Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 AM EST Sat Feb 02 2019

Valid 12Z Sat Feb 02 2019 - 12Z Tue Feb 05 2019

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

...Widespread and prolonged heavy snow across the West through
early next week...

Two distinct and significant areas of low pressure will bring
copious moisture into the West through Monday, with heavy mountain
snows early transitioning to more widespread heavy snow as snow
levels fall late in the forecast period.

The first low and its associated precipitation will spread across
California early today and then east-northeast as strong 850-700mb
flow drives anomalously high PWAT as an Atmospheric River event
persists. This combined with strong Pacific Jet energy will result
in robust lift and and a tremendous amount of orographic ascent in
the Sierra Nevada, as well as some of the other NW-SE oriented
ranges which will be orthogonal to the mean flow. This is a recipe
for very heavy snowfall, and D1-D2 snowfall totals are likely to
exceed 4 feet in the Sierras. The closed low aloft will open into
a wave as it pushes into the Intermountain West by Sunday, but an
eastward continuation of the height falls and positive vorticity
advection aloft should allow heavy snow to spread east into the
ranges of Nevada, Idaho, Utah, western Montana, western Wyoming
and western Colorado by Day 2. Snow levels will slowly fall from
5000-6000 feet today, to more widespread 3000-4000 feet Sunday, so
the heaviest snow should be confined to the higher elevations. 1-2
feet is likely in the high terrain of the San Juans, Bitterroots,
and ranges of NW Wyoming.

The second low will be associated with a strong wave digging down
the coast of British Columbia late Sunday through Monday. In the
mid-troposphere (700-500mb) a low closes off by Sunday afternoon
just off Vancouver Island, with strong height falls focused along
the coasts of OR/WA. By early Monday morning, 850-500mb heights
reach near -3 standard deviations from the climo norm, suggesting
a cold system overall. Therefore, for the Day 2-3 period, snow
levels should fall considerably over WA/OR/ID/MT leading to the
possibility of lower elevation snow well below 100 feet, and the
cities of Portland and Seattle have increasing potential to see
light snowfall accumulation. As the mid-level low continues to
drop southward to off the OR coast, strong S/SW 850-700mb flow
will focus lift into the Sierras, but additionally into the
northern California ranges of the Siskiyous, and Shasta/Trinity,
where 1-3 feet of accumulation is likely, highest across the
Sierras once again.


...Northern Plains and Great Lakes...
Days 2-3...

A two-phase snow event is likely from eastern Montana through the
Great Lakes Sunday and Monday. Phase 1 will be a synoptically
driven snowfall from eastern MT through western MN Sunday
afternoon through early Monday /Day 2/. A shortwave ejecting from
California will move quickly eastward across the Northern Plains
producing height falls locally. These height falls will interact
with an upper jet streak lifting off to the northeast as well as
an arctic surface front dropping southward from Canada to produce
lee cyclogenesis in the Central Plains late on day 2. North of
this developing low, significant moist advection on isentropic
lift will combine with a very cold airmass to produce heavy
snowfall in a swath from west to east centered over ND. Forecast
profiles depict a deep and saturated DGZ collocated with the best
omega, and SLRs around 20:1 or more in the very cold air. Guidance
is in good agreement that a stripe of heavy snow is likely, but
believe the guidance may be underdone due to the potential for
extremely high SLRs, despite only moderate snowfall rates and
short duration of forcing. WPC probabilities have increased and
show a high risk for 4 inches, and now a moderate risk for 8
inches of accumulation across this area.

Further east, phase 2 of this event will develop during Monday as
the surface low intensifies and lifts northeast towards Lake
Huron. 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. Additionally, PWATs
climb as the Pacific moisture interacts with a robust moist
advection on the SW LLJ to produce anomalies of nearly 4 sigma
above the climatological normal. Guidance is in good agreement
that heavy snow will occur, but some uncertainty still exists in
the exact placement due to warm air intrusion on the LLJ. WPC
probabilities are high for 4 inches from eastern MN through
northern WI and into the U.P. of MI, but the placement uncertainty
precludes higher chances for more significant accumulations.

There is also increasing confidence in a stripe of heavy freezing
rain southeast of the heavy snow, most likely from central WI
through the U.P. of MI. A prolonged period of WAA on the
aforementioned LLJ will drive a warm nose to above 0C, but
sub-freezing ground temperatures will keep the p-type as freezing
rain for a lengthy duration. The ground is so cold, in fact, from
the recent extreme cold outbreak, that freezing rain may continue
even with 2-meter temperatures above 0C. This is a very tricky
setup and confidence is lower than average, but a sustained period
of freezing rain is becoming more likely and WPC probabilities are
high for 0.10 inches, and up to 30 percent for 0.25 inches.


The probability for significant icing (over 0.25 inches) is less
than 10 percent on Days 1 and 2.


Weiss