Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 AM EST Sun Feb 03 2019

Valid 12Z Sun Feb 03 2019 - 12Z Wed Feb 06 2019

...Western States...
Days 1-3...

Strong moist advection from the Pacific combined with lowering
snow levels will produce widespread heavy snow across the Mountain
West through Tuesday. Although a variety of weak surface lows and
an arctic front dropping from Canada through day 2 will aid in
producing lift and snowfall, the primary forcing will involve
Pacific Jet energy lifting into California and a deep low pressure
dropping along the Pac NW coast. The latter of these will help
drive down snow levels to the surface as far south as central
CA/NV by D3, while the former will provide copious PWAT into the
coast before spilling across the remainder of the West. Focused
moist advection south of this mid-level impulse will be squeezed
out as lift due to diffluence within the RRQ of the upper jet and
pronounced mid-level height falls combines with orographic ascent
on increasing 700mb flow. Once again, the Sierras of California
appear to be favored for prolonged heavy snow with more than 6
feet of accumulation likely during the 3 days. Heavy snowfall is
also likely in the ranges of Utah, WY, and ID, where WPC
probabilities are moderate for 2 feet.

Although snow levels will initially be above 5000 feet from
Central California through central Colorado, the anomalously deep
trough digging along the coast combined with the southward shift
arctic front will drive snow levels downward each day of the
period. This will eventually also feed drier air into the region
from the north such that by D3 /Tuesday/ more modest snowfall is
expected, with the heaviest south of 40N latitude. As snow levels
drop to the surface, light accumulations are likely in the valleys
as well, including the cities of Portland and Seattle, in which
WPC probabilities show a moderate risk for 1-2" of snowfall.


...Northern Plains and Great Lakes...
Days 1-3...

Two separate systems will bring a mix of snow and ice to the
Plains and Great Lakes through the forecast period.

The first of these will begin this evening and persist into Monday
night. A shortwave trough over the Great Basin will eject
northeast into the northern Plains while a jet streak exits to the
east across Southern Canada. These features in tandem will produce
lift which will aid in lee cyclogenesis late tonight into Monday.
Across eastern MT, ND, and into northern MN, synoptic ascent will
produce a stripe of heavy snowfall, and although QPF is modest,
around 1/2 inch, very high SLRs of 20:1 are likely in the cold
airmass. Efficient snow growth is likely as the best omega appears
collocated with a deep DGZ, and WPC probabilities remain high for
6" along the Canadian border as far east as Lake of the Woods, MN,
with more than 8" possible in some locations.

Further east, the surface low intensifies and lifts towards Lake
Huron on Monday. The strongest synoptic forcing will lift away,
but more robust lift due to mesoscale dynamics including
increasing 700mb deformation and intensifying 925-700mb
frontogenesis will produce heavy snow from central MN into the
U.P. of Michigan. The DGZ is not as deep here, but snowfall rates
will likely be more intense as noted by theta-e lapse rates
approaching 0. There has been a notable northward shift in the
axis of heavy snow this morning, with a slightly warmer solution
now expected. This suggests that while heavy snowfall is still
likely, most of it will be focused north of the Canada border, and
the highest WPC probabilities for 4 inches extend from the
Arrowhead of MN east into the Keweenaw Peninsula of MI.

There is also likely to be a stripe of freezing rain across
portions of northern WI and the U.P. of MI where a prolonged
period of WAA / isentropic lift will produce a potentially lengthy
duration for precipitation to fall as freezing rain. This is
compounded by the fact that the ground surfaces are extremely cold
(due to recent cold air outbreak) and surfaces will be slow to
respond to this warming. As such, this is a tricky and lower
confidence forecast but potential exists for ice accumulations of
0.1-0.2 inches, highest across northeast WI into the U.P. of MI.

A second system is likely to race across the area on Tuesday as
another shortwave and associated jet energy interact across the
region. This system is likely to bring the heaviest precipitation
a bit further south than the first system, from IA/MO into the
L.P. of MI. A quick burst of snowfall is possible as strong WAA
occurs Tuesday, and WPC probabilities for 4 inches of snow have
been introduced across WI. Potentially more impactful, guidance is
in good agreement in significant freezing rain accretion as the
strong WAA drives a warm nose atop cold surface temperatures.
There is a clear signal for more more than 0.1" of accretion, with
several WPC super ensemble members supporting much more than that.
WPC probabilities are over 50% for 0.1 inches, and up to 30% for
0.25 inches, but uncertainty abounds in placement. This will
certainly bear watching over the next few model cycles.


Weiss