High Wind in Alaska, November 5, 2020

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
321 PM AKST Wed Nov 4 2020

.SHORT TERM...A low pressure system is quickly approaching the
region from the SW. It is expected to pass through Dixon Entrance
overnight into B.C. in the vicinity of Misty Fjords Thursday
morning. Rain started falling across the southern panhandle
around noon and as far north as Petersburg as of this writing. A
deformation band is progged to set up between Kake and Petersburg
this evening. Precip rates and cold N-NE winds are expected to
cause a change over to snow for those areas with around 4 inches
of accumulation by Thursday morning. Some rain/sleet mixing in at
times will make the snow on the ground heavy and wet and more
difficult to manage.

High pressure building over the interior/Yukon in conjunction with
the low to the south will cause north winds to continue increasing
this evening with northern Lynn Canal expected to reach Storm
Force 50kt. These winds have been causing blowing snow in Skagway
all day and likely Haines as well.

Strong northerly winds will be with us for most of Thursday then
start to ease in the afternoon/evening as both the high and low
pressures shift eastward. Clouds and precip will move out through
the morning as well, making for partly cloudy to mostly sunny
skies in the afternoon.

.LONG TERM.../Friday through Wednesday night/...An active weather
pattern continues in the long term with multiple systems to affect
the region this weekend through next week. The period begins with
a flat ridge on Friday with a weak system moving Southeast
through the gulf. Ridging sharpens on Saturday over the gulf
before the next impactful weather system arrives Sunday into
Monday. Ridging then builds back into the region behind the for
the beginning of next week.

On Friday, a weak low in the northern gulf rounds the top of a
ridge and slides SE just offshore of the eastern gulf coast. PoP
for this system is somewhat low, since much of the energy will be
dissipated getting over the top of the surface high. Cold air
already entrenched over the northern half of the Panhandle will
remain in place resulting in snow as the dominant P-type. Areas
right along the outer coast, mainly Sitka, may mix with rain
Friday afternoon due the marine influence and slightly warmer
temperatures aloft. Snow accumulations are expected to be light
with 6 hour snowfall totals around an inch or less for the central
Panhandle and Icy Strait corridor. Northerly winds will pick up
to 15 to 20 kt in the Inner Channels and small craft conditions
will develop in the gulf as this system passes by. Overall, a low
impact system compared to the one coming later in the weekend.

Immediately after the passage of this system, a ridge will rotate
around the top of the surface high and produce another dry and
sunny day for Southeast Alaska on Saturday. Change will occur soon
thereafter, as a developing low and associated front south of the
AK Peninsula tracks rapidly over the northern Gulf and displaces
the surface high south and east. Current long range models are
indicating high QPF values with this system, so it bear close
watching. There is model spread regarding the track of the low
with the GFS solution bringing it into the northern gulf while the
EC has it moving into the Interior and the Canadian into Prince
William Sound. Both the GFS and EC ensembles are more in line with
the GFS solution, taking the low towards the northern gulf.
Trended the forecast towards this solution.

There are indications that this system could pack a punch as both
the GFS and Canadian models show a storm to hurricane force low
and associated front. Currently have a high end gale force front
pushing into the Panhandle Sunday evening. Did not go as high as
the deterministic models just yet, however, trends in the wind
forecast, strength, and track of this system will be monitored
closely over the next few days. Not only could winds be impactful,
but also QPF as ensemble QPF M-climate data is showing around Max
values for 6 hour, 12 hour, and 24 hour totals Sunday through
Monday. Current deterministic models are showing 24 hour precip
totals around 1 to 3 inches for the northern half of the Panhandle
and NE Gulf Coast. The big question will be if this falls as rain,
snow, or a mix. With cold air already in place on Saturday, there
will be a battle between how strong the warm air advection is
ahead of this system and how deep the cold air is at the surface.
The EC Extreme Forecast Index has portions of the northern
Panhandle highlighted for snow during this timeframe. WPC guidance
along with the GFS and Euro keep surface temperatures marginal
(low to mid 30s) for accumulating snow. Temps aloft around 850 mb
will be marginal as well starting out around -6C to -8C Sunday and
rising to around -4C as the system passes. Current forecast does
have accumulating snow for Juneau and much of the northern Inner
Channels; however, this is a low confidence forecast and more
analysis will be done in the coming days. What is certain is that
an impactful weather system will affect the region this coming
Sunday and Monday. The uncertainties lie with P-type, the track
and strength of the low, and strength of the warm air ahead of
it.

Overall forecast confidence is high through Saturday then drops
off toward Sunday and next week.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory from 6 PM this evening to 6 AM AKST
     Thursday for AKZ026.
     Strong Wind from 6 PM this evening to midnight AKST tonight for
     AKZ025.
     Strong Wind from late tonight through Thursday afternoon for
     AKZ025.
     Strong Wind through late tonight for AKZ019.
     Strong Wind through Thursday afternoon for AKZ018.
MARINE...Storm Warning for PKZ012.
     Gale Warning for PKZ011-013-022-031>033-041-042-051.
     Small Craft Advisory for PKZ021-034>036-043-052-053.

&&

Ferrin/CM

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