Heavy Snow in Alaska, December 17 to 19, 2020

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
703 AM AKST Thu Dec 17 2020

.SHORT TERM...The forecast starts off on an active note with a
gale force front bearing down on the panhandle from the SW which
will bring more precipitation and wind for the area. This system
will bring at least one additional wave with it on Friday,
followed by another low approaching the panhandle by late Fri
night into Sat morning. Models continue to struggle with the
placement of this last system, though they have been trending the
low track further north, with a broad area between Anchorage and
Cape Fairweather favored for landfall. The main problem continues
to be where the rain snow line will set up, and just how much snow
the panhandle is going to get. Models have come into increasingly
better agreement on a push of warm air reaching into the central
and northern panhandle on Friday, which could warm temperatures
enough to cause a changeover to rain. Have warmed up temperatures
Friday afternoon into Friday night across the central and northern
panhandle as a result. This would see most areas switch over to
rain or a rain snow mix across the central panhandle, though the
northern panhandle could see snow last longer. With the
uncertainty in when the warmer air will arrive, chose to nudge
down the snow ratios slightly, but continue to expect broadly
similar totals to previously anticipated for Haines, Skagway,
Gustavus, and Juneau until the transition to rain occurs. Hyder
will also need to be watched, as cold air could linger longer
there than in other locations.

Winds will also be of some concern, with gale force winds for the
outer coastal waters and small craft to gale force for the inner
channels as these features move through. Highest winds will likely
be Thu night with the initial front, and Fri as then next wave
moves in. Forecast confidence is below average, giving timing
concerns on the anticipated warmup and the transition to rain.

.LONG TERM.../Saturday through Thursday as of 10 PM
Wednesday/...The long term period continues to be filled with
uncertainty as model guidance provides little agreement from model
to model. When you find yourself in this scenario, changes are
made where confidence is highest. Through this time period, there
are two areas where confidence is high enough to add valued
changes to the forecast. One, on Saturday, a warming trend takes
place as southerly flow increases through the inner channels. Two,
on Tuesday, confidence has increased that the panhandle will
likely see a 12 to 24 hour break in precip as high pressure builds
across the panhandle before quickly being displaced by yet
another low pressure system. With this break, cold air advection
aloft pared with decreasing cloud cover, overnight temperatures
will plummet.

Back to Saturday, the storm expected to impact the panhandle will
bring gale force winds along the coast, heavy rain for the
central and southern panhandle, and snow across upper lynn canal.
The track of this low will dictate on how the forecast progresses.
For now, the low is forecasted to move through the Icy Strait
corridor. With this type of track, pressure gradients support
southerly flow, warming temperatures, allowing for rain to be the
main precip type from Icy Strait south. Areas including Haines and
Skagway hold on to the cold air the longest allowing for snow. Now
if the track of the low shifts further south, the forecast changes
allowing for snow through the Icy Strait corridor. Ultimately,
further analysis of later model runs will be needed before
forecast confidence can being increased.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Warning from 9 AM Thursday to 4 PM AKST Friday for
     AKZ025.
     Winter Storm Warning from 9 AM Thursday to 5 AM AKST Saturday
     for AKZ018-019.
     Winter Weather Advisory from 9 AM Thursday to 4 PM AKST Friday
     for AKZ017-020-021.
     Winter Weather Advisory from noon Thursday to noon AKST Friday
     for AKZ029.
MARINE...Gale Warning for PKZ043-051-052.
     Small Craft Advisory for PKZ022-031-033>036-041-042-053.

&&

GFS/CDC

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