Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 220058
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 PM AKDT WED SEP 21 2016
.ANALYSIS AND UPPER LEVELS...
The strong front over the Gulf of Alaska continues to shift eastward
this afternoon, extending from Prince William Sound southward
through the central and eastern Gulf. The front is bringing steady
rain and gusty easterly winds to the eastern Prince William Sound
coastal area. The majority of the moisture is staying on the
coastal side of the mountains however, and areas further north in
the Copper River Valley are only seeing spotty showers and
relatively lighter wind gusts. Showers further west over the Cook
Inlet Region are tapering off this morning as residual gusty winds
downsloping over the coastal mountains are limiting the
availability of Gulf moisture. A surface low is developing along
the front south of Middleton Island and tracking northward toward
the northern Gulf Coast, poised to move inland tonight.
Further west...a much quieter weather pattern is in place as the
main stacked low gradually weakens over the northern Bering Sea.
This low is keeping unstable showery conditions over the Bering
Sea and Aleutians as cold air wraps around the low.
The most notable discrepancy in the models today is in the handling
of a wave developing along the front in the northern Gulf. As this
wave moves inland over the northern Gulf Coast tonight, it will
bring in some additional rainfall and gusty winds behind the
frontal passage. The GFS looks to have initialized the best with
the placement of the wave on satellite imagery this morning; the
thinking is that the GEM is tracking the low northward too fast
while the ECMWF looks to be a little slow. The middle-road and
better initialized GFS solution was used in the timing and
intensity of the precipitation and the winds associated with
PANC...Rain will develop this evening as an upper level front
moves into the area. Conditions could drop to MVFR by late evening
and continue into the morning, though there is some uncertainty
in the extent of cross-barrier flow which will greatly affect
ceilings and rainfall intensity/visibility. Rain will diminish to
showers by late morning as the front moves north. Winds will be
fairly light and variable until late morning, when some southerly
winds could work their way into the airport as the front moves
north. Overall confidence is higher than it has been the last 24
hours, but still below normal.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Wet and windy conditions will continue in the near term before
a small pattern change Thursday evening. The frontal boundary
associated with the large scale system is currently stalled from
the Susitna Valley south-southeastward into the eastern gulf. An
active jet and shortwave trough within it will first ramp up
winds in the low levels again, mainly along the north gulf coast,
continuing storm force winds in Prince William Sound, possibly
into Passage Canal. The strongest winds should remain in the Sound
with a little poking into Portage Valley. As this disturbance
curves along the front northwestward it will spread moisture up
and over the Chugach through the Susitna Valley, despite all the
cross-barrier flow which usually keeps the lee- side dry. Steady
rain will continue through Thursday morning when the upper trough
will push through to the northeast. This is the small pattern
change that will stay around through the weekend. The Southcentral
will be under the influence of the same longwave trough, but no
big disturbances are in the flow upstream. Cold air aloft and
seasonably warm sea surface temperatures will keep the chance for
showers in most areas, especially upslope areas such as the north
gulf coast, and northern Susitna Valley.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A more benign and showery regime has set up over Southwest Alaska,
yielding widespread cloudy and showery conditions over the area
through Thursday night in persistent onshore flow ahead of the
sprawling Bering low as it gradually approaches the coast. The
heaviest shower activity and gusty winds will remain confined to
the Southwest coast and along the Alaska Peninsula, with a slight
chance for a few thunderstorms along the immediate coast of the
Kuskokwim Delta and the Western Capes tonight. The potential for
thunderstorms will then spread to the Bristol Bay coastline by
Thursday as the pool of cold air aloft associated with the low
center shifts eastward towards the mainland.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The large and stacked low west of Nunivak Island will remain the
dominant weather feature through Friday, as it continues to fill
and make slow eastward progress towards the southwest coastline.
This will keep widespread showers and small craft advisory-level
winds over much of the Bering for the next few days in broad
cyclonic flow around the low. There will also continue to be a
slight chance for thunderstorms over a large swath of the eastern
Bering in this timeframe as cold air aloft moving over the
relatively warm waters continues to generate modest instability.
By Friday, ridging builds into the Western Bering/Aleutians from
the west, bringing in some overall drier conditions to the area,
and the potential for some marine fog to develop.
.LONG TERM FORECAST (Days 3 through 7)...
Beginning the extended period forecast Friday evening, a broad
upper level trough will encompass all of the Bering Sea and
southern Alaska. A digging arctic trough will be in the process
of moving into northwest Alaska and will eventually merge with
another separate trough over the North Pacific and Gulf of Alaska.
Showers and light precipitation will consolidate along the digging
trough Saturday and Saturday night and will continue to move east
through Sunday with precip chances ending from west to east.
Slightly cooler air will settle in behind the trough, especially
across southwest Alaska.
Monday and beyond, the upper level flow will become highly
progressive and low amplitude as a series of shortwave troughs
cross the Bering Strait into southwest Alaska. The track and
timing of these lows in somewhat uncertain with the first moving
into southwest sometime Sunday night into early Monday. With a
trough to the east, this should incite an offshore flow pattern
over Southcentral with rain and moisture being blocked/downsloped
from the Alaska Range, thus most rain chances will remain over
southwest Alaska through most of Tuesday. After that, much
uncertainty develops by midweek as a strong shortwave trough moves
out of eastern Siberia. The ECMWF favors more ridge amplification
over the Bering and a track that remains closer to the Brooks
Range whereas the GFS is quite a bit farther south. The current
forecast favors an ECMWF/ECMWF ensemble approach from WPC which
highlights greater ridge amplification and lower precip chances by
midweek and into the latter part of the week. No major storms or
anomalous temperatures are expected during this period as the
North Pacific subtropical moisture remains displaced well to the
PUBLIC...Small Stream Flood Advisory 125 161 171.
MARINE...Storm Warning 119 125.
Gale Warning 128 129.
SYNOPSIS AND MODEL DISCUSSION...RD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP