High Wind in Alaska, April 5 to 6, 2018

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
239 PM AKDT Wed Apr 4 2018

.SYNOPSIS...As a low moves into the outer Gulf of Alaska a gale
force front will approach the southeast gulf and southern
Panhandle Thursday. High pressure will remain across NW Canada.

&&

.SHORT TERM...A front associated with a cold core low in south
central Alaska brought more organized snow showers to the
northeast gulf coast region than previously thought. Around 1 to 2
inches fell. Snow has now ended, but the rest of the Panhandle
has had a stellar, albeit slightly cooler day than normal where
temperatures have remained in the mid to high 30s to the lower 40s
through the lunchtime hour. Meanwhile, outflow winds have
slackened in comparison to yesterday, largely due to the
weakening of the low pressure from yesterday and the building
ridge aloft.

This all changes tonight as an organized front pushes north into
the southeast gulf through Thursday. This front will mainly incite
stronger winds throughout much of the Panhandle. Perhaps most
significantly, our confidence has increased for a genuine Taku
set-up for Downtown Juneau and Douglas. A period of very parallel
cross- barrier flow, an inversion setting up nicely at ridgetop
level due ridging aloft, and a tight gradient will all contribute
to moderate to high potential of high Taku winds of 60 mph or
higher. We will issue a high wind warning to cover the Taku winds
Thursday. Likewise, winds will increase through favored channels
to high- end small craft and gales through Thursday. This includes
Lynn Canal, Cross Sound, and Sumner Strait. We also suspect
Prince of Wales Island and Ketchikan may see stiff easterlies
Thursday. Winds will be tricky for the Ketchikan area as stiff
northwesterlies look to switch directions sometime from midday
Thursday into Thursday evening. MOS is indicating many locations
with higher winds than normal for such a frontal passage. While we
have general breezy conditions, some areas to watch on Thursday
are Gustavus, Tenakee Springs, Wrangell, Kake, and even
Petersburg beyond the normally active Downtown Juneau, Haines,
and Skagway.

Precipitation will also arrive Thursday. But a check of model
soundings indicate a very dry atmosphere. Wide dewpoint
depressions persist aloft over Ketchikan well through mid-
afternoon Thursday before the possibility of light precipitation.
Due to such a dry atmosphere, despite afternoon temperatures in
the low 40s, we feel strongly that precipitation will be mainly
light snow entering the southern Panhandle. But because of the
temperatures, we cannot rule out some rain drops mixed in. As a
result, we carry rain/snow chances through Friday morning for much
of the southern Panhandle. Increasing warming from southerly flow
and higher saturation will finally help the rain/snow line push
into the south-central Panhandle by Friday, just as most models
pull the precipitation back offshore as a new low center directs
the energy into Vancouver Island.

The few adjustments made relied on NAM/GFS for gulf winds with
more NAM high res for Inner Channels. We relied on
GFS/NAM/ECMWF/GEM for Pop and raised model fields to categorical
Thursday afternoon/evening across the southeast gulf and southern
Panhandle.

.LONG TERM.../Friday through Wednesday as of 10 pm Tuesday
evening/...Extended range forecast begins with a Rex block aloft
that will remain in place through Saturday afternoon: High
pressure ridge will extend from southern British Columbia
northwest into the Yukon. Associated upper low will be located
over the far southern Gulf of Alaska near 50N/140W. The ridge will
pinch off a closed high and retrograde west across the southern
interior of Alaska through saturday evening. The low will weaken
nearly in place before becoming absorbed by a digging upper
trough that spins up another upper low southwest of the gulf
Saturday afternoon/evening. This second upper low will remain
south of the Gulf until Monday, possibly Tuesday, but at least two
strong short waves will rotate around the base of the low and
track north along the west side of an induced upper ridge with
axis that will extend from central california all the way up to
the Yukon. The low will swing north over the central or eastern
gulf by Tuesday morning.

At the surface, a low associated with the Rex Block low will be
vertically stacked. Surface high pressure extending from the high
Arctic all the way down to the Great Plains will maintain an
outflow pattern over the panhandle early on. This northerly
gradient will begin to relax Saturday as both the low in the gulf
and high pressure over western Canada weaken and should break
down late Sunday night or early Monday. At that time, a strong
surface low will be tracking north into the central or eastern
gulf and then retrograde west toward Kodiak on Tuesday.

Precipitation rotating about the first low over the south central
gulf will be largely confined to the southern half of the
panhandle through Friday evening. Dry weather is expected Saturday
and Saturday night across all of Southeast Alaska. Precipitation
re-enters the forecast on Sunday with a weak frontal boundary
moving north over the southern panhandle, and then a stronger and
better organized front Sunday night into Monday.

Northerly gales over Lynn Canal will continue through at least
Friday morning and then diminish as the offshore pattern finally
weakens. Small craft winds will persist through Saturday night.
Not a lot of wind over the inner channels otherwise. Over the
gulf, Outflow wind plumes will persist vicinity Cross Sound,
Situk, and the Alsek through Friday afternoon. Elsewhere over the
gulf, easterly gales over the southern coastal zones and offshore
zone 310 will diminish to small craft intensity by late Friday
night and the general wind flow briefly shifts northerly on
Saturday, and then back to the south or southeasterly by Sunday.

&&

.AVIATION...

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind from Thursday morning through Thursday afternoon for
     AKZ027.
     High Wind Warning from 4 AM to 6 PM AKDT Thursday for AKZ025.
     Strong Wind from Thursday morning through Thursday afternoon for
     AKZ018.
     Strong Wind Thursday afternoon for AKZ028.
MARINE...Gale Warning for PKZ012-013-022-041>043.
     Small Craft Advisory for PKZ011-021-031-032-034>036-051-052.

&&

JWA/Fritsch

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