Small Craft Advisory 580, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
344 PM AKDT MON SEP 19 2016

.SHORT TERM...A very persistent weak low still continues to
circulate off the central outer coast but is steadily tracking to
the south and looks like it will finally either dissipate or
depart the forecast area. The low is taking with it low to mid
level cloud cover and visible satellite imagery is showing
clearing condition over the central inner channels and spreading
south. Areas of low marine stratus remain, however, over some
areas of the central and northern inner channels and this is
visible in both web cams as well as visible imagery. In place of
the low, high pressure will shift east over southeast Alaska
beginning tonight. High dewpoints combined with mostly clear skies
and light winds tonight suggests that and extensive marine layer
or areas of radiation fog will develop again tonight. Assuming the
marine layer and fog dissipates, tomorrow should be another mostly
sunny day. Expected low clouds tonight should keep overnight lows
in the mid 40s or higher. Depending on how long it takes the
marine stratus to dissipate, tomorrow's highs should get into the
upper 50s or low 60s.

Virtually no change to the previous forecast apart from some
tweaks to overnight lows and re-characterization of some of the
areas of fog. Overall forecast confidence is average.

.LONG TERM...Main focus for the long range is the strong front
that is approaching for mid to late week. Confidence continues to
build for storm force winds to occur Wed in the northern gulf
with mid to high gales for the rest of the gulf through at least
Thu. Seas will also be as high as 23 feet during this period.

Now, there is also increasing model support for a possible second
period of storm force winds encompassing the ne and northern gulf
from late Wed night into Thu. This is the result of a triple point
low developing on the front well South of the gulf wed afternoon
from a combo of the left exit region of a jet streak and an area
of upper level vorticity. This low will rapidly move north along
the front through Thu. Some model disagreement on track (NAM much
slower then ECMWF and GFS, and the ECMWF is a little farther east
then the GFS), and strength (Morning model runs ranged from 976 to
992 mb), but they are showing signs of coming into better
agreement (afternoon model run strengths were within 5 mb of each
other as far as strength). These higher winds will crash into the
northern gulf coast Wed night into Thu with strong gusty winds
possibly visiting Yakutat, Yakutat Bay, and the northeast gulf
coast.

The appearance of this low also brings a few changes to other
parts of the forecast. Namely how fast the front moves through the
rest of the panhandle. the stronger westerly flow on the south
side of the low will push the front to the east faster then what
was shown in the previous forecast. As a result, gusty winds and
rain will likely arrive a few hours earlier then the previous
forecast due to the faster eastward progress of the front. This
faster speed has the front clear the panhandle by late Thu evening
leaving showers in SW flow behind. Exact timing will depend on how
fast the low moves north along the front in the gulf.

The rest of the extended period is rather quiet with generally SW
flow and showers being the rule. There are some shortwaves that
will be moving through the area during this time but when and
where are not well indicated with position ranging from the
northern gulf to near Haida Gwaii depending on the model you look
at.

&&

.AVIATION...VMC tonight...becoming IMC to very low IMC due to CIGs
and VSBY across all of southeast Alaska by tomorrow morning.
Conditions expected to improve by mid day with some lingering
pockets of low clouds into the afternoon. Light winds and no turb
for one more day.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...None.
&&

Fritsch/EAL

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