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FXAK67 PAJK 211424

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
624 AM AKDT Thu Sep 21 2017

.SYNOPSIS...A front will track eastward across the gulf Thursday,
and it will pass over the panhandle Thursday night through
Friday. A weak low pressure system will pass over the southern
Panhandle Friday night. Another front will pass over the
panhandle later this weekend into early Monday.


.SHORT TERM.../Thursday through Friday as of 530 am/ A dry morning
will change to a wet evening Thursday. A low lying stratus deck is
currently passing over the panhandle ahead of a weather front
located over the central gulf. Some foggy conditions are expected
this morning with dense fog present near Petersburg and Wrangell.
Fog is not expected to be as widespread or as long lasting as
yesterday due to the passage of an upper level shortwave trough
which acted to provide some vertical mixing of the air inhibiting
fog formation/persistence.

A ridge of high pressure over the panhandle will weaken today as a
front approaches from the west but will sharpen up tonight as it
gets pushed up against the coast mountains. Cloud cover will
increase through the day as the front tracks east. Yakutat will
see the earliest onset of precipitation associated with the front
with rain expected to begin during the early afternoon hours. The
outer coast stays dry a bit longer where rain will begin sometime
during the late afternoon and early evening hours. This rain
quickly spreads over the extent of the panhandle with rain in the
forecast everywhere by late Thursday night.

Tomorrow will begin with widespread showers across the panhandle.
Shower activity decreases through the day and becomes isolated to
scattered over the northern panhandle by tomorrow night. The
southern panhandle follows the same trend of decreasing showers
through the day, but a weak low pressure system will bring a
second wave of rain into the southern panhandle Friday night.

Winds will increase as the front progresses east with strongest
winds expected over the northeastern gulf where gales are
expected between Icy Cape to Cape Suckling. Winds will increase
in the inner channels as well; however, winds in excess of 15 kts
are not expected outside ocean entrances and Cross Sound. Some
vertical shear is also possible along the outer coast this evening
as the front passes.

Winds were increased slightly over the gulf with gales now in the
forecast for zone 52. In addition, land winds were reduced
slightly and PoP values were increased along the front. A blend of
the NAM and GFS were used as primary guidance, but some hi-res
model input was used to help with winds over the inner channels.

.LONG TERM...Saturday through Thursday...The upper level ridge of
high pressure, currently positioned over the gulf, will continue
to weaken and move east into Canada and the western CONUS, while
an upper level trough continues to dig into the the Alaska
mainland on Saturday. An upper level shortwave will develop out
of this parent trough and lift northeast across the northern gulf,
impacting portions of SE Alaska. This system and its associated
weather front will bring increased precipitation across the
panhandle, with increasing intensity beginning Sunday and
continuing into early Monday. A subtropical jet feature coupled
with this shortwave should advect large amounts of moisture,
mainly aimed for Haida Gwaii and western portions of British
Columbia, but we expect southern portions of the panhandle to see
some heavier precipitation with this setup. Models continue to
indicate anomalously high precipitable water amounts with this

For adjustments to the forecast, we used a combination of the
ECMWF, Canadian, and the NAM during the weekend and into the early
part of next week, leaning more towards the EC. We trended
more towards WPC guidance after the Tuesday period. After the Day
6 timeframe, models have had a more difficult time agreeing on
the synoptic pattern, so a lot of uncertainty remains in our
forecast past the Tuesday/Wednesday period.


.AJK Watches/Warnings/Advisories...
PUBLIC...Dense Fog Advisory until 10 AM AKDT this morning for AKZ026.
MARINE...Gale Warning for PKZ052.
     Small Craft Advisory for PKZ041>043-051.




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