Gale Warning 293, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
234 PM AKST Sun Dec 20 2020

.SHORT TERM.../Sunday night through Tuesday night/...Scattered
thunderstorms have been occurring since last night and should
continue though the evening. This convection is supported by cold
air advection moving in from the west. CAPE and Lifted Index values
in the 12Z Yakutat and Annette soundings support convective
activity. Thunderstorms are expected to continue this evening along
the gulf coast from Yakutat to just south of Port Alexander.

As for models, they are in generally good agreement for the short-
term forecast period. The majority of the Panhandle will experience
relatively calm weather until a storm force front approaches the
area for the Tuesday into Wednesday timeframe. Precipitation will be
on the lighter side with showers continuing through Sunday, then
diminishing on Monday. For the southern Panhandle, a waves moving
into this area will increase shower coverage and intensity at times.

Precipitation rates pick up Tuesday into Wednesday as a quick
moving atmospheric river associated with the frontal system slides
through the area. Rainfall amounts around 1 to 2 inches expected
with this next system with higher amounts possible in upslope
favored areas. Forecast Integrated Water Vapor Transport(IVT) values
approach the 700 to 800 kg/(ms) range for the Tuesday into Wednesday
timeframe. This is due to a long fetch of deep moisture from the
central Pacific region being carried toward southeast Alaska. Warm
air advection will be moving toward the Panhandle with this system
keeping the wintry precipitation to a minimum. P-type is expected to
be mainly rain, with the exception of the Haines and Klondike
highways where accumulating snow is likely.

The outside waters, inner channels, and the Panhandle will see windy
conditions with this frontal system due to a tight pressure gradient
between the system and a relatively strong high in British Columbia.
Storm to gale force winds expected in the coastal waters as the
front approaches while small crafts and gales will develop in the
Inner Channels Tuesday into Wednesday. These winds will bring
appreciably higher waves. The associated low will move inland over
south-central Alaska and weaken as the front passes through the
Panhandle after the end of the short-term forecast period.

There is a chance of fog for the Monday evening into Tuesday morning
for the southern Panhandle and areas along the outer coast due to
decreased cloudiness and lighter winds allowing for good radiational
cooling.

.LONG TERM.../ Tuesday to Sunday as of 9 pm Saturday / Long term
timeframe starts with a high amplitude upper level ridge moving
east through the panhandle and then anchoring over western Canada.
This results in a continued southerly becoming more, southwest
direction for the end of the week. In a general sense a broad
upper level trough moves to the western part of the gulf, although
the operations solutions have different ideas where to put the
various upper trough for the weekend.

From a deep (lower 950 mb) parent low over the Bering Sea a
moisture laden front will sweep across the Gulf on Tuesday and
into Southeast Alaska. The front will move through the area
briskly, which will limit rain and or snowfall amounts. This
transitory feature has forecast IVT values that classify it as
weak atmospheric river. The best rainfall amounts expected, at
present west of Yakutat at over 2 inches, while the panhandle
should see 1 to 2 inches of rain.

Across colder sections of northern Lynn Canal this may fall as
snow with potential range of 3 to 12 inches, with the larger
amounts near the Canadian borders. Misty Fjords may be another
area that could see significant snowfall ( up to a foot or so )
Wednesday. Ensembles have fair confidence in this system spread a
better than average precipitation event.

The front on Tuesday will bring Gale to locally Storm force winds
to the outer coastal waters and then diminishing to 15 to 20 kt
by Wednesday.

Late Friday night into the weekend the model agreement continues
to be poor with individual features over the Gulf of Alaska. With
slightly better confidence we can say a broad area of lower
pressure over the gulf will persist through the weekend.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Strong Wind until 3 PM AKST this afternoon for AKZ019.
     Strong Wind until 6 PM AKST this evening for AKZ018.
MARINE...Small Craft Advisory for PKZ012-022-041>043-051>053.

&&

JLC/Bezenek

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