Small Craft Advisory 479, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
500 AM AKDT Tue Jun 20 2017

.ANALYSIS AND UPPER LEVELS...
There are two main lows over the area. One sits over the northwest
mainland while the other is over the southern Gulf of Alaska.
Between these two lows ridging extends over much of mainland
Alaska from Canada. Out west, more ridging is slowly tracking east
across the central Bering.

&&

.MODEL DISCUSSION...
The models continue to be in good agreement with the main synoptic
features but struggle with some of the details. While they now
have a good handle on the easterly wave pushing through
Southcentral Alaska, there are differences with what follows. Some
bring another weak wave through which would bring more rain while
others keep the rain off to the east.

&&

.AVIATION...
PANC...South to southeast gusty winds will continue through the
morning but will taper down and become westerly in the afternoon.
Rain will continue through much of the day before diminishing in
the afternoon. However, showers around the area will remain likely
through much of Tuesday night.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Steady rain out ahead of an upper level easterly wave will spread
southwestward across the Susitna Valley and Cook Inlet corridor
this morning and persist through the afternoon hours. Thus, expect
a cool day with temperatures maxing out in the 50s. Meanwhile,
back behind the wave rain will gradually taper off this morning
from east to west across the Copper River Basin and Eastern Prince
William Sound. Weak instability may produce just a few light showers
for these areas this afternoon.

The easterly wave will rotate southward tonight across the western
Gulf, bringing rain to Kodiak Island. Meanwhile, another wave
rounding the north side of the retreating upper low over the
Gulf will track in the vicinity of the southeastern Copper River
Basin and northeast Gulf tonight through Wednesday. Forecast
confidence in the track of this feature is low. The most likely
area to see another round of rain is the southeast Copper River
Basin on down to the Cordova area in southeastern Prince William
Sound. A very broad brush was used for the precip forecast until
we get a better idea of where the wave will track.

For the remainder of Southcentral, mostly cloudy skies will
continue tonight. There should then be some breaks of sun on
Wednesday as an upper ridge builds southward from interior Alaska.
This ridge along with low level northerly flow will allow
temperatures to quickly rebound into the 60s to lower 70s. A
thermal trough will set up over interior Southcentral and serve
as the focus for afternoon/evening convection.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Tue and Wed)...
The area will remain mostly dry for the next couple of days with
mild temperatures. The dominant feature for today will be the low
in the Gulf of AK. It is keeping generally north to northeast flow
across much of the area. It is also keeping some very weak
instability right along the spine of the AK Range. Thus, have
opted to leave some isolated thunderstorms in that area for this
afternoon-evening. The Kuskokwim Delta will hang on to some low
clouds through much of the day today with onshore flow keeping a
steady supply of moisture into the area. This will limit
temperatures from climbing too high, similar to what happened on
Mon.

By late tonight into Wed, some weak ridging from the Bering will
start to join forces with a stronger ridge near the ALCAN border.
Even though there could still be some low clouds along the coast
and into the Delta Wed AM, the building high pressure should help
to clear out most places by the afternoon. The lone exception will
be the AK Range where weak instability could still produce a few
pop-up showers.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Tue and
Wed)...
A transient shortwave ridge will keep quiet weather over the
Eastern Bering while a more active pattern brings some rain and
winds to the Western Aleutians. The gusty north-northwest winds
along the AK Peninsula should diminish through the day as the
gradient relaxes. A weak shortwave will ride down from the north
today and could help to keep some light showers from Dutch Harbor
through the Alaska Peninsula, especially on the Bering side.
Meanwhile, a warm occlusion will keep rain falling over Shemya
until this evening.

By Wed, the ridge will build even more over the Eastern Bering.
This should lead to some widespread marine stratus and some patchy
fog, especially during the overnight hours. Over the Western
Aleutians, a deepening low on Wednesday will bring a gale-force
front into the area by the afternoon. This front will have a deep
moisture tap, so expect a swath of moderate rain as well.

&&

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

The upper level ridge from south of the Aleutians into southwest
Alaska across the interior will split into two ridges as an upper
level low over the Gulf of Alaska pushes north Friday and the
large upper level Arctic trough sweeps across the state from the
Brooks Range north. The upper level ridging remains over the
eastern Aleutians and eastern Bering Sea through Saturday then
builds back into southwest Alaska and the interior Sunday as the
upper level low over the Gulf moves off to the southeast. A strong
upper level low will track into the western and central Bering
Sea through the weekend behind the aforementioned ridge. This
upper low will track to the north toward the Bering Strait as the
upper ridge extends from the Gulf of Alaska into the central and
eastern interior by late Monday and Tuesday.

This will usher in a return of the thermal trough and warmer
temperatures over much of the southern mainland by this weekend
with increased afternoon and evening convection through the
weekend. Over the Bering Sea and Aleutians the weather will
transition from marine stratus and fog typically seen under the
surface high to a wetter and windier regime one would expect with
the surface based low moving across the Bering and the associated
front that sweeps along the Aleutians through the weekend.

Model agreement has remained consistent through the long term and
the national guidance remained heavily weighted toward the
operational model guidance through the weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales...178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...SA