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FXAK67 PAJK 171416

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
616 AM AKDT Sat Mar 17 2018

.SHORT TERM.../Saturday through Sunday night/...An inverted trough
extend northwest across the panhandle from a low situated over
southwestern British Columbia. To the west, a weak north-south
ridge lies over the eastern gulf. Farther west, a low near
50N/150W will track southeast and undercut the ridge Saturday
night. As this low moves off to the southeast, a stronger and
larger ridge will build northeast across the gulf, though the
parent surface high is expected to remain south and west of the
gulf. The new ridge axis will become very nearly east-west late
Sunday night as a low and associated front tracks east across the
northern periphery of the ridge and along the peninsula and north
gulf coast.

Weather for Saturday and Saturday night continues to be fairly
mundane and will be very nearly the same as it has for the past
couple days. Specifically, very little wind with low clouds and
fog developing at night and then clearing by the afternoon.
Probably the biggest difference in the latest short range forecast
is the inclusion of small craft seas along the outer coast
resulting from a SW swell for Saturday and Saturday night.

By Sunday, some changes begin to occur as the front discussed
above transit across the northern periphery of the surface ridge.
The chance of precipitation will increase from west to east
slowly but steadily through Sunday night. Likely Pops now forecast
from Cape Fairweather west with chance PoPs down to Wrangell and
Cape Ommaney by the end of Sunday night. Temperatures were
adjusted down by a few degrees based on model spectrum spreads,
but still, overnight lows in the current forecast are at or
slightly above freezing, so precip type being snow is not a slam-
dunk. Worst case at this time looks like up to an inch through
Sunday night for Yakutat if temperatures get low enough.

Apart from aforementioned changes to temperatures, little to no
changes made to the previous forecast. Overall forecast confidence
is average to above average.

.LONG TERM /Monday through Saturday/...As of 10pm Friday...On
Monday a WSW jet will move into the northern gulf and jet dynamics
will help to spawn a surface low near PWS. The jet turns out of
the north and the whole system shifts to the south through Monday
night as a ridge builds to the west with a trough digging into the
Gulf of Alaska. This will cause the surface low to shift south as
well through Tuesday. However, a closed upper level low will form
a the base of the trough and linger over the far SE Gulf or North
Pacific through the end of the week.

Confidence has increased in the low for the start of the week, so
winds over the outside waters have been increased to gale force.
These will be out of the W-SW on the south side of the low, then
shift to the NW and eventually NE after the front and low passes.
The onshore flow ahead of the front will be warm enough for rain
to occur on Monday, starting in Yakutat in the morning then
spreading SE across the panhandle through the night. Behind the
front/low there will be fairly strong CAA, so expect precip to
transition to snow showers briefly before ending over the northern
panhandle on Tuesday. Some light accumulations are possible. The
southern panhandle will transition to showers as well, but will
likely be more of a mix or all rain.

With the low shifted to the south, expect winds to increase out
of the north through the inner channels for Wed/Thurs. Have
continued to limit POP Wed-Fri across the northern panhandle due
to flow becoming more offshore as the low shifts to the south and
due to dry air advecting in from the north at the surface.
However, over the southern panhandle, the surface and upper level
low will be close enough to draw up moisture from the south. It
also looks to be fairly convective, so will need to watch for
afternoon thunderstorms.

Since models now have the upper level trough developing into a
closed low and has shifted it ever so to the west, this places SE
Alaska under S-SELY flow aloft, this should help to mitigate the
CAA from the northerly surface winds and overall colder
temperatures at 850mb. The biggest model differences continue to
be how the upper level trough moves out on Saturday and whether
there will be precip assoc with the upper level low. Have left
some in across the south with at least mostly cloudy skies since
we`ve been burned before by "east movers". Overall forecast
confidence is above average and models remain in good agreement.


.AJK Watches/Warnings/Advisories...
MARINE...Small Craft Advisory due to Seas for PKZ041>043-051.




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