Weather Service Report


498 
FXAK67 PAJK 251348
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
548 AM AKDT THU AUG 25 2016

.SHORT TERM...Quiet morning for most of the panhandle as a ridge
over the eastern gulf keeps most of the wet weather confined to
the northern gulf coast. Yakutat has been raining all night with
rainfall amounts there over the last 24 hours reaching around 1
inch. Rest of SE alaska has been rather dry with low clouds, some
fog, and isolated light showers observed north of Frederick
Sound.

The next 24 hours will be somewhat of a transition period as the
ridge that has influenced the weather over the last several days
flattens and starts to drift to the SW. Over the top of it comes a
trough that will start to spread rain back to the E tonight.
Yakutat, which is already in the rain, will continue raining
through tonight with the heaviest rainfall this afternoon and
evening as the trough begins to move. Expected rainfall amounts
will be around 1 inch. Rain begins to invade the NE gulf coast and
northern inner channels this evening and will spread south and
eastward through the night. The extreme southern panhandle will
mostly be spared from getting wet through late tonight but the
rain will be on their doorstep. Rainfall amounts will range from
nothing to around a half an inch. Due to the flow coming from the
W and SW, the highest amounts will be along the NE outer coast
and the northern coast range.

The other aspect of this trough that we will need to watch is the
wind in a few areas. The trough is dragging a weak occluded front
through the gulf tonight as it moves east. short range models are
suggesting a band of 25 kt small craft winds will form along it
tonight in the central and eastern gulf. As such small craft
advisories have been hoisted tonight for those areas. Into the
inner channels, the trough will form a surface low in the southern
Yukon tonight. The interaction between that and the gulf ridge
will kick up some gusty winds in Lynn Canal and Skagway. S winds
of around 30 kt are expected in Lynn Canal Thu night with gusts
approaching 30 to 35 mph in Skagway. Decided to not go with a
strong wind headline for the Skagway area late tonight due to not
quite enough pressure gradient and a rather weak low level jet
feeding higher winds into the area. However, it will still be
close so some isolated gusts to 40 mph are not out of the question
late tonight. Otherwise winds will be less then 20 kt for most
places through tonight.

Some model differences noted on wind magnitudes in the gulf as
the trough comes through. Nam and ECMWF were suggesting 20 kt
while the GFS and Canadian voted for some slightly higher 25 kt
winds. Favored the GFS and some Canadian for guidance as the
higher wind speeds make a little more sense in this situation.

.LONG TERM...Upper trof will move east then southeast across the
area Thu night through Fri night. There are some timing
differences between the GFS/NAM and EC/GEM, with the GFS/NAM
slower moving the trof through. Was leaning toward slower idea
with support from vast majority of GFS ensemble members and UKMET. 
Some ridging will build across the area behind the trof Sat.
After Sat, models begin to diverge on evolution of larger scale
upper trof over the area. Models do agree for the most part on
upper ridge building over southwest AK which would favor shortwave
energy digging into the gulf, making for increased threat for
some unsettled weather early next week. Used 00z GFS/NAM blend for
Fri-Sat night, then mainly left Sun onward as is.

For Fri into Fri night, rain will diminish from the northwest as
upper and surface trof pass by the area. Shortwave ridging will
help induce low level ridge over the Yukon, which will increase
northerly flow significantly. Ended up going with 20-30 KT north
winds over much of the north Fri night. Clearing trend will
develop over much of the north late Fri night.

For Sat and Sat night, the clearing trend will move south and
affect all but the far south by afternoon. Looking like a fairly
warm day over the north half of the area with plenty of sunshine
and offshore flow. Ended up boosting max temps several degrees
over much of the north Sat. Did drop temps a couple degrees over
the south due to clouds being more pesky there. There may even be
some spotty light precip near Hyder through Sat night. The 20-30
Kt north winds will diminish slowly Sat and Sat night as the low
level ridging over the Yukon weakens some.

For Sun onward, effects of more shortwave energy diving into the
gulf will increase threat for precip. Should be cooler as well
with more clouds around on Sun, although the far north could be
warmer than current forecast has it.

&&

.HYDROLOGY...The increasing rainfall tonight will cause area
rivers and streams to rise once again. The Taiya river is again a
cause for concern as the rainfall may cause it to rise up to
bankfull but below minor flood stage on Fri. Some concerns on
whether there will be enough rainfall to cause that great of a
rise as overall flow is out of the west and SW rather then the
south that is ideal for higher rainfall amounts in the Taiya
basin. We will keep an eye on it as rain start up across the basin
tonight. Elsewhere, other rivers are expected to remain within
their banks.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-022-041-043-051. 
&&

$$

EAL/RWT

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