Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
301 PM EST Mon Feb 06 2023

Valid 00Z Tue Feb 07 2023 - 00Z Fri Feb 10 2023

...Pacific Northwest to the Northern & Central Rockies...
Days 1-3...

An upper trough responsible for the development of a storm system
tracking through the northeast Pacific will deliver a slug of
850-700mb moisture into the Pacific Northwest starting tonight.
The Olympics and Cascades will be the first impacted with rounds
of heavy snow tonight and into Tuesday. Snow levels originally
around 4000' initially will drop to as low as 2000' once the cold
front moves through. Mean 850-300mb winds are generally be out of
the WSW, so while not oriented orthogonally to the Cascades,
strong upslope enhancement along with the influx of 850-700mb
moisture will result in heavy snowfall totals through Tuesday
evening. Latest WPC PWPF shows high probabilities (<70%) for
snowfall >12" in the Washington Cascades, while there are moderate
chances (40-60%) for snowfall >8" in the Oregon Cascades. These
totals, along with the heavy/wet snow-type expected, have led to
the WSSI depicting some Major impacts in portions of the Olympics
and Washington Cascades, largely driven by a combination of
snowfall amount and snow load. Travel will be hazardous in these
areas, including some of the passes of the Washington cascades
from a combination of snowfall accumulating on roads and snowfall
rates >1"/hr throughout much of Tuesday (>80% chance of these
rates in the Olympics and both Washington and Oregon Cascades much
of the day Tuesday.

This upper trough is set to direct the same slug of Pacific
moisture into the Northern Rockies Tuesday night and into
Wednesday. The Bitterroot, Lewis, and Teton Ranges of northern ID,
western MT, and western WY respectively feature the best odds for
>8" of snowfall with probabilities ranging between 50-80% from
Tuesday evening through Wednesday evening. The tallest peaks of
the ranges are likely to see Moderate to even Major impacts
according to the WSSI. As the cold front plunges south and east
along with the shield of 700mb moisture, the Central Rockies and
Black Hills become favored to pick up heavy snowfall Wednesday
evening and into early Thursday morning. While not likely to see
as much snow as their neighbors to the north and west, latest WPC
PWPF does show 30-50% probabilities for >6" in the Black Hills and
similar probabilities for >4" in the northern Colorado Rockies.
Snow totals will be lighter across the Northern Plains on Thursday
as the upper trough provides sufficient vertical ascent within the
column, but its progressive nature should keep totals to <4".

...Upper Midwest to Interior Northeast...
Day 3...

A slow moving upper low moving across the southwestern U.S. is
forecast to eject east into the Southern Plains by Wednesday. This
upper trough will tap into the Gulf of Mexico to usher a tongue of
rich 850-700mb moisture north within a 50-60 knot LLJ Wednesday
evening. The surface cyclone will strengthen Wednesday night
beneath the diffluent right-entrance region of a 130-150 knot
250mb jet streak. This warm conveyor belt of moisture will wrap
around the 700mb low and place a deformation zone of precipitation
from eastern IA early Thursday morning into WI and northern MI by
late morning into midday Thursday. There will be no shortage of
available moisture (NAEFS shows 500-700mb specific humidity >90th
climatological percentile by 12Z Thursday in these aforementioned
areas), but it is lacking sufficiently sub-freezing cold air
within the boundary layer with a narrow DGZ below 500mb. There is
a path for heavy snowfall totals with snowfall rates of 1-2"/hr
given the strong dynamics at play, and these kinds of rates can
dynamically cool the column enough to cause rapidly accumulating
snowfall. However, in areas where the mesoscale dynamics are not
as robust, snow could easily melt and be rain within a boundary
layer with marginal thermals for snow. WPC super ensemble plumes
show some "boom" scenario for snowfall with totals >8", but there
are plenty of plumes that are <2" as well, showing just how
volatile and sensitive both the storm track and the mesoscale
dynamics at play can wreak havoc on modeled snowfall forecasts. At
this time, the latest WPC PWPF shows 10-20% probabilities of
snowfall >4" from central WI to northern MI and the U.P. of MI
through Thursday evening.

As the Gulf of Mexico moisture continues its push north and east
towards the Mid-Atlantic on Thursday, eventually it will reach the
Northeast where wet bulb temperatures are cold enough at the onset
to lead to snowfall Thursday afternoon. However, a dry slot in the
700-300mb layer will quickly work its way behind the initial thump
from strong 290-300K isentropic ascent. With boundary layer temps
having moderated to also marginal levels, the setup here could
lead to snow quickly transitioning to sleet and freezing rain, and
even a changeover to plain rain by late Thursday afternoon. Latest
WPC PWPF shows the Adirondacks with a 10-20% chance for ice
accumulations >0.1", making them most susceptible to hazardous
travel conditions. With the marginal boundary layer temperatures
and the quick drying of the column in the 700-300mb layer, this
setup would support any notable impacts predominately due to ice
rather than snow.

Mullinax