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FXAK67 PAJK 171353

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
553 AM AKDT Thu Aug 17 2017

.SHORT TERM.../Through Fri night/ A surface low pressure wave is
located 100 mi west of Sitka this morning moving NE around 30 to
40 kt. This feature will fill and dissipate over the coastal
mountains by this evening. Satellite imagery depicts a long
moisture plume of highly saturated air extending from SEAK to the
SW almost to the dateline. The center of counter-clockwise
circulation for the broader upper air low pressure center is over
Prince William Sound this morning. Lightning stikes were detected
overnight in the vicinity of Cape Suckling west of Yakutat. The
area most favorable for thunderstorms will gradually expand SE to
aound Sitka so isolated t-storms remain in north coastal
forecasts today.

Forecast models consistently advertise wavy frontal systems will
continue to march along the moisture plume over the Pacific and
into SEAK through Fri night. I perferred the GFS model for
adjusting previous forecasts to current conditions. These
adjustments did not change the screaming message of continued
rain. I did not change rainfall amounts but the rain will come in
surges and pin pointing amounts will be very difficult.

The combination of increasing wind speeds aloft and fluctuating
surface pressure gradients is causing periods of higher winds and
resulting seas. For today...Lynn Canal and Stephens Passage winds
were in the 20-30 range last evening but will subside through the
day. However expect SW winds to increase some this evening over
the southern inner channels and southern outer coast when high
pressure builds in after todays weather front. Seas along the
southern outer coast will remain at or above 8 ft through Thurs

Temperatures will be rather stationary the next 24 to 36 hours.
Clouds, rain, and overall lack of change in air mass
characteristics will keep daytime and nighttime temps generally
within 5-7 degrees.

.LONG TERM.../Saturday through Thursday/ Extended forecast
continues the trend of wet and cool through next week. A broad
upper level trough remains planted over Alaska through much of the
period supporting SW flow at most levels of the atmosphere. This
is briefly broken up mid week as an upper low breaks off from the
main trough and dives SE.

Main concern is for rainfall Sun into Mon as deep tropical
moisture from former tropical storm Banyan gets advected into the
area. Current model performance on this system is not that great
right now. First off, the low that is drawing this moisture
northeastward is not being handled very well by the various
forecast models. ECMWF and Canadian are trended slower then what
the GFS and some of the ensemble means are showing for low track.
Further more the track of the moisture plume itself is still up in
the air. Currently it appears that the moisture will first arrive
with a front on Sun followed by a second surge of moisture on Mon
morning associated with a following shortwave. Recent model runs
are starting to trend that second surge farther south with most of
the moisture impacting British Columbia rather then SE Alaska.
Elected to not do much with the current forecast to await to see
if the southward trend holds out.

Into mid next week, the upper low dropping southeastward toward
Haida Gwaii will cut off the SW flow in favor of a more
southeasterly flow over the area. Showers expected to remain in
the area but will not be as heavy as recently. Also the farther
north you get the drier it will be. Mainly used WPC guidance for
updates where needed.


.HYDROLOGY...Gauged rivers are rising as you would expect given
the persistent and widespread precipitation...some on the order
of a foot or two since Wed morning. Rivers in the southern
Panhandle crested Wed evening after a period of very heavy rain
there. Northern Panahandle streams are rising this morning due to
heavy showers overnight. Montana Creek in Juneau and Taiya River
near Skagway are among those that bear watching today as they may
approach Action Stage. we are also keeping an eye on a potential
heavy rain event in the Sun to Mon time frame. This system is
coming from the sub-tropical western Pacific so it is associated
with a warm air mass. Freezing levels could rise to 10K feet
meaning plenty of rain runoff rather than high elevation snow.

.AVIATION...Showers are once again causing highly variable
ceilings and visibility values between VFR and IFR at the
terminals. With the shower bands in this system, it was difficult
to time the heavier precip for the the lower conditions, but one
look at satellite and anyone would say wet with impacts to flight
operations. The next TAF issuance will have to consider pockets of
LLWS as some moderate winds at 2000 FT develop with timing in the
evening or even later.


.AJK Watches/Warnings/Advisories...
MARINE...Small Craft Advisory for PKZ012-013-042.
     Small Craft Advisory due to Seas for PKZ022-041-043-051.




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