Small Craft Advisory 728, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
500 AM AKDT Thu Sep 28 2017

.ANALYSIS AND UPPER LEVELS...
An upper level low near Kodiak continues to dominate the pattern.
Its associated warm occlusion that was formerly over the Gulf of
AK is now aligned along the AK Range into the Aleutian Range. It
continues to push some showers across the Susitna Valley and parts
of the Cook Inlet, but the bulk of the steady precipitation has
been relegated to the North Gulf Coast. A modest push of pressure
rises behind the front has allowed for some gusty southeast winds
through the typical gaps (Portage/Whittier, Turnagain Arm, etc.)
overnight. Meanwhile, a colder trough is dropping in from the
Arctic over the northern half of the state. This is working in
tandem with a ridge over the western Bering to reinforce northerly
flow across much of the Bering and just about all of SW AK. This
is leading to some gusty winds through the Alaska Peninsula and
also dragging down some low stratus from the Bering Strait into
the YK-Delta. The jet activity is really only working to support
the Arctic trough to our north and the Bering ridge way out west.
Otherwise, the subtropical jet continues to bring steady rain to
Southeast AK.

&&

.MODEL DISCUSSION...
Models are in fair agreement this morning. The most notable
difference is how far south they dig the energy and associated
cold air from the Arctic Trough into the weekend. The EC (and to
some extent the GEM) were the most bullish with this feature on
Wed. However, the 00z run overnight trended more towards the
American models which advertise not much more than a glancing blow
along the AK Range and the Copper River Basin. The American models
were preferred with this package. The NAM Nest and abundant hand
edits were used in trying to depict the various local effect winds
across South Central for today. Forecast confidence is moderate to
high heading into the weekend.

&&

.AVIATION...
PANC...Winds are going to be very tricky today. The brief push of
southeasterly flow overnight should be over as the "down-Inlet"
gradient takes over at the surface. However, low-level wind shear
will remain probable as southeast flow continues just off the
surface. Another piece of energy should bring southeast winds back
into the airport off the Turnagain Arm for a period this afternoon
and evening. So whether it is gusty at the surface or just above,
expect some bumpy conditions at and around the terminal today.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The very slowly weakening Gulf low located just east of Kodiak
Island will be the primary driver of gusty gap winds throughout
the Gulf Coast this morning. Overnight this low produced a barrier
jet that brought gale-force gusts from the North Gulf into Prince
William Sound and Passage Canal, all of which will be diminishing
as the morning progresses. Strong gap winds will also continue to
blow through Portage Valley and the Turnagain for much of the
morning. Forecast confidence further west over the Anchorage Bowl
is however much less, especially this morning as a weak down-
inlet should cause the Turnagain wind the fade in and out into
south and west Anchorage. By this afternoon the models are in
reasonable agreement that the down-inlet flow will weaken enough
to bring these winds into the Anchorage Bowl, along with gusty
winds out of the Knik River and Copper River Valleys. By this
evening the Gulf low should weaken enough to allow most of the gap
winds diminish into Friday.

The second impact from this low will be with rain showers, which
should mainly occur along the North Gulf Coast and Prince William
Sound as strong cross barrier flow will limit any showers on the
lee side of the Chugach Mountains. A better chance of rain for
Southcentral will come Friday evening, after the cross barrier
flow has weakened and a trough begins to approach the area from
the Interior. The good news is that there is very good agreement
that this trough will quickly move though the area, and allow for
high pressure to move into most of Southcentral for the Saturday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Rain continues to move along and just west of the Alaska/Aleutian
Ranges this morning in association with an upper level deformation
zone connected to a low over the Gulf of Alaska. This feature will
remain in place through Thursday night before weakening and
falling apart. Otherwise, the remainder of the region will remain
under the influence of dominate and dry northerly flow. Some low
clouds off the Bering Strait will drift over the Kuskokwim Delta,
otherwise expect quiescent and fair conditions.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Ridging over the western Bering will slowly track east into the
central Bering on Friday, extending over southwest Alaska as
well. This will keep northerly flow for most locations in the
Bering and Aleutians before a stronger gale force front moves over
the Western Bering Sea and Aleutians Friday and Friday night.

&&

.LONG TERM FORECAST (Days 3 through 7)...

It looks like a sunny weekend is on tap for much of the Alaska
mainland this weekend as high pressure builds across southern
Alaska from the eastern Bering Sea. In the meantime, a pattern
change will begin to take hold across the western Bering Sea as
the arctic jet begins to strengthen as cold air builds over
eastern Russia. A the jet strengthens, the upper level pattern
will become more "zonal" and allow for multiple disturbances to
track off the coast of Russia and into the Bering Sea and
ultimately into Southwest Alaska. This pattern change should also
keep the subtropical jet well south of Alaska and Alaskan waters
which means that the warm and moist air that brought rainfall much
of the summer/fall will now be cut off from Alaska. Models are
struggling with timing, placement and intensity of features moving
off the coast of Russia but a very active pattern looks likely by
Monday of next week. Looking back at the nice weekend,
temperatures will likely be near or below freezing across most
inland locations while high temperatures climb into the upper 40s
to upper 50s.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 178.

&&

SYNOPSIS AND MODEL DISCUSSION...MSO
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...MC