Small Craft Advisory 639, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
506 PM AKST Tue Dec 21 2021

.SHORT TERM.../ Through Thursday / The short term forecast has a
change in the weather ongoing. The surface low over the eastern
gulf is Southwest of Sitka and it will continue to move southeast
this evening to be west of Haida Gwaii by late evening. Snow
ending across the northern panhandle as the low moves southward.
By midnight the snows should be ending leaving clearing conditions
and cold outflow patterns establishing itself over the panhandle
and the onset of cold arctic push.

Winter storm warnings and Advisories will be expiring through
afternoon and evenings. Numerous reports of 6 to 10 inches of
fluffy early and wet snow in the afternoon. The snow reports were
from Yakutat, Haines, Gustavus, Juneau, Petersburg, and Klawock.

Tightening pressure gradient along the northern coastal range will
cause north winds to increase this evening Lynn Canal to increase
to gale force 40 to 45 kt, and gusts in the Haines and Skagway
regions of 45 to 55 mph this tonight, and diminishing to 40 to 45
mph for Wednesday. Part of the Canadian high pressure will be
sliding into northern British Columbia, so gap flow will increase
assisting with the cold outflow the northeast pressure gradient
begins to increase.

.LONG TERM...All guidance continues to point toward prolonged
northerly outflow winds and bitterly cold temperatures over the
Panhandle through the period, especially toward the holiday
weekend. A low in the northeast Gulf will quickly move
southeastward and remain just off of the Outer Coast, bringing up
to 50 percent PoPs and up to a quick 1 to 2 inches late Wednesday
night through Thursday morning. There looks to be only chances for
increased cloudiness and scattered flurries elsewhere in
southeast Alaska as the low moves by. For the rest of the period,
it looks to remain dry throughout the Panhandle with decreased
clouds and continued windy conditions as our weather will, once
again, be completely dominated by a strong area of high pressure
over the Yukon region of Canada.

As for temperatures for the period, we chose to generally go
between 2 and 8 degrees below MOS guidance due to MOS generally
not handling prolonged cold outflow events very well. Model
forecast 850mb temperatures between negative 20 and negative 30
degrees Celsius advecting toward the Panhandle from the north and
east from the relatively higher pressure located over the Yukon
and British Columbia, Canada translate to overnight low
temperatures at the surface potentially dipping down as low as
negative 10 in some areas in the northeast Panhandle. The
prolonged decreased cloudiness toward the weekend will contribute
to the extra cooling, as well. Use extra caution and dress warmly
if you must be outside for any prolonged period of time in these
dangerously cold temperatures and breezy conditions as frostbite
can occur on exposed skin in a matter of minutes.

As for winds, strong winds will be possible in the Skagway and
Haines areas associated with the aforementioned northerly outflow
winds. Mariners should remain alert to the risk of enhanced seas
and freezing spray build-up associated with the strong outflow
winds, especially over the north and central north-south oriented
Inner Channels. This will allow seas to build over distance due to
the long fetch of the wind. Near Gales are possible over Lynn
Canal through the weekend. Up to small craft winds are possible
over adjacent northern and central Inner Channels and gustier
winds are likely out of interior passes oriented perpendicular to
the pressure gradient. The offshore winds will keep us dry for the
majority of the period.

Forecast confidence was generally above average for the period as
the models are in much better agreement than they were, yesterday,
about the track of the late Wednesday night into Thursday low in
the Gulf.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Warning until 7 PM AKST this evening for AKZ026.
     Winter Weather Advisory until 9 PM AKST this evening for AKZ027-
     029.
     Winter Weather Advisory until 4 PM AKST this afternoon for
     AKZ024.
     Winter Storm Warning until 9 PM AKST this evening for AKZ028.
     Strong Wind through Wednesday morning for AKZ018.
MARINE...Heavy Freezing Spray Warning for PKZ012.
     Gale Warning for PKZ012-013-022-031-032-041>043-051-052-310.
     Small Craft Advisory for PKZ011-021-033-034-036.

&&

Bezenek/JLC

Visit us at http://www.weather.gov/Juneau