Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 160010
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
410 PM AKDT Fri Sep 15 2017

.ANALYSIS AND UPPER LEVELS...
A deep storm force cyclone has reached it`s lowest pressure,
estimated at 969mb, in the vicinity of the Pribilof Islands.
Although the low is now fully occluded, low level winds continue
to strengthen on the south to west side of the low, as colder
more unstable dives southward across the central Bering Sea. This
core of strongest winds is shifting eastward with the low center,
from the south central Bering Sea toward the Pribilof Islands,
Eastern Aleutians, and southern Alaska Peninsula. The occluded
front out ahead of the storm is pushing onshore along the
Southwest Alaska coast, bringing rain and gusty southeast winds.

Meanwhile, downstream of this storm, an upper level short-wave
is exiting northward through the Copper River Basin, producing
mainly mountain showers. Lingering instability and upslope flow
are producing showers across much of the Prince William region,
while downslope flow in the lee of the mountains is producing
dry conditions and some sunshine across much of the rest of
Southcentral.

&&

.MODEL DISCUSSION...
Model guidance is in excellent agreement with handling of major
features over the next couple days and forecast confidence is
generally high. The main forecast challenges include determining
how much wind will mix down to the surface with Bering Sea storm
system and strength of barrier jet along north Gulf coast as
occluded front/triple point low approach on Saturday.

&&

.AVIATION...
PANC...With persistent low level southeast flow, VFR conditions
will persist. Surface pressure gradients are marginal for bending
the Turnagain wind into the terminal this afternoon and evening,
but do expect some gusty winds to make it in. Increasing down
inlet flow ahead of the occluded front moving into the Gulf
overnight will then lead to development of a north surface wind,
which should hold all day Saturday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The Anchorage Bowl remains in downsloping drier conditions with
the cross-barrier flow. While the northern coastal communities,
and the western Cook Inland extending into the Susitna Valley
remain under a showery regime. This weather phenomena is being
influenced by an upper level shortwave as it moves through the
aforementioned region before exiting to the north this evening.
This briefly gives way to partly cloudy skies for inland areas,
but more moisture will be streaming northward from the Gulf
during the overnight hours tonight. There is a triple point low
developing over Kodiak Island by Saturday morning with an
associated weather front entering the northern Gulf. This leads
to increased pressure gradients with a barrier jet developing
along the northern coast with strong gusty easterly winds. These
winds will extend through the Hinchinbrook Entrance into Prince
William Sound. This coincides with strong gusty southeasterly
Turnagain Arm winds, and higher elevations, occurring by Saturday
midday. Look for the winds to peak in the late afternoon hours
before tapering-off by late Saturday evening time-frame.
Otherwise, the bulk of the precipitation with this synoptic
feature will be along the northern coastal communities. By Sunday
a weak high pressure looks to briefly build over the Anchorage
Bowl bringing mild weather with partly cloudy skies as we head
into Monday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The previously mentioned Bering Sea front has now started to push
inland this evening bringing rain and gusty winds to much of the
southwest Mainland through this evening. The front quickly washes
out late tonight leaving much of the mainland in moist cyclonic
flow with showery and locally gusty conditions, as the parent low
tracks into Bristol Bay and crossed the Alaska Peninsula Saturday.
The responsible upper level trough lingers through the weekend
leaving a chance of showers and relatively light northerly winds.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The focus remains on the Bering Sea storm pushing over the
Pribilof Islands this evening and tracking eastward into Bristol
Bay by Saturday afternoon. The swath of strongest winds still
looks to clip the Islands, especially Saint George as the system
pulls to the east this evening. The High Wind Warning remains in
effect mainly for Saint George Island through Saturday morning.
Rain will come with the winds on the backside of the system during
the overnight period, then switch to more showery conditions by
mid-day Saturday.

The strongest winds and precipitation will quickly shift toward
the Alaska Peninsula overnight tonight into Saturday before
slowly tapering down through the weekend. The High Wind Watch was
upgraded to a High Wind Warning for the Alaska Peninsula late
tonight through Saturday evening, as the core of the strongest
winds track over the area.

&&

.LONG TERM FORECAST (Days 3 through 7...Mon through Fri)...
The week of September 18th will start out relatively quietly. One
area of low pressure will be pulling away from the North Gulf
Coast and another area will be moving into Bristol Bay. This will
allow for a brief period of drying for most of South Central on
Monday. For Southwest AK, things will once again be wet and a
little breezy. The low over SW pivots and develops a triple point
surface low in the Gulf of AK late Mon into Tue. It will quickly
exit towards SE AK on Tue, once again leaving much of the southern
half of AK in drying offshore flow. Then yet another low follows
a very similar path from the Bering Strait towards western AK,
possibly bringing some gale-force winds to the northern Bering.

After this point, model discrepancies start to increase. The
pattern will essentially be driven by how far the south the
energy from the Arctic can make it and what sort of interaction it
will have with a 100-150 kt zonal jet positioned over the North
Pacific. Right now, models do not show signs of any strong phasing
that would result in big storms. The quasi-stationary "warm
front" sits south of the Aleutian chain while various troughs
dropping in to SW AK from the Arctic only bring glancing blows of
cold air to the mainland. However, it is worth noting that much of
the warm air and moisture south of the jet is tropical in nature
and global models generally do not handle those features well.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 181 195.
MARINE...Marine Wx Statement 155 165 170 179.
    Hurricane Force Wind 165 170.
    Storm Warning 155 179.
    Gale Warning 119 131 150 160 171>175 185.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PEPE
LONG TERM...MSO



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