to warnings due to the
increased confidence as reflected by WPC probabilities as high as
40% for more than 6 inches of snow. Where this band pivots,
double-digit snowfall totals are possible.


...Great Lakes...
Days 2-3...

Northern and southern stream shortwaves will phase near the
western Great Lakes Saturday, merging into a deep closed low over
the area by Sunday morning. This deepening trough will drive an
intensifying and poleward arcing southern stream jet streak into
the Mid-Atlantic/Ohio Valley, placing the intense and favorable
LFQ for diffluent ascent over the greatest mid-level height falls.
This will help to rapidly deepen a surface low lifting northeast
from the Mid-Mississippi Valley, and this low will likely track
across eastern MI and then into Ontario by the end of the forecast
period. While there remains substantial longitudinal spread in the
track of this low amongst the various global members, both the
GEFS and ECENS means are west of the operational runs, which
suggests a slightly warmer solution than current progs suggest.
Despite that, the intense synoptic ascent moving atop an
increasingly moist column reflected by robust theta-e advection
which may lift into a modest TROWAL will support heavy
precipitation surging across the area.

While the eastern half of the Great Lakes will likely be primarily
rain, an intensifying deformation axis on the NW side of the low
could cause a p-type changeover from rain to heavy snow through
dynamic cooling later in the event Saturday evening. If this
occurs it could produce at least modest snowfall accumulations in
lower Michigan, but confidence in this is quite low, especially
since it would need to occur at night to be most efficient.
However, farther NW into the U.P. of MI, colder air in place will
allow for moisture spreading NW to fall as snow, with more
impressive accumulations probable later D3 as the low departs
leaving cold N/NW flow in its wake across Lake Superior. Lapse
rates over the lake appear impressive, but forcing generally
appears focused beneath the DGZ so accumulations may be moderated.
Current WPC probabilities for more than 4 inches of snow peak
around 20%, highest in the central U.P. due to LES addition.


...Northeast...
Day 3...

The same surface low moving northeast through the Great Lakes will
extend a triple point and warm front eastward into the
Mid-Atlantic along which a secondary low may form and shift east
across New England. The poleward shift of the accompanying jet
streak will place increasingly favorable LFQ diffluence for ascent
over the Mid-Atlantic and shifting into the Northeast late
Saturday, occurring concurrently with increasing mid-level
divergence downstream of the potent closed low ejecting from the
Great Lakes. Waves of PVA within this mid-level flow will
additionally enhance lift, while potent S/SW 700-500mb advects
copious moisture northward noted by PW anomalies reaching above +2
sigma according to NAEFS. This deep layer ascent acting upon the
robust moisture plume will be further enhanced by strengthening
WAA along the warm front, and an expanding area of heavy
precipitation will likely surge into the Northeast late Saturday
night into Sunday. The antecedent thermal structure is quite
marginal for snowfall, and although snow levels just before precip
onset may be low across northern New England, they should continue
to rise on the strengthening WAA. Precip will likely be heavy, but
at this time heavy snow accumulations appear confined to the
highest terrain of Northern New England as reflected by WPC
probabilities for more than 4 inches as high as 60-80% for the
eastern Adirondacks into the Whites of NH. However, this event
needs to be monitored, as in some areas, especially if the cold
air can remain entrenched longer, could produce significant
accumulations of heavy and wet snow leading to considerable
impacts.


..West Coast...
Day 3...

A shortwave moving onshore Saturday morning will be followed
almost immediately but a second, but more intense, impulse which
will drop out of the Gulf of Alaska and approach the WA coast by
the end of D3. Downstream WAA ahead of this second impulse will
advect high probabilities for IVT exceeding 250 kg/ms onshore,
with the accompanying WAA, height falls, and modest diffluence
within the LFQ of a weak Pacific jet streak producing intensifying
ascent into the Pacific Northwest. Moisture surging onshore will
only drive PW anomalies to slightly above normal, but the
accompanying cold front to help lower snow levels from around 4000
ft to 2500 ft will allow snowfall to expand across much of the
west coast terrain from the Shasta/Siskiyou region northward along
the Cascades and into the Olympics. At this time the forcing
appears transient, snow snowfall totals should be moderate, but
WPC probabilities indicate a high chance (>80%) for more than 6
inches of snow in the Olympics and WA Cascades, with probabilities
for more than 6 inches falling to around 20-30% in northern CA.


Weiss