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FXAK67 PAJK 180108

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
408 PM AKST Sat Feb 17 2018

.SYNOPSIS...High pressure will build over NW Canada through
tonight, then gradually weaken Sunday. Outflow winds will
gradually decrease through Sunday.


.SHORT TERM...Not many adjustments needed for this forecast
period. Remaining mainly clear through tonight and Sunday with
dominating high pressure. Currently have some gusty north winds
due to high pressure in the Yukon and an inverted trough extending
north over the southern inner channels. Decreased lows for
tonight since it should be able to get colder than this morning
based on clear skies and gradually decreasing winds. Decreased
wind speeds across the board late Sunday into Monday as the
inverted trough weakens off to the SE and the pressure gradient
becomes nearly non-existent over the inner channels. Expect many
places to have light winds with variable directions dominated by
diurnal thermal changes.

On Monday a weak system will be passing across the central gulf
NW to SE. Latest model trends have pulled it further offshore, but
opted to leave in a mention of some flurries for the outer coast
for now. Preferred the NAM for changes that were made.


.AVIATION...Northerly outflow is causing LLWS and/or gusty winds
late this afternoon at many locations across SE. Expect to see a
decrease in the gusts after sunset. Otherwise remaining VFR through
the next 24 to 48hrs.


.PREV DISCUSSION......ISSUED AT 554 AM Saturday...

SHORT TERM.../Through Sunday night/ Upper ridge will build over
the gulf into interior AK through tonight...then remain there into
Sun night. A couple of shortwaves will dive SE through the area
today on front side of building ridge. Upper trof to the W of the
ridge will lift out and split into several pieces as it tries to
move through the ridge. Models are struggling on how the
individual pieces of the trof evolve and where they move. Looks
like strongest leftover of the trof will move into Bristol Bay
then into interior AK. Models hint at a vort maxima splitting and
moving into the N gulf, with another vort breaking off several
hundred miles to the S or SW over the far S part of the gulf. Run
to run consistency has been poor with the more southern pieces of
the upper trof, and confidence in any model solution on them is
low. After coord with WFO PAFC, ended up using the 00z GFS to
handle this through Sun night, but did incorporate the 06z
NAMnest to handle outflow winds through daytime Sun. This also
made for a smoother transition into the longer range period.

Main forecast concern will be winds. Fairly strong deep layer
N-NW flow will be over the area through tonight, with some
weakening of this flow for Sun and Sun night. Strong high pressure
will build over NW Canada and E interior AK through tonight, then
weaken Sun. This strong flow will support SCA to gale force N
winds over much of the inner channels through tonight. There is a
small burst of wind accompanying a shortwave moving S through the
area this morning, with the stronger winds with it lasting for an
hour or less. Overall, did drop winds off a little bit based on
the pressure gradient being weaker than forecasted especially over
the northern area. Still may get strong winds in downtown Juneau
and Douglas today, especially once some daytime heating weakens
the low level inversion. The wind should gradually weaken over the
area later tonight through Sun as stronger flow aloft shifts
further E into Canada.

Otherwise, watching a small band of clouds that formed ahead of a
shortwave that is entering the N-central gulf. Some of these have
moved into the Yakutat area early this morning. Think these clouds
will slowly push S of that area later this morning as shortwave
moves by them to the S and E. Otherwise looking mainly sunny
today and clear tonight. Next significant band of clouds will be
associated with the splitting upper trof moving through the ridge,
and those will be mainly high clouds moving in Sun afternoon and
night, with mid-level clouds reaching the E gulf and possibly
outer coast Sun night. Keeping any precip chances with this
system well offshore through Sun night. Temps will be near to
slightly below normal through the weekend, except some wind
sheltered areas will get fairly chilly during tonight, with some
single digit lows possible in these areas over the N.

LONG TERM... /Monday through Friday as of 9 PM Friday/ An area of
high pressure will hover over the Gulf at the beginning of the
long term period. This ridge seems strong to block or weaken most
features. But, it does look like a few shortwaves will be able to
squeeze past the ridge, and thus, weakening these potential
shortwaves. One wave of particular note is that heading into
Tuesday night/ early Wednesday. This low will slide north to south
along the panhandle and increase wind speeds after its passage.
The ridge seems to be weakening by this time. Thursday looks like
another good day for a potential shortwave.

At this point, several models are picking up that there is a low
tracking through the Gulf on Monday, but are having difficulty
with pinpointing an exact position. Previous runs of the NAM were
bringing in precipitation closer to the coast, but now most models
keep the low well out over the central gulf and keep the panhandle
dry. For the Tue/Wed low, the latest ECMWF sped things up while
the previous run looked more in line with the more consistent GFS,
so the 12/18z runs of the GFS were used for updates. Based on
this pressure gradient, this increased wind speeds behind the low
(especially over the Gulf). We went from having 25 kt previously
to having 30-35 kt near the center of the low, and even this may
need to be increased further if this solution pans out. We`ll
begin to see wind directions shift from N to S, and back again
over the inner channels as the low passes across the region.
Local edits were also made.

A NAM/SREF blend early and a WPC blend for POP was used to
decrease POP`s over the panhandle and reduce "likely"
precipitation to chances, seeing as it is still 5 or 6 days out.
Mainly the GFS was used for QPF. Other models wanted to keep
higher amounts, but we opted to trend on the lighter side
throughout the week. Temperatures will remain "above average"
through at least mid-week, but will gradually decrease through
the extended as the ridge flattens and allows colder Arctic air to
pass. The NBM kept this trend for both max and min temps, as was
consistent with MOS Guidance and a good in-between model for the
WPC, NAM, and previous forecast grids.

Forecast confidence has improved slightly from yesterday, albeit
the location and timing of the various lows is still relatively
uncertain. After Tuesday, model agreement increases somewhat.




.AJK Watches/Warnings/Advisories...
MARINE...Gale Warning for PKZ012-013-031.
     Small Craft Advisory for PKZ022-032>036-041>043-051.




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