Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 111227
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
427 AM AKDT Mon Jun 11 2018

.ANALYSIS AND UPPER LEVELS...
A pattern change is well underway across mainland Alaska this
morning as the leading edge of an Arctic trough centered over the
Beaufort Sea descends upon Southcentral, spreading rain over much
of the area from Anchorage and the Mat-Su to the Copper River
Basin and Prince William Sound. Southwest AK remains mostly on the
dry side, however increased offshore flow and more extensive
cloud cover due to the digging trough has helped to mitigate any
of the marine stratus and fog issues which had been observed in
recent nights. Widespread stratus and fog does remain locked in
across much of the Bering beneath high pressure extending out of
eastern Russia. Satellite imagery reveals that the eastern edge of
the cloud shield is starting to be eroded somewhat as drier
northeast flow increases off the Kuskokwim/Bristol Bay coast.

&&

.MODEL DISCUSSION...
Models are in good synoptic agreement for the short term as the
Arctic trough continues to dig into the northern Gulf through
Tuesday morning. The trough axis will then slowly shift eastward
towards the Alcan border through Wednesday morning, leaving
Southcentral in drier northwest flow aloft and bringing a reprieve
from the rainy conditions by the middle of the week. Given the
good agreement, mainly utilized higher resolution NAM guidance to
make minor adjustments to the forecast.

&&

.AVIATION...
PANC...The heaviest rain associated with the advancing trough
should clear out of the terminal by early to mid morning, leaving
more showery conditions in its wake. VFR conditions have persisted
through the heaviest rain overnight and are expected to continue
for the remainder of the TAF period, though brief MVFR conditions
cannot be ruled out through mid morning. Weak Turnagain Arm winds
should redevelop in the afternoon as the coastal pressure gradient
strengthens once again, with southeast winds potentially making
it back into the terminal given a relatively flat gradient over
Upper Cook Inlet.

&&

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

A strong upper trough digging across Southcentral today is the
main weather maker for the next day or so. The trough holds
roughly along 150W today then moves to the eastern Copper River
Basin/eastern Gulf later in the day Tuesday. This feature will
bring cooler temperatures and wet conditions across much of
Southcentral through tonight with gradual drying from the west and
showery conditions tonight and Tuesday. The western Susitna
Valley and Cook Inlet however will see the least amount of
precipitation as these areas will generally be on the back-side
of the trough. General low pressure over the mainland and ridging
along the coast today will give rise to easterly gap winds across
Turnagain Arm and southerly winds along the Copper River.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

The region today is getting sandwiched between a mid-level to
upper-level high pressure system over Siberia and an unusually
deep trough diving south through Southcentral Alaska. This places
southwest Alaska in northerly flow with the two systems, with the
pressure gradient increasing because of such. What this means for
us is look for somewhat gusty winds today with mainly clear skies.
This trend will continue for the next few days, but winds will
weaken as the initial trough wobbles off into the northeastern
Gulf.

Another sizable mid-level low developing over the North Slope will
attempt to send a reinforcing shot of cold air southward,
this time towards southwest Alaska. However, a bubble of high
pressure briefly developing overhead will stop this, as the
remnants of a western Pacific tropical storm head towards the
central Aleutian chain. This will maintain the dry weather through
Thursday before things begin to change.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

The calm before the storm best sums up the weather over the next
few days, as a ridge of high pressure keeps winds light and
mainly dry conditions. The exception to this is the central
Aleutians into the AKPEN, where onshore flow and orographic lift
has been sufficient to produce isolated showers at Nelson Lagoon
and Dutch Harbor. This has also resulted in some fog across this
region, with it being locally dense at Nelson Lagoon.

Aside from that, dry offshore flow will develop over the next few
days, strengthening especially tomorrow. This will allow portions
for the eastern Bering to clear out, possibly reaching the
Pribilof Islands as well. However, the remnants of a tropical
system will move from the western Pacific into the east central
Aleutians just beyond the forecast period, making things more
interesting.

&&

.LONG TERM FORECAST (Days 3 through 7)...
By Wednesday, much of southern Alaska will be located in a col
between a large upper level low centered over the Arctic Ocean,
another big upper low well south of the Aleutians and weaker
ridging to the east and west. A trough stretching from the
northern to southeastern Gulf will weaken rapidly and dissipate as
a low to the south swings east and inland into British Columbia.
The arctic low will dig south into northwestern Alaska Thursday
and Friday, while the North Pacific low lifts up towards the
eastern Aleutians and Alaska Peninsula. In comparison to
yesterday, all the models have shifted further west with the track
of this southern low with the GFS remaining the farthest east
solution. Currently there is a roughly 500 mile surface low
position difference by Saturday morning, but all models are still
in good agreement with regards to a significant pattern shift to
wetter, southerly flow for Friday through Sunday and likely
continuing into next week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None
MARINE...None

&&

$$

SYNOPSIS AND MODEL DISCUSSION...CB
AVIATION...CB
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...JR



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