Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
452 PM EST Tue Nov 19 2019

Valid 00Z Wed Nov 20 2019 - 00Z Sat Nov 23 2019

...Southwest from California across the Central Rockies...
Days 1-3...
Prolonged precipitation event from multiple upper lows will occur
across the region through Thursday.

Low pressure crosses northeast over the northern Baja Peninsula
tonight and ejects across the Four Corners region through
Wednesday. Meanwhile, a more potent upper low will develop over
northern California this evening and merely drift east over the
southern Great Basin through Thursday before shifting east across
the across the south-central Great Plains. As the initial low
fills and lifts northeast toward the Central Plains, snow-levels
will be above 10,000ft in this tropically sourced air mass.
However, height falls under the remnant trough allow the 700mb
temp (near 10,000ft) to drop below 0C and allow snow to overspread
the highest terrain in AZ, southern UT, and southern CO in the
widespread heavy precipitation. The Day 1 risk for 8 inches or
more is moderate to high in the San Juans and southern Wasatch.
Snow levels drop to around 6000ft under the second low across
southern and central CA where Day 1 probabilities for 4 inches are
moderate to high across the other Sierra Nevada, the Spring Mtns
west of Las Vegas and the highest San Bernadino Mtns.

The California closed low will push into the southern Great Basin
Wednesday night, keeping the tropical Pacific moisture feed into
the Four Corners region. Snow levels continue to fall to around
6000ft just ahead of the low with heavy snow expected for the
higher terrain in these states. Widespread moderate to high
probabilities exist for 8 inches above 6000 ft from the Mogollon
Rim across the Wasatch to the Unitas, the San Juans, Sangre De
Cristos. More moderate 8 inches probabilities extend across the
Great Basin ranges, and ranges and even high plains of southern
Wyoming. This feature will begin to eject eastward on Day 3, and
the weaker forcing will produce moderate continued snowfall in the
highest terrain of CO and NM, with light snow spreading eastward
into the Plains.


...Northern Rockies to Lake Superior...
Days 1-2...
Northern stream trough digging southward from Alberta will drive a
cold front into the Northern Plains while moisture streams
overtop. Robust ascent through the LFQ of a 110kt upper jet streak
combined with upslope enhancement on low-level E/NE flow will
produce precipitation, which will likely quickly change from rain
to snow as cold air floods southward and ProbSnow increases
towards 100%. WPC probabilities indicate a high risk for 4 inches
from near Glacier NP eastward to the MT/ND border along the
international line.

At the same time, a southern stream system will eject out of the
Southwest spawning surface cyclogenesis racing northeastward
towards the Great Lakes. This system will be accompanied by
significant moisture, height falls, and modest PVA to provide
ascent and moderate snowfall from the central Plains into
Minnesota. However, interaction of these two shortwaves will
likely create an area of intense deformation, and a band of
frontogenesis where thickness lines contract between the two,
collocated with the upper jet streak. In this area, which is most
likely in a SW to NE band in the vicinity of the MN Arrowhead,
heavier snow is forecast, and WPC probabilities are moderate for 6
inches.

Weiss