Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
407 AM EST Fri Dec 27 2019

Valid 12Z Fri Dec 27 2019 - 12Z Mon Dec 30 2019

...Southwest U.S....
Day 1...

A closed 700 mb low moving over Arizona today moves northeast
overnight and reforms over the central high plains by Sat morning.
Snows continue early in the mountains of central to northern AZ
taper as the upper trough moves further east.  The heaviest snow
is over the CO San Juan mountains, in windward locations in
sustained upslope flow thru today. Average liquid equivalent QPF
near an inch supports Day 1 WPC probabilities are high for 12
inches or more over the San Juans of southwest CO to northern NM.

...Central and Northern Great Plains...
Days 1-3...

...Major winter storm forecast to produce a foot or more of snow
in portions of the northern Plains...

Lee side surface low pressure develops over eastern NM late Friday
and moves northeast across KS Saturday, IA Sat night, and slowing
down across southern MN or adjacent areas Sunday.
Confidence in the track of the surface low has increased with some
uncertainty still with the depth of the mid-level low and axis of
comma head precip over the northern Plains. A preferred blend of
the 12Z ECMWF/ECENS/UKMET/21z SREF Mean was used for the low
track, QPF, and precip types.

This results in high confidence for 4 inches and low to moderate
confidence for 8 inches of snow from northeast CO across
western/central NE, central SD, and southeast ND.

With the cyclone stalling over southwest MN on Day 3, bands of
snow northwest of the low persist in northern MN to southeastern
ND and eastern SD. Probabilities for 4 inches are high in these
areas complemented by low to moderate probabilities for 8 inches,
and low for 12 inches.

A precip type transition zone will occur along the 850 mb low
track, resulting in freezing rain accumulations given the cooling
and drying antecedent conditions.  The day 1 axis of freezing rain
extends from southwest to central NE, southeast SD, and to
northwest IA/southern MN.
On day 2, the icing extends from eastern SD and southern MN to
northern WI/possibly the adjacent UP of MI.
 
...Northeast...

Day 1...
Surface temperatures area below freezing ahead of a warm front
crossing northern New York into New England. Warm low level air
advection produces light precip that results in a period of
freezing rain or drizzle across northern NH and most of Maine. 
The moisture and lift depart Maine into the Atlantic Canada
tonight, bringing a decrease in coverage of pockets of freezing
drizzle. The accumulations near a tenth of an inch are possible in
interior Maine.

Day 3...
High pressure over Quebec and nosing down into eastern New England
on Sunday gives way to low level warm advection, with  warm front
drifts north across NY State.  Light to moderate icing
accumulations are possible where cold air becomes trapped in the
NY Catskills and Adirondacks.
The warm advection continues downstream into the MA Berkshires and
VT Green Mountains, where light icing is possible.


...Pacific Northwest to California...

A progressive upper trough with weakening surface low pressure
moves from the northeast Pacific towards the northwest US Sunday. 
The surge in moisture advection and lift crosses the Pacific
northwest early Sunday, continuing downstream into northwest
California as the day progresses, and then across central CA
Sunday night.  Higher elevations along this trajectory are
expected to receive several inches of snow, including the OR
Cascades, CA Siskiyous, and northern to central CA Sierra. The
lack of a long duration snow limits the event potential, .


Petersen