Small Craft Advisory 405, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
218 PM AKDT Wed Aug 23 2017

.SHORT TERM.../through Friday night/ A low over the far SE gulf
will drift ESE across Haida Gwaii this evening before dissipating
late tonight. A weak upper trof will move SE across the far N area
this evening. An occluded front will move E into the eastern gulf
by Thu evening, then push across SE AK Thu night and Fri. A
series of low pressure trofs will move across the area Fri
afternoon and Fri night. Blended in 12z GFS with previous forecast
for 12z Thu to 12z Fri period, mainly to increase winds a bit
ahead of the front. Used 15z SREF to adjust POPs some, with manual
edits to help fix any issues.

Showers with the low over the SE gulf will move slowly out of the
area tonight. Some showers with weak upper trof will brush by the
N area mainly this evening. Then, well ahead of next front,
showers will increase in coverage Thu as low level convergence
increases within a shallow convective airmass over SE AK. Rain
with the front will move into the outer coast and W parts of the
inner channels late in the afternoon, then spread further inland
Thu evening. After the front moves by late Thu night and Fri,
precip will become showery again, but with several trofs moving
across the area Fri into Fri night, expect shower coverage to
remain on the high side with a moist low level airmass moving
across the area.

Band of 20-30 kt winds on E side of low over the SE gulf should
diminish late this afternoon, although it may take a bit longer
for diminishing trend to occur over the far S inner channels.
Winds will ramp up over the gulf again Thu as next front moves in.
Winds ahead of the front could reach min gales late in the
afternoon or early Thu evening over the far N-central gulf, but
for now going with 30 kt. SCA level winds should affect most gulf
zones Thu afternoon and evening. After front moves by, winds
should drop below SCA levels over the gulf. On the inner channels,
expect most places to peak in the 15-20 KT range ahead of the
front from Thu afternoon into early Fri, but would not be
surprised if 20-25 kt winds show up in a few spots. After front
moves through inner channels, winds should still remain mostly in
the 15-20 kt range but locally stronger winds could occur as any
low pressure trof moves through.

.LONG TERM.../Saturday through Wednesday/ Upper level low will
drop over the Aleutian chain into the western gulf then broaden
over the central and eastern gulf through Saturday. By Sunday this
low will track north while weakening as high pressure builds over
the panhandle and eastern gulf through late Monday. Another upper
low forms over the western gulf by mid week, however its
interaction with the high pressure ridge and eventual track is
uncertain. At the surface frontal band will have moved into the
panhandle with rain across the entire panhandle. Series of surface
waves rotating under the upper low will move over the panhandle
into Saturday before starting to diminish on Sunday. Heaviest
rain fall likely to occur into Saturday morning. More model
spread starting Sunday with the position of the surface features.
ECMWF a more northerly low track and faster drying trend for the
south while GFS moves low center much further south with higher
rainfall amounts there. This variation lasts into Monday, but by
Tuesday operational models fall more in line with high pressure
over the panhandle keeping most fronts/precip off to the west.
Due to uncertainty into Sunday kept with a broad brush approach
to wind fields.

Inherited forecast and new model runs for early Saturday were
still in line so little change was made. Due to large operational
spread starting Sunday kept with WPC ensemble approach, which also
showed little variation from previous runs. Initially higher
confidence but this quickly drops through the weekend.

&&

.HYDROLOGY...Most area streams and rivers have fallen somewhat,
but some localized rises are occurring over the southern panhandle
with band of heavier showers moving through. Do not expect any
flooding issues from this however, and streams should drop quickly
tonight across the south. Not seeing any issues for the area for
Thu night into Fri with next front, as streams should have plenty
of time to fall quite a bit before rain moves in with next front.
Also, rainfall with next front will be much less than the system
from early in the week.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ035-036-043-051-052.
     Small Craft Advisory due to Seas for PKZ041-042.

&&

RWT/PRB

Visit us at http://www.weather.gov/Juneau