Blizzard in Alaska, December 30 to 31, 2016

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
623 AM AKST Fri Dec 30 2016

.SHORT TERM...Strong shortwave trof will move E across the N gulf
today then SE across the E gulf and SE AK tonight. Cold front will
move across the area just ahead of the shortwave trof through
tonight, reaching the far S late tonight.

Main forecast concerns will be winds and precip potential/type/amounts.
The shortwave/front are very energetic with jet stream level winds
in excess of 140 KT. The pressure gradient will increase
significantly today as cold front approaches, and expect most of
the N inner channels and N gulf to see gale force S-W winds ahead
of the front. Stronger winds will spread SE to the SE gulf and S
inner channels this afternoon, but they will not be as strong as
the pre-frontal winds over the N. Still...25-35 KT S-W winds are
likely over the SE gulf and S inner channels this afternoon and
evening. After the cold front moves through, winds will shift to
N-NW and gale force winds are expected over the gulf and N half of
the inner channels tonight. There may be a 2-3 hour window on
inner channels where winds will drop to 10-20 KT between the
strongest pre and post frontal winds. Have strong wind headline for
zone 25 this afternoon. Zone 18 will see strong winds as well but
they will have a more significant concern, which will be discussed
below. Zone 27 will likely see strong winds later tonight after
cold front has passed and strong W-NW winds develop in cold
advection behind the front.

Precip will be increasing over the area from NW to SE today. Am
expecting the heaviest precip just ahead of the cold front, and in
areas that upslope on SW flow. Temps will be rising in the strong
onshore flow so most places near sea level should see mainly rain.
However, a few less windy areas over the N could see some snow at
precip onset but it will become rain in the afternoon. Snowfall
near sea level will generally be an inch or less. At higher
elevations, the combination of wind and snow will cause blowing
snow, and blizzard conditions are likely along the Klondike
highway beginning late this morning and continuing into early
evening. Issued blizzard warning for White Pass area along the
Klondike highway from 18z-06z today. The precip will rapidly
diminish from the NW tonight a few hours after the cold front has
passed through any given area. Clearing should develop over the N
half of the area late tonight as colder air slowly filters in.

.LONG TERM...The front discussed above will be largely south of
the area by Saturday morning with just a few left over showers for
the far southern panhandle up to Hyder. North winds will continue
into the morning but ease through the day as the pressure gradient
slackens.

A strong northerly jet extending from the arctic will bring cold air
aloft across the region Saturday and Sunday. This flow pattern will
keep things dry through the week. However, at the lower levels there
looks to be a layer of much warmer air. Light winds and mostly clear
skies will allow for good radiational cooling at night. The
combination of these things will create an inversion and high
pressure building over the gulf will increase the likelihood of a
marine layer and fog to develop for the first couple days of the
week. Have continued with mention of patchy fog across the panhandle
for now, but this may need to be expanded once it starts to form.

Flow starts to back more NWLY Monday into Tuesday which would help
to push a marine layer of clouds inland if one were to develop. A
short wave passing over the ridge and across the panhandle will
likely helps mix out the marine layer on Wed with another burst of
North winds behind it. Things cool off again for the end of the week
as the upper level jet looks to tap back into the northerly arctic
airmass.

Used mainly the GFS for adjustments to pressure through the mid
range. This slackened winds speeds across the central inner channels
through mid week. Forecast confidence is average.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind late tonight for AKZ027.
     Strong Wind from noon AKST today through this afternoon for
     AKZ025.
     Blizzard Warning from 9 AM this morning to 9 PM AKST this
     evening for AKZ018.
MARINE...Gale Warning for PKZ012-013-021-022-031>035-041>043-051-052.
     Small Craft Advisory for PKZ011-036.
     Small Craft Advisory due to Seas for PKZ053.
&&

RWT/Ferrin

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