Winter Storm Watch 11, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
702 AM AKST Sun Nov 14 2021

.SYNOPSIS...Gulf lows continue to support periodic showers
through the next 48 hours for Southeast Alaska with marginally
cold temperatures for snow. See the short term for details.

HOWEVER, our focus centers from Tuesday onward as multiple
systems could deliver early season significant winter
precipitation for much of the Panhandle. Even the southern
Panhandle locations could see impactful snow at times towards late
week in some scenarios. Please monitor the forecast as details
emerge.

&&

.SHORT TERM...A complex family of lows in the Gulf of Alaska and
their eventual tracks will dictate interesting weather for
Southeast Alaska during the next 48 hours. Weak low pressure over
Dixon Entrance has supported rain showers across much of the
south. Earlier yesterday evening, rain/snow showers ended across
the north as offshore flow in the lower levels began to dry the
atmosphere. Already, northerlies have begun to tick up through
Lynn Canal, Stephens Passage, and Glacier Bay. We expect this
trend to continue through tonight--culminating in gales through
Lynn Canal and windy, cold conditions in Haines and Skagway.
Metrics for outflow winds in Downtown Juneau continue to decline,
so we drastically lowered wind speeds. Farther south, marginal
thicknesses across the central Panhandle may result in some snow
mixing into the rain showers at times, particularly away from sea
level, but no accumulation is expected as temperatures at the
surface will be too warm. Rising instability across the far south
with a low level jet may incite some thunderstorm potential for
the far south this afternoon. Should lightning occur, we may
extend the threat of the thunder through the evening.

Sunday night, a short-wave rounding from the base of the upper low
in the central gulf will begin to lift the band of showers
northward. As it encounters colder air across the central
Panhandle, light snowfall accumulations are likely from Wrangell
west to the coast and north to Angoon. While there are some
solutions that would bring this north to Juneau, we think the
drier atmosphere wins out. However, with MOS chances of
precipitation pushing into the likely category, particularly in
the ECMWF, we did extend the chances of precipitation north into
Juneau. Lower chances of outflow winds help the case. We expect
accumulations to range from a half to 2 inches.

The precipitation looks to diminish Monday, especially across the
central areas, but as the upper low presses southeast, we expect
more cold air advection from the west and northwest to filter
through the region. Highs for most of the Panhandle will not
manage to exit the 30s. It will feel cold and crisp, and help set
us up for light snow beginning Monday for the north. A weak system
then arrives from the gulf with little wind to speak of to support
mixing, which is bad for snowfall in Southeast Alaska. While cold
air advection begins aloft, whereby 850 temperatures reach -8 C
to -9 C in some cases, we raised temperatures Monday night given
increasing cloud cover. Still, temperatures near 30 degrees with
lighter winds will support light accumulations from Yakutat to the
Icy Strait region including the capital city. These areas could
see an inch or two.

.LONG TERM.../ Tuesday through Sunday as of 10 PM Saturday / At
the 500 Mb level, a closed low drops out of interior of the state
to just south of Prince William Sound, while a short wave moves
in from the west south of the gulf as develops into a small closed
upper center in the trough that is aimed at the Pacific Northwest.
Flow over the gulf continues to remain onshore into early Friday.
The closed low over the northeast gulf move into western Canada
through the weekend.

Tuesday, ensembles continue to support high probabilities of 2 to
5 inches of snowfall for much of the northern Panhandle
Tuesday/Tuesday night. However, given efficient overrunning and
more moisture, some areas could receive more, potentially giving
the region our first major winter storm of 6 plus inches.
Contrarily, warm air will follow the system. Should it follow too
quickly, snowflakes will collapse into grains and accumulations
will be much lighter. Winds could also introduce mixing that kills
higher accumulations. However, for the threat a special weather
statement has been posted, which includes a late week threat of
snow for the entire Panhandle.

The surface features on Tuesday has a frontal band lifting
northeast across the gulf into the the Panhandle area. The remains
of the surface low will move to the northeast gulf. Over the
Arctic Slope a cold high pressure system build in and shift south
and southeast into Northwest Canada. Thursday into Friday, the
pressure gradient along the northern panhandle increases by Friday
night and will likely continue through much of the weekend.
Colder air will spill into the northern gulf Friday and Saturday,
with the colder air stretches into the central panhandle. Have
increased the outflow winds for Friday into Sunday especially over
the northern inner channels marine zones. Land areas also got
slight adjustment up on wind speeds too. The temperatures looked
close so didn't make much in way of changes.

The interesting forecast challenge for the next weekend is that
with the flow pattern, chance to likely precipitation is expected
to continue Friday into the weekend, however with the cooling
temperatures, the P-type is trending towards snow or a mix. This
situation will need to watched to see where the cold air goes and
if there are any impulses and bands of moisture to enhance
snowfall. For now looking and light to moderate snowfall
accumulations. Even Ketchikan and Prince of Wales Island could
receive significant winter weather late in the week.

The model agreement stayed about the same or increased a little
over the last 24 hours.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Strong Wind from this evening through late tonight for AKZ018.
MARINE...Gale Warning for PKZ012.
     Small Craft Advisory for PKZ013-041-042.

&&

JWA/Bezenek

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