Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 300014
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
414 PM AKDT Sat Apr 29 2017
.ANALYSIS AND UPPER LEVELS...
A vertically stacked low is near Haida Gwaii this afternoon. A
couple upper waves on the north side of the low are being pulled
westward across the Gulf as a trough axis extending from the
eastern Bering Sea to the southern Gulf of Alaska lifts northward.
Middleton Island radar shows rain over the northern Gulf
spreading toward the Kenai Peninsula. A separate weak wave out
ahead of this is bringing rain to Kodiak Island. Otherwise, much
of Southcentral and Southwest Alaska have started the day out with
sunshine. A look at area soundings from this morning show
continued instability, particularly in the low levels of the
atmosphere where lapse rates are between 7 and 8 degrees per
kilometer. Combination of upper short-waves and instability means
conditions across the region remain favorable for the growth of
cumulus clouds through the afternoon and evening clouds, with some
showers, especially over and near the mountains.
Meanwhile, out west a large upper level low is centered over the
Western Aleutians with a weak surface low just to the south and
headed eastward. A nearly stationary front extends from the
Eastern Aleutians northwestward across the Bering Sea. The front
had been weakening, but now a surface low tracking up the front
is causing some tightening of pressure gradients and stronger
winds right out ahead of the front.
Models remain in good agreement with large scale features through
Monday. The primary forecast challenges (which are typical for
this time of year) are areal coverage of convection over
Southcentral and Southwest Alaska along with strength of gap
winds over Southcentral.
Focus then shifts to a low developing over the north central
Pacific Monday and tracking into the Gulf of Alaska on Tuesday.
Prior to the 12z model cycle this morning, there was a large
spread in track and intensity of this low among medium range
guidance. The GFS, ECMWF, and Canadian solutions are much closer
this morning, indicating a low over the south-central Gulf with
central pressure in the 960s or 970s. With increasing confidence
in a deep late season storm system, will trend toward the GFS
solution and bring widespread gale force winds into Gulf. If the
deeper solutions verify there is good potential for storm force
PANC...Chances of the Turnagain wind coming into the airport this
afternoon and evening are marginal. There will be a bit of a down
Inlet pressure gradient, which means most of the wind coming out
of Turnagain Arm should remain over the Inlet. However, with
some sunshine over interior areas along with an unstable air mass
it is possible for some southeast wind to come in - later and
weaker than normal. Winds look notably stronger Sunday
afternoon/evening with a stronger coastal ridge. Otherwise, VFR
conditions will persist.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Sunny skies over Southcentral today have given way to diurnal
rain showers over the MatSu Valley`s that will continue this
evening as showers continue to reform over the Talkeetna
Mountains. The Anchorage Bowl will however likely remain just far
south of these showers to remain dry through the evening. Further
south, a weakening low near the Barren Islands will bring rain to
the southern Kenai Peninsula into Sunday. Beginning on Sunday, a
frontal system moving through the Gulf will bring more active
weather to the Gulf Coast and Southcentral into early next week.
The bulk of the rainfall from this system should be confined to
the Gulf Coast as downsloping along the Chugach Mountains should
limit any precipitation to a few passing rain showers. Instead,
gap winds will develop along Turnagain Arm and the Knik River
Valley, which will also bring gusty southeasterly winds into the
Anchorage Bowl on Sunday. This cloudier pattern will then continue
into Monday as the front begins to dissipate over the north Gulf
Coast, allowing gap winds to begin to diminish.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
With high temps around 50 of higher this afternoon convective
showers are expected to last into the evening, but diminish
through the night. Sunday afternoon the front moving through the
Bering will move in over southwest Alaska bringing some heavier
rainfall to the area that will last into Monday as the front
stalls over the area. Winds will also increase somewhat out of a
southeast direction as the front first pushes into southwest
Alaska but will gradually diminish moving towards Monday.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The main low over the central Bering is pushing a front across the
Bering that is draped over the Pribilof Islands and Alaska
Peninsula. The front will continue its easterly track overnight
which will leave showery conditions for most of the Bering and
Aleutians in its wake. The low will track south on Sunday and as
it passes over the Central Aleutians Sunday night it will change
course and head east. This will bring northerly winds and
continued light showers to the Bering and Aleutians.
.LONG TERM FORECAST (Days 3 through 7)...
There is fair agreement amongst the models of a deep low moving
into the Gulf around Tuesday evening. However, they are struggling
with the intensity of the low which could be the difference
between a gale force low and a storm force low. For now there is
higher confidence in the low remaining gale force. Regardless,
this system looks to bring rain to areas around Southcentral
Alaska into Thursday, with the heaviest for the north Gulf coast.
Meanwhile ridging will have moved in over the Bering with another
system trying to approach the Aleutians from the west, but being
largely held at bay by the ridge.
MARINE...Gale Warning 130.
SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK