Small Craft Advisory 526, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
346 PM AKDT SUN AUG 21 2016

.SHORT TERM...High pressure centered along the SE AK coast Sunday
resulted in sunny dry weather for nearly the entire panhandle. Temps
reached into the 70s around Ketchikan and 60s elsewhere. This
thermal enhancement helped NW winds along the coast and into
coastal bays blow stronger today with Cape Spencer area gusting to
near 30 kts by 3 pm. Winds may continue to rise elsewhere such as
Lynn Canal until temps start to cool off later this evening.

The exception to the sunny weather today was the north coast
including Yakutat. This area had less influence from the pressure
ridge and more influence from the approaching moist SW flow
aloft. This approaching front is the main message of the forecast
for Monday. Rain reached as far east as Cordova Sunday and fell at
moderate rates at times. This precip will slowly edge eastward
the next 24 hrs as the front is somewhat parallel to the wind flow
aloft. Rain is expected to reach Yakutat Monday morning and then
to roughly Gustavus and Sitka by mid afternoon. The prevailing NW
surface winds Sat will reverse to pre-frontal southerlies late
tonight and during the day Sun.

Clouds are expected to slowly increase over the southern Panhandle
tonight in time to limit formation of radiation fog. Patchy fog is
mentioned for the Kake-Petersburg-Wrangell area Sun morning. The
effects of the front will be felt over the southeast half of the
panhandle with increasing clouds Sun and precip not arriving there
until the start of the extended forecast period.

.LONG TERM...Monday evening southerly flow across the Gulf of
Alaska will spray Yakutat, Pelican, and Sitka with light rain.
This moisture plume advects rain eastward overnight into the rest
of the Northern Panhandle including Juneau, Skagway, and Kake. As
a barrier jet associated with a front making landfall weakens
through Monday night across the northern gulf, models are
indicating a wave along a stalled front to re-strengthen winds
along the northeast gulf Tuesday. Its track would seem to take it
as far west as Yakutat Bay and as far east as Cross Sound. And as
such, it will also enhance rainfall amounts which is also why this
wave is important to track. The latest NAM guidance has veered it
toward the east. This still works with our high confidence for
rain across the northern Panhandle Monday night into Tuesday.
Right now, the best area for moderate to heavy rain lies from
Yakutat to Lituya Bay, but this is in flux. The best chance for
rain across the southern Panhandle will be Tuesday morning, but
areas south or Point Baker will merely see a few drops, if at all.

Predicting the movement of moisture plumes can somewhat be
analogous to forecasting the behavior of a wild hose, albeit, in
much slower motion. We are certain that models have a difficult
time anticipating all the variables that control its motions. Any
undetected kink can lead to a jog to the east or to the west which
can potentially have great impacts on the forecast. Confidence the
plume's behavior begins to melt by mid-week as the ECMWF appears
to break off a piece of energy from the longwave trough oriented
from South-Central Alaska southwest along the Alaska Peninsula
and along 170 W deep into the Pacific. This piece of energy in the
ECMWF causes the ridge to pump northeastward into Northwest
Canada. Meanwhile the GFS, Canadian, and GFS Ensemble Mean
all transfer this energy eastward across southern Alaska. This
keeps the ridge flattened, thus carrying precipitation faster
across the Panhandle Thursday into Thursday night. The ECMWF is
therefore slower to eject the stronger piece of energy from the
southwest. For this reason, we have expanded the chance of rain
from new WPC Guidance Thursday into Friday. The surface pattern
also makes clear that there will be a point where southerly winds
through the Inside Channels veer to northerly. And therefore,
Thursday and Friday's wind directions may be 180 degrees from
correct. The weekend looks dry and pleasant.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Gale Warning for PKZ052.
     Small Craft Advisory for PKZ022-033-035-041>043-051.
&&

TA/JWA

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