Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 261243
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 AM AKDT TUE JUL 26 2016
.ANALYSIS AND UPPER LEVELS...
The synoptic pattern has pretty much remained steady state
through the overnight and early morning hours with the lack of
large scale features to mess around with the upper levels. A
vertically stacked low remains centered just to the southeast of
the Shumagin Islands this morning with rainfall beginning to re-
develop along the Alaska Peninsula. In addition...scatterometer
data over the Alaska Peninsula and Eastern Aleutians shows gusty
northwesterly winds pushing through channeled terrain. Looking
further east toward the Gulf of Alaska, a very weak and diffuse
area of low pressure is situated just to the south of Middleton
Island. This low in conjunction with a building coastal ridge
along the northern gulf coast has created a weak barrier jet that
is also evident on scatterometer data.
Latest radar trends show much of the rainfall across interior
locations of Southcentral Alaska tapering off while continued
showers have been observed along the coastal range due to the
proximity of the two lows in the Gulf of Alaska. Shower activity
across Southwest Alaska is increasing as the low south of the
Shumagin Islands pumps moisture northward and lift associated with
the upper level low helps to utilize this moisture.
Models are in overall good agreement through Thursday but continue
struggling with the broad and elongated area of low pressure
residing over western Alaska and how it evolves in the coming
days. Models have high confidence a weak shortwave currently on
the western periphery of the upper level low over western Alaska
rotates into the gulf of Alaska on Wednesday from the North
Pacific and spins up a new surface low near Middleton Island.
However, timing differences remain quite large and also the
intensity of the low is in question. Models have made a large leap
forward with the 00Z runs as they now paint this closed low moving
into the gulf as opposed to half of the earlier runs keeping this
low a trough while the other half depicted the low. Otherwise,
high pressure will continue along the northern gulf coast and keep
gusty winds through Portage Valley and Along Turnagain Arm.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Broad and deep southeasterly flow will provide for a drier day
today across inland portions of Southcentral. The atmosphere
remains quite moist with very weak instability and the downslope
component will be on the weak side, so still expect isolated to
scattered showers moving through. An upper level short-wave moving
onshore combined with upslope flow will maintain wet conditions
across the Prince William Sound zones and the southern Kenai
Peninsula. A weak warm front behind the short-wave will lift
northward across the Gulf today. High pressure will build out
ahead of it late this afternoon and evening producing gusty winds
through the usual gaps (Turnagain Arm/Knik Arm/Copper River).
These winds may be just enough to help generate a few breaks
The warm front starts out quite weak. However, a short-wave
digging south of the upper level low south of the Alaska
Peninsula will amplify the pattern today. This will send a
plethora of short-waves shooting out of the trough northward
toward Southcentral tonight, with the low center to follow. This
will reinvigorate the front along the north to west Gulf coast.
Have trended toward more widespread Small Craft Advisories, though
guidance is hinting at potential for gales along the north Gulf
coast on Wednesday. Expect continuous rain along the coast Tuesday
Night through Wednesday Night, with rain likely spreading inland
across Southcentral Wednesday Night. This looks like another
potent upper level low with impressive instability and a decent
jet streak on the east side of the low. Thus, expect a transition
to heavy showers, or maybe even a thunderstorm, as the low reaches
the north Gulf coast Wednesday Night.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
As the upper low stalls south of the Akpen plenty of afternoon
and evening showers are expected over the region into Thu. A pair
of easterly waves moving through the area will also move into the
region of instability over the Kuskokwim delta. This interaction
will give a slight chance of afternoon and evening thunderstorms
today, and when the interaction also includes an approaching
Bering front another slight chance on Wed as well.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Varying amounts of fog are expected across the Bering and Aleutians
into Thu with light but steady west surface flow. A front moving
across the Bering through Wed will increase the chance of rain. On
Wed night flow shifts to the northwest and some showers will be
possible over the central aleutians.
.LONG TERM FORECAST (Days 3 through 7)...
The forecast on Thursday and Friday keeps the highest precipitation
amounts along the northern Gulf and Prince William Sound as an
upper low moves east across the southern gulf. A North Pacific
ridge to the west keeps the Bering and western portion of the
Aleutian Chain under low clouds and stratus through much of the
weekend. Model solutions differ heading into Saturday as an upper
low digs from the Arctic. This system is progged to bring a front
into the Southwest Friday night. The ECMWF is currently the most
aggressive solution bringing a very wet pattern to Southcentral
Saturday night. In comparison, the GFS would lag the front across
the Southwest with a triple point low developing over the eastern
Aleutian Chain Saturday night. The forecast in the extended range
was adjusted with the ECMWF and GEFS Mean as it offered the best
run to run consistency. Even though this solution was favored,
there is still a bit of uncertainty for rain and wind trends.
SYNOPSIS AND MODEL DISCUSSION...MMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DS