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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0800Z Oct 17, 2018)
 
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Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 AM EDT Wed Oct 17 2018

Valid 12Z Wed Oct 17 2018 - 12Z Sat Oct 20 2018


Day 1...

...Southern and Central Rockies...
A closed mid-level low across AZ will begin to open into a
positively tilted trough and gradually lift northeast into Utah by
the end of Day 1. Lift associated with this feature due to jet
level diffluence and weak mid-level vorticity will interact with
moisture being advected into the Rockies on S/SE 700mb flow.
Upslope enhancement is likely, especially into the San Juans of
Colorado and the Wasatch of Utah, with the best moisture/lift
combination expected across the former. Snow levels will remain
high, above 7000 feet, and while some discrepancy still exists in
the guidance into where the strongest lift will occur, it appears
the San Juans will be favored for the heaviest snow where upslope
is most intense. Across these mountains, WPC probabilities show a
high risk for 4 inches of snow or more, with 8 inches or more
possible in the highest terrain. Elsewhere, WPC probabilities are
moderate for 4 inches.

...Great Lakes...
Upper trough centered across the Hudson Bay will push eastward
through Day 1 while a shortwave digs across the Great Lakes. This
shortwave will initiate cold air advection across the Lakes,
producing enough instability to create lake effect snows downwind.
Unidirectional winds from the W/NW will support lake effect
precipitation SE of most of the Lakes, but the heaviest
precipitation is likely east of Lake Ontario where moisture from
Lake Huron will combine with the fetch, and be enhanced by upslope
into the Adirondacks and Tug Hill Plateau. Temperatures will be
marginal however, so snow is expected primarily above 1000 feet,
and WPC probabilities are only mentionable for 2 inches of snow.

...New Mexico...
Shallow cold air will remain across New Mexico early Wednesday
before warm advection erodes the shallow cold layer. Overrunning
precipitation will fall into a sub-freezing surface layer evident
on area soundings to produce some light freezing rain accretion
across the Sacramento Mountains. A few hundredths of an inch are
possible before the column warms too much to support freezing rain
during the afternoon.

Day 2...

...Mountains of Utah and Colorado...
Mid-level positively tilted trough will fill and eject eastward
into the Plains during Day 2. Moisture and lift will combine early
across the Mountains to produce snow showers, before dry advection
and warming temperatures bring an end to the snow. The longest
duration of snowfall will be across the San Juans where WPC
probabilities are moderate for 4 inches of snow. Elsewhere across
the high terrain of Utah, including the Wasatch Range, a few
inches of snow are possible above 9000 feet.

The probability of significant icing (0.25 inches or more) is less
than 10 percent.

Day 3...

The probability of significant snow (4 inches or more) is less
than 10 percent.

The probability of significant icing (0.25 inches or more) is less
than 10 percent.


Weiss