Small Craft Advisory 509, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
304 PM AKDT FRI AUG 12 2016

.SHORT TERM...Wet and breezy weekend conditions begin late Friday
night after a brief break Friday afternoon and evening.

Upper level ridge over the panhandle moves east into British
Columbia as long wave trough extending from a closed upper low in
the Bering Sea moves across the AK Gulf and then over the
panhandle. The closed upper low in the Bering Sea will drop south
over the AK peninsula and into the western gulf through Saturday.
At the surface first of the weekend fronts is moving up along the
windward side of a surface ridge currently over the panhandle. As
the surface ridge follows the track of the upper ridge eastward
the front will move the eastern gulf and into the panhandle. Brief
break in rain fall this afternoon as ridge axis moved over and
flow shifted to the southeast. Breaks in cloud cover will be short
lived however with rain covering the entire area by tomorrow
afternoon. Long fetch along the front bringing in ample moisture
with precipitable water values of 150 to 200% above normal.
Heaviest rain will be just west of Yakutat this evening spreading
east late tonight and into tomorrow.

Low level jet along the front with 925 mb winds upwards of 50 kt
in the northern gulf coast resulting in narrow band of gale force
winds at the surface. N Lynn Canal small craft winds will drop
this evening as pressure gradient eases and mid level westerly
flow shifts to the south. For the rest of the inner channels winds
will pick up as the front moves inland. As of now have inland winds
below small craft with the exception of Clarence Strait which will
get more channeled southerly flow tomorrow afternoon.

Models not in exact alignment with position of the front this
evening. There were enough minor differences in position of heavy
rain fall and winds that keeping with an ensemble forecast for
press and precip seemed best option. Did use more high-res nmm which
brought up winds over the gulf just enough to reach gale force
and bumped up inner channel winds a bit. Forecast confidence is
average.

.LONG TERM...Made mostly minor forecast adjustments for Sat night
through Mon. Low (remnant of Omais) will push into the northwest
gulf Sun. There will be one or more fronts/trofs that move across
the area during this time. Looking like highest rainfall amounts
will be along the northeast gulf coastal area although northern
panhandle will have bursts of locally heavy rain as well. See
hydrology section for effects on local streams and rivers.

Winds were nudged upward about 5 KT over the inner channels for
Sat night into Sun night based on strength of 925 mb flow and
pressure gradient. Much of the inner channels will see 20-25 KT
winds especially Sun. The eastern gulf will have 20-25 KT winds
for much of the Sat night-Mon time frame due to influence of low
over the northwest gulf.

Still a hint of a drier spell toward midweek with ridge building
in but models differ on how long this will last. WPC used an
ensemble approach, which looked reasonable, so basically used
them for Mon night onward.

&&

.AVIATION...IFR conditions due to low cloud cover and fog from stable
airmass under the panhandle ridge finally beginning to lift.
Expecting another reduction in visibility tonight for the southern
panhandle before the front advances there. Do expect ceilings and
visibility to drop for the rest of the region as the front moves
in but with visibility above 2 miles for most of the time. For the
coastal areas forecast low level wind shear starting this evening
into tonight. Buff Kit soundings kept any shear just along the
coast at PAYA and PASI.

&&

.HYDROLOGY...Local rivers and streams showed rising levels, some to
bankfull, from Thursday rain and were starting to drop with the
break in precip. Heavy rain fall expected for the weekend will
again cause river to rise and due to already saturated ground and
some waterways at higher starting points there is potential for
some to again reach bankfull. The Taiya river near Skagway is the
main issue with enough rainfall in the basin that river levels
could reach moderate flood stage by Sunday. A flood watch was
issued after consulting with the AK River Forecast Center. Jordan
Creek is forecast to rise to bankfull level by Mon morning with
potential to go higher, but will not issue any hydrologic products
for that stream at this time.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Flood Watch from Saturday afternoon through late Sunday night
     for AKZ018.
MARINE...Gale Warning for PKZ043-051-052.
     Small Craft Advisory for PKZ012-036-041-042-053.
     Small Craft Advisory due to Seas for PKZ022.
&&

PRB/RWT

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