FXAK67 PAJK 171543

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
643 AM AKST Sun Dec 17 2017

.SHORT TERM.../Today through Monday night/As of 0600 Sunday
morning, a weakening upper level ridge currently positioned over
western Canada has begun to break down even further and
transition east, while a broad upper level trough extends over
the western gulf. A shortwave embedded in this broad circulation
can be seen on WV satellite imagery lifting northeast across the
northern gulf, east of the Aleutian Islands. As the shortwave
began to lift overnight, the associated unstable air mass
triggered a number of lightning strikes this morning, especially
along the coastline near Mt Fairweather and along the western
coastline of Baranof Island. Forecast soundings for Sitka and
Klawock continue to show decent instability and sfc to 3km lapse
rates in excess of 7 degrees C/km. In addition, this system has
the support of the left exit region of an upper level jet streak
of 100 kt, which is providing divergence aloft to assist with lift
and with triggering the instability. In addition to the Sitka
area, we've added an area of possible isolated thunderstorm
development to the Klawock and Hydaburg areas for this afternoon,
as the the area of instability will shift southeast as the feature
moves east.

A weak surface low associated with this shortwave will slowly move
east across the northern gulf and weaken today, continuing into
Monday. Due to the relaxing pressure gradient as this feature
weakens, we'll see winds continue to diminish today and see this
trend continue into Monday. Potential temperature analysis shows
much colder air aloft with this system, so we'll expect
precipitation to be more shower like in nature, which is difficult
to pinpoint where we'll see more precip accumulation today and
tomorrow. As this surface low moves southeast along the eastern
gulf coastline Monday night, we'll begin to see a surface ridge of
high pressure build in behind, and we'll see a shift to offshore
flow. With this change, we'll see precipitation chances diminish,
more breaks in cloud cover, and potential fog development. The
only locations where wind may be a concern during this timeframe
will be the Lynn Canal and Skagway and White Pass areas. As the
low kicks south, the pressure gradient will tighten between it and
a strong surface high over the Yukon Territories, bringing
increased northerly winds, possibly gales to Lynn Canal marine

Generally, we did not make many changes to the current forecast.
We mainly used the NAM and NAMnest for any needed changes. We also
added in some additional areas for thunderstorm development,
which was mentioned previously.

.LONG TERM...Tuesday through Saturday/ as of 10 PM Saturday...An
upper level high pressure ridge will build across the panhandle
this next week and slowly propagate from west to east. The upper
level ridge will result in drier conditions occurring across the
panhandle with fairly dry conditions occurring on Tuesday across
most areas of the panhandle. Wednesday will be fairly dry however
moisture will begin to increase primarily across northern areas
of the panhandle. Thursday and Friday a series of weak fronts
will move near the panhandle gradually increasing the moisture
across the region. The chances for precipitation will begin
increasing on Thursday and increase further on Friday. Cloudiness
will diminish on Tuesday with the ridge building across the
region. However cloudiness will begin increasing by Wednesday
afternoon and then increase further on Thursday with the
increasing moisture advected across the area by approaching weak

Temperatures will remain cool across the panhandle but above
seasonal normal values for this time of the year.
Primarily used the GFS model for the long term forecast with only
minor changes if any.


.AJK Watches/Warnings/Advisories...
MARINE...Small Craft Advisory for PKZ012-013-021-022-041>043.
Small Craft Advisory due to Seas for PKZ051>053.




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