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

Valid 00Z Sun Feb 03 2019 - 00Z Wed Feb 06 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.

A strong storm system currently moving into California will
continue through the Day 1 period and this robust energy/lift will
occur over the Sierras as well as some of the other NW-SE oriented
ranges that will be orthogonal to the mean flow. D1 totals will be
measured in feet in some places. As the system spreads into the
interior Rockies, the mountain ranges of ID, western WY, UT, and
CO will likely see 1 to 2 feet through Day 2.

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 1-2...

A strong shortwave trough currently pushing into California will
lift through the central and northern Rockies before crossing into
the northern Plains and Upper Midwest Sunday night into Monday.
Surface cyclogenesis in response in the lee of the Rockies will
take a track from the central Plains toward west-central
Wisconsin. Very strong isentropic lift will produce a swath of
precipitation north of the low, with heavy snow likely across
portions of central/eastern MT into ND and northern MN. 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 for 6" amounts across
eastern MT and ND are high and are now moderate for 8" totals.

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 a concern for a stripe of freezing rain across
portions of MN into north central 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.

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


Weiss/Taylor