Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 PM EST Fri Feb 01 2019

Valid 00Z Sat Feb 02 2019 - 00Z Tue Feb 05 2019

...Western States...

Two primary waves of precipitation are expected across the West
over the next few days, closely tied to a pair of strong lows. The
first will begin to ramp up tonight in California due to a rapidly
deepening and occluding low approaching the coast, and will
gradually spread into the Intermountain West by Sunday. This low
will be quite strong relative to climatology, with model forecast
MSLP on the central California coast falling into the lowest
percentile of the distribution for this time of year, and
850-700mb winds above the 99th percentile. The result should be a
tremendous amount of orographic ascent in the Sierra Nevada;
coupled with precipitable water values in the southern portion of
the range above the 90th percentile, this is a recipe for very
heavy snowfall. Most of the snow should be focused within a 12-15
hour window and several feet of snow could fall in the Day 1
period alone, creating a strong likelihood of tremendous snow
rates above the snow elevations. The closed low aloft will open
into a wave as it pushes into the Intermountain West Saturday
Night and 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 should be at least 5000-6000 feet across most of that
region, confining heavy snow to the highest elevations.

The second low will be associated with a strong wave digging down
the coast of British Columbia this weekend, and toward the Pacific
Northwest coast on Monday. In the mid-troposphere (700-500mb) a
low closes off by Sunday afternoon just off Vancouver Island, with
strong height falls (over 100m in 12hr) focused along the coasts
of OR/WA. By early Monday morning (06-12Z Monday), 850-500mb
heights are in the lowest percentile of the climatological
distribution for mid-winter, suggesting a cold system overall.
Therefore, for the Day 3 period from Sunday Night into Monday,
snow levels should fall considerably over WA, N OR, N ID, and W MT
leading to the possibility of lower elevation snow well below 100
feet. The structure of the low will be somewhat diffuse by the
time it reaches WA/OR, with most models indicating a broad
depression with multiple low centers spread over several hundred
miles. Therefore, precipitation may not be focused in particular
locations. Orographic ascent should also be weaker over the
Northwest on Day 3, This should lead to a broader distribution of
lighter snow amounts, but the potential for accumulations even
into populated corridors like Seattle and Portland along
Interstate-5. On the southern fringes of the digging low, stronger
westerly flow and access to more substantial moisture should lead
to renewed snow potential in the ranges of California -- not only
the Sierra Nevada, but also the Klamath and Shasta Siskiyous. Snow
levels should be falling in these areas as well, but will not be
as low as the Northwest.

Over the course of the next three days, WPC snow probabilities
indicate a near certainty that the Sierra Nevada receives over 2
feet of snow (greater than 90 percent), with over 1 foot of snow
likely in numerous other ranges of the western US, especially
above 6000-7000 feet in elevation.


...Northern Plains and Great Lakes...

The initial wave in the West (the one impacting California on Day
1) will reach the Plains by late Sunday. This will lead to lee
cyclogenesis and a surface low tracking from the central Plains to
the Great Lakes on Day 3. To the north of the low track, a swath
of accumulating snow and ice is forecast. The snow should extend
from central and eastern Montana (late on Day 2) into North Dakota
and northern Minnesota. Very cold air will arrive from Canada
later in the weekend; this will aid in cyclogenesis but also keep
the temperature profiles in the areas of snow quite cold. Forecast
soundings from the aforementioned area (particularly ND and N MN)
show a layer about 4km deep, extending from the surface upward, in
the -10C to -20C layer. This will favor high snow ratios, and
forecast snow amounts were adjusted upward to account for this.
The result is a high likelihood (over 70 percent chance) of at
least 4 inches of snow from central Montana all the way to the
immediate vicinity of Lake Superior. The probability of over 8
inches of snow in the same area is also over 30 percent,
indicating a reasonable chance for a significant snowfall event.

Just to the south of that, there will be a chance for freezing
rain, from South Dakota east-northeast into the southern half of
Minnesota, northern Wisconsin, Upper Michigan and northern Lower
Michigan. For western parts of that region (SD, S/C MN), this will
likely be due to a layer of dry air initially around 800-700mb
that will favor supercooled water droplets reaching the ground.
However, the wet bulb profile in these areas is mostly below
freezing, so as the cyclone becomes better developed, a changeover
to snow is expected. To the east, and generally along and east of
an inverted trough axis Monday morning (N WI, N MI), freezing rain
may be sustained for a longer period of time due to more robust
warm-air advection aloft. From northeast Wisconsin into eastern
Upper Michigan, at least 0.10 inch of ice accumulation is more
likely than not (over a 50 percent chance), while the probability
of 0.25 inches of ice is over 10 percent.


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

Lamers