Area Forecast Discussion
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FXAK67 PAJK 191404

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
604 AM AKDT Thu Oct 19 2017

.SYNOPSIS...A low offshore of Prince of Wales Island will move
through Dixon Entrance today on its way to southern Clarence
Strait late this afternoon. Meanwhile an elongated area of low
pressure over the northeast gulf will move ashore Thu night. A
ridge of high pressure will build over the Panhandle on Friday.


.SHORT TERM...As the southeast gulf low slowly churns through
Dixon Entrance and into the southern portions of Clarence Strait
today, offshore flow will increase in the lower levels across
the northern Panhandle. We have all but erased pops in much of
this region through this afternoon. But the duration and strength
of this event will do little to clear skies, even as many areas
will experience sun breaks. Weak cool air advection will be
countered easily by a little more insolation, therefore, we expect
temperatures to vary little from yesterday.

Meanwhile high lapse rates in the vicinity of the low sparked
lightning Wednesday evening along the eastern shores of southern
Prince of Wales Island. It also has resulted in moderate rain
showers across the far south. As the low draws nearer to
Ketchikan, we expect a wind shift through Clarence Strait and
likelier through Ketchikan as well. After gustiness early this
morning, winds will calm down across the south.

To the north, northern Lynn Canal has reacted to a tightening
pressure gradient with a strong 25 kt northerly. In addition, high
seas with a southwest swell will keep small craft advisories
posted for the outside waters through tonight at least.

After the low moves ashore, we expect strengthening onshore flow
as the longwave trough itself prepares to move eastward. This in
turn will expand the area of showers Panhandle wide yet again.
However, due to a deep system approaching Southeast from the
western gulf, the ridge begins to negatively tilt Friday night as
first wind and then rain north and eastward from the coast.

The 06Z NAM provided a basis for any changes. Forecast confidence
is generally good.

.LONG TERM.../Saturday through Thursday/Model agreement is fair
for the weekend, then they differ quite a bit next week. Main
issue is timing of systems moving across the NPAC, potential for
at least 1 tropical system getting into the westerlies, and
interaction between these systems and an upper trof over W AK.
Decided to use the 12z/18 ECMWF to handle Sat, then went with
mainly WPC for Sun onward.

First main system will be a complex low moving into the gulf by
Sat. Looking like an occluded front will move N across the area
Sat but will weaken as it moves across the N half of the area.
This front will most likely bring at least gale force winds with
it over the gulf, with at least SCA level and possibly gale force
winds to mainly the more E-W inner channels. Should see some rain
with the front Sat, although as front weakens over the N, precip
may become more spotty as it encounters offshore low level flow.
The main low appears like it will move to the far N-central gulf
by late Sat night, then weaken there Sun. As this happens, looking
more showery for the area especially later Sun.

While model agreement is not good next week, they suggest at least
1 strong system will affect the area. This is when some effect
from what will be remnant of Typhoon Lan will come into play. The
model timing on how fast the typhoon moves into the westerlies
varies somewhat, with GFS/GEM faster than the EC by about 1 day.
(Of interest...the Joint Typhoon Warning Center timing is right
in the middle of the 2 extremes) It is this feature that the
GFS/GEM spins up into a powerful storm S of the Aleutians early
next week, then moves it into the gulf by late week. EC also shows
a fairly strong system but it does not have remnants of Lan in
it, and is somewhat weaker than the GFS/GEM. Still plenty of time
to monitor this situation. In between this larger system and the
one over the weekend, models suggest another low may try to move
NE into the gulf early next week but vast model differences exist
on this, so later forecasts may need to be updated to reflect this
system more than current forecast has it.


.AJK Watches/Warnings/Advisories...
MARINE...Small Craft Advisory for PKZ012-041-042.
     Small Craft Advisory due to Seas for PKZ022-043-051>053.




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