Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 261242
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
442 AM AKDT MON SEP 26 2016
.ANALYSIS AND UPPER LEVELS....
The upper level trough that has been nearly stationary over the
southern mainland for 2 days now has weakened substantially since
yesterday. Ahead of the trough axis over northern Cook
Inlet/Southcentral, clouds and showers continue to form, keeping
temperatures warm and preventing widespread fog/stratus from
developing. This is a slightly different story further down Cook
Inlet behind the trough axis where skies have mostly cleared,
allowing for patchy fog/stratus to quickly develop overnight along
the coast with light winds. This trough will continue weakening
today as it moves eastward ahead of the next low over the Bering.
Overnight ship reports from this Bering low have confirmed that
gale-force winds have developed along the front on the southern
edge just north of Dutch Harbor. This front will continue moving
southward today, spreading gales and rain throughout the region.
Behind this low, a 100-kt jet streak can be seen diving to the
southeast through the Bering, which will help gusty northwesterly
winds to continue after the front passes today. To the west of
this system a ridge of high pressure is beginning to spread
through the western Bering, allowing for widespread fog and
stratus to develop over the western Bering/Aleutians.
The models remain in very good agreement with track and intensity
of two systems that will move through the Bering over the next 2
days. The GFS in particular initialized the best with winds over
the Bering, and was generally preferred overnight to increase
winds from this system. Over Southcentral, the main challenge
will come from how quickly clouds around Cook Inlet will clear out
overnight, and the potential for fog/stratus development. None of
the models are handling this clearing very well based on overnight
satellite trends, so forecast confidence in the details on
exactly when and where fog may develop, and its longevity (such as
in the the Cook Inlet area TAF`s) is low.
PANC...Preferred the 06z raw model (NAM) low-level moisture
profiles for the 12z TAF (especially over statistical guidance)
package in trending more pessimistic with MVFR stratus deck
remaining around through the morning (and possibly longer). Lower
cigs may become scattered with VFR conditions especially from the
airport west this afternoon, with continued MVFR conditions likely
to the east toward the mountains. Big question remains to what
extent mid-level clouds may prevent widespread IFR or MVFR
conditions from developing around the airport tonight. For now,
thinking the mid-level clouds provide enough insolation to
prevent that from happening, but this is low confidence at this
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level trough currently stretches directly over the
Kenai Peninsula. The trough has been providing sufficient upper
level support for overnight showers across the Kenai Peninsula
and along Cook Inlet. In particular, showers have been forming
along the eastern Inlet, where the contrast between the relatively
warm Cook Inlet waters and cooler nighttime land temperatures is
providing low level support for shower development. Generally
these showers have helped to keep the boundary layer relatively
mixed overnight and have prevented the development of fog.
However, fog has been forming further inland within the Matanuska
valley, where showers have not yet developed. Skies are expected
to continue to clear today as the trough pushes eastward and a
fast moving shortwave ridge pushes in.
There is a chance for fog to develop again Monday night into
Tuesday morning ahead of the next approaching upper level trough.
With lower pressure in the Gulf and a surface ridge in place,
light offshore flow may be sufficient to mix out developing fog in
the Anchorage Bowl and Kenai Peninsula. However, fog could be
more established in eastern portions of the zone such as Prince
William Sound and the Copper River Basin.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An upper level closed low continues to track through the eastern
Bering before crossing the Eastern Aleutians into the North
Pacific Ocean by Monday afternoon. This feature will bring
rain along the Southwest Alaska coastline today before tapering-
off as an upstream amplified ridge builds into the AOR heading
into Tuesday. Therefore, look for drier conditions except for
patchy fog developing under the upper level ridge for Tuesday, and
Wednesday morning. Meanwhile, the Kuskokwim Delta region will have
a trough pushing into the northern Bering resulting in a swath of
moisture accompanied by rain/stratus entering the Kuskokwim Delta
region by early Wednesday morning.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
As the surface 1004 mb low tracks through the eastern Bering this
morning, this feature produces a showery weather regime through
the central/eastern Bering before spilling over into the Central
to Eastern Aleutians today. The gale-force westerly winds
associated with this surface low will diminish to small craft
advisory winds by Monday afternoon. While, a weak ridge builds
into the central Bering/Aleutians by Monday afternoon. A storm-force
low drops down off the Kamchatka Peninsula by early Tuesday
morning. The associated weather front with this system enters the
Western Aleutians/Bering during the same time frame. This brings
a swath of rain/stratus with gale-force southwesterly winds to
the aforementioned region. By Tuesday afternoon this low pressure
system deepens, and strengthens resulting in storm-force
southwesterly winds entering the western Bering. As the low
pressure moves farther north the storm-force winds weaken to
gale-force as they enter the northern Bering by late Tuesday
evening. As we head into Wednesday, the majority of the
Bering/Aleutians will be under the influence of a high pressure
ridge accompanied by marine layer stratus/fog.
.LONG TERM FORECAST (Days 3 through 7)...
The extended term forecast will begin on Tuesday night with
transitory upper level ridging in place over mainland Alaska,
promoting mainly dry conditions across the Southcentral region
beneath clearing skies and offshore flow. A shortwave trough
moving across the northern half of the mainland on Wednesday will
flatten out the ridge and drag a front through the southwest part
of the state, bringing a chance of rain before quickly moving off
to the east. The pattern then becomes much more amplified during
the latter half of the week as an upper level trough deepens over
eastern Russia and a high amplitude ridge builds over the mainland
from the central Pacific. This will support a continuation of the
mostly dry and clear conditions across the southern mainland
through the end of the work week, allowing overnight low
temperatures to dip near or below freezing in many locations as
conditions become optimal for radiational cooling. Meanwhile,
another shortwave rounding the base of the eastern Russia trough
will help to spin up a deep surface low moving into the Bering in
the Thursday/Friday timeframe, bringing rainy and windy conditions
to much of the Bering region. This system also looks to swing a
front over the southwest mainland Friday night and into the Gulf
of Alaska by Saturday, bringing a return of precipitation to the
forecast for Southcentral by next weekend.
Gales...155 172 178 185 412 413.
SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD