Area Forecast Discussion
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FXAK67 PAJK 221553

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
653 AM AKST Thu Feb 22 2018

.SHORT TERM...A gale force low will track southeast across the
panhandle today and is expected to produce heavy snow over
portions of Southeast Alaska. The decision was made to issue a
Winter Storm Warning for this afternoon into early tomorrow
morning for Juneau, Douglas, Petersburg, and Wrangell based on
snowfall amounts models were depicting. Snowfall will generally
begin around 9 am this morning, with the heaviest amounts being
produced around 6 pm this evening. By late Friday evening,
lingering snow showers should produce minimal snow accumulations.

Our main concern for this morning was to focus on this storm. A
NAM/GFS/Canadian blend was used to make pressure changes over the
Gulf. The progression of the front was also delayed by 3 hours,
with precipitation now expected to begin around 9 am local time
along the northeast gulf coast. Wind speeds over the gulf were
increased to a 40 kt westerly gale over the gulf. Winds on the
inside for both marine and land locations generally unchanged.
Maintained southerly gales over northern and southern Lynn Canal.
Did decreased winds slightly over northern Stephens Passage
vicinity Young Bay and kept gusts below 40 mph for Skagway.

POP and QPF were adjusted using a blend of the NAM12 and the 3-hour
GFS, resulting in some timing changes as indicated above. Model
agreement on PoP and QPF was very good and boosted confidence in
timing and locations expected to see precipitation over the short
term forecast period. Changes to temperatures and snow levels were
more problematic. Most locations across the panhandle were
within model spectrum spreads for the tonight period, with the
exception of Juneau. Models wanted to keep Juneau in the low to
mid 20 range, while the GFS MOS was keeping Juneau hovering at or
slightly above freezing. Modest warm air advection will accompany
this front and there will be an increase in temperatures. This and
the resulting impacts on the snow level has been the most
difficult part of the forecast and has been the case for the past
few days. High confidence that there will be some locations that
transition to a rain/snow mix before any significant accumulations
occur - Elfin Cove, Angoon, Kake. Additionally, there will be
some places that see just rain - Sitka, the outer coast of Prince
of Wales, and possibly Ketchikan. In all cases, what snow that
does accumulate will have a fairly high water content and
difficult to clear where accumulations are more than a couple
inches. Used the Canadian NH as a foundation for snow ratios. Heaviest
accumulations expected farther inland, from Juneau south to the
Petersburg area where around 8 inches are forecast. In these
locations, Omega is maximized near, but slightly below the snow
growth zone. Storm totals will depend greatly on actual low level
temperatures below about 5000 feet elevation.

Confidence on timing is average trending above average. Confidence
in the actual location of the rain snow line, how it changes, and
storm total snowfall is below average.

.LONG TERM...Saturday through Thursday...A much more active
pattern can definitely be expected for SE Alaska. For the early
part of the long term on Saturday, we`ll start off with a
developing shortwave out of mainland Alaska that will strengthen
over the central gulf and impact the panhandle. Overall, our
confidence has increased regarding this feature, and deterministic
models have come into better agreement. Model 500 mb vorticity
and winds indicate additional strengthening from increasing
vorticity and jet dynamics. It appears to take a similar track as
some of our previous lows, and if we go off precedence, the
precipitation type with this system will most likely remain as snow,
and model 925 mb temperatures confirm our suspicions. The more
difficult aspect with this shortwave will be QPF or how much
precipitation we can expect. Due to the origin of this shortwave,
moisture may initially be lacking, but the marine environment may
alleviate this, as there appear to be relatively strong west,
southwesterlies aloft to advect in additional moisture.

On Sunday, model analysis continues to show this low weakening as
it tracks southeast along the panhandle and a weak ridge of high
pressure quickly building in behind Sunday night into Monday.
Another shortwave then appears develop Monday, again over the
central gulf, before possibly impacting the panhandle Monday
night. However, there is quite a bit of disagreement among the
deterministic models regarding surface features and timing. Seeing
this, we tended to gravitate towards WPC guidance Tuesday onward
to account for this uncertainty.

The main changes we made to the long term forecast consisted of
increasing chances for precipitation on Saturday and continuing
into Sunday. In addition, we increased winds over the gulf and
some of the inner channels Saturday night into Sunday as that
developing low mentioned previously strengthens and the pressure
gradient tightens between it and a high pressure over British
Columbia, Canada. We tended to gravitate towards the NAM for the
12Z Saturday to 12Z Sunday period, as models were in pretty good
agreement at this point. After this period, we mainly chose to use
WPC guidance and NBM analysis.


.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Storm Warning from 3 PM this afternoon to 9 AM AKST
     Friday for AKZ025-026.
MARINE...Gale Warning for PKZ012-013-041>043-051-052.
     Small Craft Advisory for PKZ021-022-031>036.




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