Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
443 PM EDT Sat Nov 03 2018

Valid 00Z Sun Nov 04 2018 - 00Z Wed Nov 07 2018

...Northern and Central Rockies...

Days 1-3...

Shortwaves embedded within confluent 500mb flow across the Pacific
Northwest will advect moisture east-southeast atop the mid-level
ridge to produce widespread and frequent mountain snows from the
northern Cascades and through the northern and central Rockies.

The first shortwave and associated jet streak will drop across the
northern Rockies on day 1 before shifting eastward into the Plains
on day 2. The heaviest snow associated with this feature will be
across Idaho, northwest Wyoming, and western Montana, where WPC
probabilities are high for 8 inches of snow, with over 12 inches
likely in the Grand Tetons. The best 700mb omega and moisture
shift southeast into day 2 and the highest probabilities for more
than 8 inches of snow are across the Rockies Sunday night into
Monday. The highest snowfall totals will be above 8000 feet, but
lesser amounts are likely down to 4000 feet.

A second impulse will drop along a similar trajectory during day 3
producing another round of heavy snow where the best combination
of diffluence and upslope enhancement come together. This is
likely to occur across the Bitterroots of Idaho southeast towards
the ranges in NW Wyoming. An additional 4-8" of snow is possible
in the highest terrain within this second impulse.

Although these primary shortwaves will be the trigger for heavy
snow, several weaker mid-level pieces of energy combined with
periodic jet-level diffluence will allow for snow to continue
across many of the peaks above 6000 feet through the entire
period. This will produce widespread 1-2 feet of snow in many of
the mountain ranges, with the highest amounts likely in the
Absarokas, Grand Tetons, Northern Rockies, and Bitterroots. Lesser
but still significant snowfall is likely into early next week
within the Northern Cascades, as well as some of the other
mountain ranges including the Big Horns and Uintas.


...Western Great Lakes...

Days 1-2...

An area of low pressure will strengthen as it moves northeast from
Iowa towards Lake Superior Sunday into Monday. This low will
strengthen in response to a shortwave digging southeast through
the Upper Plains, with a negative tilt developing into Monday.
Precipitation will expand and overspread the area from the south
on increasing warm and moist advection, and temperatures initially
will be cold enough for snow through the column despite barely
sub-freezing surface temperatures. As the 700mb low tracks
northward across far eastern MN, 850-700mb temps will climb above
0C and the precipitation will gradually change over to rain during
the second half of day 1 across Wisconsin and the U.P. of
Michigan, with snow lingering into day 2 only across the Arrowhead
of MN.

Thermal profiles are marginal for significant snow as the
dendritic growth zone is well elevated and surface temperatures
will hover near freezing south of Lake Superior. This suggests
that heavy rates will be required for significant accumulation.
However, briefly heavy snowfall is likely as an area of strongly
sloped 925-600mb frontogenesis lifts northward producing intense
omega despite a lack of -EPV. The most likely location for 4
inches of snow or more will be across the Arrowhead of MN which
will experience a pivot of mid-level Fgen as the negatively tilted
low slows down, and it is from Duluth points northeast where WPC
probabilities are moderate for 4 inches of snow. Elsewhere, 2-4"
is possible across far northern Wisconsin and into the U.P. of
Michigan before precipitation type transitions to rain.


The probability of significant icing is less than 10 percent days
1-3.


Weiss