Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
425 AM EST Wed Feb 05 2020

Valid 12Z Wed Feb 05 2020 - 12Z Sat Feb 08 2020

...Midwest through the Great Lakes...
Days 1-2...

Low pressure developing across eastern Texas will strengthen and
lift northeast into the Ohio Valley through Thursday night. This
low will intensify in response to height falls aloft as a large
positively tilted trough ejects slowly eastward from the Four
Corners region this morning to the Missouri Valley Thursday
morning. Significant moisture flux ahead of this system will
spread precipitation northward on warm/moist advection, with
synoptic ascent being provided through the aforementioned height
falls, as well as strong jet level diffluence and PVA as several
impulses rotate through the longwave trough. Additionally, an
elongated area of mid-level frontogenesis will arc SW to NE while
drifting slowly eastward behind a surface cold front, further
aiding in already robust omega. All of these features together
will produce a large swath of rain, snow, and mixed precipitation
from Oklahoma northeast into lower Michigan.

The heaviest snow is likely in a band well NW of the developing
surface low, where the atmospheric column is coldest and can
support all snow. This is most likely from far NE OK, through
central MO, and then northeast through IL, northern IN, and lower
MI. The heaviest snowfall is expected where the fgen lingers the
longest, across Missouri, where WPC probabilities reach 40-50% for
6 inches. SW and NE of this axis, a stripe of high WPC
probabilities for 4 inches exists. Since this band is expected to
be so narrow, minor fluctuations in the track or placement of the
best fgen could lead to more sleet than snow, and this could lower
accums, especially on the SE edge of this max swath.

While the guidance still offers some different solutions/evolution
of the thermal structure, there is reasonable agreement that a
stripe of moderate to heavy freezing rain will occur along and
southeast of the heavy snow and just NW of the low track. While
strong forcing could create some heavier rainfall rates limiting
the accretion efficiency, there is a persistent signal in the HREF
mean for a stripe of heavy freezing rain from southern Missouri
northeast into Ohio, with a relative maxima possible in Ohio where
rates may be a little lighter, but the duration is expected to be
longer due to the surface low weakening and shifting eastward late
on Day 2. WPC probabilities are moderate to high for 0.25" in
these areas.


...Northeast...
Days 1-3...

Two rounds of heavy wintry precipitation, possibly with a brief
lull in between, are expected the next 3 days across the
northeast.

The first is likely beginning tonight as the low pressure from the
Ohio Valley extends a warm front northeast towards the
Mid-Atlantic and New England. WAA associated with this front will
overrun a cold airmass due to high pressure across SE Canada.
Brief mid-level confluence combined with precipitation onset will
reinforce the high early tonight, but increasing divergence and
strong WAA/lift will gradually erode this cold air, last at the
surface. Before this occurs, a period of moderate to potentially
heavy snow is likely from the Poconos of PA northeast through New
England and Upstate New York. With the rapid advancement of the
low-level warm nose, significant snow accumulations should be
confined to the Adirondacks and other terrain of northern New
England, where WPC probabilities are moderate to high for 4
inches, with more than 6 inches possible across Maine through Day
2.

South of this heavy snow, a prolonged period of freezing rain is
possible as the warm nose quickly overwhelms the cold air, which
remains entrenched only at the surface and in the terrain. Most of
the model guidance suggests wet bulb temperature will remain below
freezing for a considerable portion of the precipitation beginning
Wednesday night through Thursday night across Southern and Central
New England. Chances for significant freezing rain accretions have
increased this morning, and are now 50% or more on Day 2 for the
Berkshires, northern Worcester Hills, and Southern Greens, with
high probabilities for 0.1" or more from the Laurel Highlands and
the Poconos through the Catskills and into other, lower terrain,
of Central New England.

A more significant low pressure system is likely to lift out of
the southeast on Day 3, and bring impressive wintry precipitation
to the region again on Friday. This low is expected to intensify
rapidly as it moves across Southern New England in response to
phasing mid-level vort energy and an extremely favorable jet
position. While there remains considerable spread in the thermal
structure, it is likely that at least Northern New England and
Upstate New York will receive significant snowfall Friday. WPC
probabilities are moderate for 8 inches across these areas on day
3.


...Mountain West...
Days 1-3...

A modest but persistent Atmospheric River (AR, IVT 500-600 Kg/m/s)
will advect into the Pacific Northwest the next 60 hours, with
attendant moisture and jet dynamics shifting southeast around a
ridge of high pressure off the Pacific Coast. Within this AR,
periodic mid-level impulses are likely to interact with a
sharpening low/mid level fgen boundary which should manifest as a
surface stationary front through the end of the week. Along this
front, waves of low pressure beneath the embedded shortwaves will
enhance ascent, and waves of moderate to heavy snow are likely
from the PacNW through the Colorado Rockies this period. Snow
levels are expected to rise today into Thursday as the moist
Pacific air is tropically sourced. Snow levels should remain
relatively consistent on Thursday before slowly falling on Friday
as the nearly stationary front sinks a bit southward and a
stronger mid-level impulse with accompanying height falls moves
onshore.

Today, the heaviest snow is expected from the Washington Cascades
through the Blue Mountains of Oregon, the Sawtooth and Bitterroots
of ID, and into western MT/NW WY. Here, WPC probabilities for 8
inches are high, above 3000 ft in MT/WY, climbing to above 6000 ft
in the Olympics of WA. Snow amounts of 2 feet are possible in the
highest terrain. By D2 heavy snow will persist across much of
these same ranges, but also extend down into the Wasatch and
Colorado Rockies. Snow levels will be generally unchanged from day
1, and WPC probabilities are high for 12 inches Thursday, with
amounts over 2 feet again possible in isolated locations. On
Friday, the forcing begins to weaken across most of the Mountain
West, but another shortwave will approach the PacNW coast. This
leaves the heaviest snow accumulations as most likely in the PacNW
on Friday, where WPC probabilities are moderate for 12 inches
again, with some lingering heavy snow likely across the Colorado
Rockies as well.

Weiss