Area Forecast Discussion
Issued by NWS Anchorage, AK

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182
FXAK68 PAFC 221657 AAA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
857 AM AKDT Tue Aug 22 2017

.ANALYSIS AND UPPER LEVELS...
Current analysis shows a 999 mb surface low spinning over the
southern Gulf of Alaska which has become cutoff from the main
upper level trough to the north. Scattered rain showers along the
Gulf Coast and Prince William Sound are still being picked up on
radar but are slowly diminishing as the low center continues to
track off to the east. Rain has finally ended for most other
locations across southcentral as upper level ridging centered over
the Bering Sea builds in across west and central parts of the
state. Further west, a deepening low pressure system is located
just to the southwest of Attu. A well defined triple point and
baroclinic leaf, as seen on satellite imagery, is pushing north
across the western Aleutians with a line of thunderstorms observed
this evening along the frontal boundary.

&&

.MODEL DISCUSSION...
Models continue to show solid agreement through the short term in
depicting the low over the Gulf as well as the upper level ridge
building over the southern half of the state. The models have
been more inconsistent in dealing with the low developing over the
Bering and its track east into mainland Alaska, with successive
model runs over the past 24 hours slowing down the progression
of the system. The latest model runs do show some better
consistency with this feature increasing forecaster confidence.

&&

.AVIATION...
PANC...Very moist low levels from the previous week of rain and
building high pressure has led to the development of patchy fog
in the Anchorage Bowl this morning, as seen in local web cams and
METARS. The fog first developed north of the terminal over Bryant
Army Airfield on JBER as well as over Birchwood airport, with
occasional LIFR cigs/vis at these locations. Light northerly flow
pushed the fog south over Elmendorf AFB and eventually down to
Merill Field. So far, the fog bank has not moved over the
Anchorage terminal and most recent obs and webcams show the fog
bank diminishing and shouldn`t pose an issue for the airport. High
pressure continues to build in today leading to VFR conditions
and light winds.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

An upper level ridge will finally provide some relief from the
soggy pattern that has brought persistent rain over the past
several days across the Southcentral Region. Any lingering showers
along the north Gulf coast will diminish throughout the day into
the evening hours as the low near Kodiak drifts to the eastern
Gulf. However, drier weather will only last into Wednesday night.
A low pressure system will develop and push inland near Dillingham
by Thursday morning. Its associated occluded front will support a
coastal jet and bring gale force winds and plenty of moisture
along the north Gulf Coast. There is still some uncertainty in
the intensity of precipitation. While steady rainfall is likely to
occur across Prince William Sound and the eastern Kenai
Peninsula, perpendicular downslope flow may partly limit rainfall
across the Anchorage Bowl and western Kenai Peninsula. The extent
of this precipitation limitation is highly dependent on the track
of this front. While models are now more consistent with this
front, even small changes in wind orientation may limit rain.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The Southwest mainland looks to remain mostly dry through
Wednesday morning, as weak ridging builds into much of the area
today. The exception will be across the northern extent of the
area, as the base of an upper level trough tracks out of the Delta
region and into the Lower Kuskokwim Valley this morning, bringing
a chance for some light rain to these areas. There is also some
fog potential across the Delta early this morning as onshore flow
helps to advect the marine layer inland. The arrival of a front
from the Bering on Wednesday afternoon will bring the next round
of widespread rain to the area, with rain continuing into Thursday
as yet another Arctic trough digs into the region.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Gusty northwest winds will continue to subside this morning along
the Alaska Peninsula, as the upper low moves farther away from
the area. This will usher in a period of benign cloudy conditions
today across the eastern half of the Bering as high pressure
builds overhead. Further west, a front stretched over the western
Bering and central Aleutians/Bering today will slowly push to the
north and east through Wednesday, bringing rain and Small Craft
Advisory level winds to much of the Bering by Wednesday evening.
The parent low will track through the western and central Bering
tonight into early Wednesday morning then into the eastern Bering
by Wednesday afternoon. This will spread precipitation across much
of the Bering and Aleutians through Thursday morning when the
system tracks inland over the Bristol Bay area.

&&

.LONG TERM FORECAST (Days 4 through 7)...

Beginning the extended period forecast Thursday night, the weather
pattern across the region will be dominated by a large trough over
the state, stretching from the Arctic Circle to the North Pacific.
A highly amplified upper ridge will be in place over the Western
Bering, so this will be a rather slow moving pattern. As one may
expect, such amplified troughing will result in a continuation of
typical Alaska early fall weather with clouds and rain impacting
most of Southern Alaska. On Friday and Saturday, the surface low
will move from eastern Bristol Bay to the Gulf of Alaska.
The forecast gets a little more tricky here as weak pieces of
shortwave energy will meander around the larger upper trough, so
it seems that there may be waves of rain instead of continuous
rainfall, but it will still be wetter than drier, especially
across Southcentral and the Gulf Coast as Southwest AK slowly
dries out Sunday.

By Monday, the pattern will become more dominated by a zonally
driven North Pacific jet stream which will cross the southern
Bering Sea and eventually amplify as it moves south of the Alaska
Peninsula. This should result in a brief break in the weather
(note, rainfall) across Southern Alaska sometime early next week
before troughing over the North Pacific moves back in sometime
Tuesday and Tuesday night. In other words, it seems that summer is
effectively over.

&&


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

&&

$$

SYNOPSIS AND MODEL DISCUSSION...KVP
SOUTHCENTRAL ALASKA...ELN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PEPE
LONG TERM...JA



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