Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 240003
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
403 PM AKDT Sat Sep 23 2017

.ANALYSIS AND UPPER LEVELS...
There is an upper level low over southwest Alaska and the eastern
Bering Sea with two centers. One center is near Nunivak Island,
with the other center is just southwest of Kodiak Island. The
upper low near Kodiak has an associated 989 mb surface low, and
there is another 989 mb surface low centered near King Salmon.
Plenty of moisture and showers are associated with these lows.
There is an occluded front stretching from the Kenai peninsula
southeast across the Gulf of Alaska. This is bringing a decent
amount of rain into the eastern Kenai Peninsula. High pressure is
over the western and Central Bering Sea. There is also a weak
negatively tilted high over the Copper River basin.

&&

.MODEL DISCUSSION...
The numerical models are in decent agreement for the short term
portion of the forecast (through Monday afternoon). There are some
minor differences in how they handle the low moving towards Prince
William Sound, but the agreement is better than in previous runs.
There is a split in how the surface low over southwest Alaska
moves into and across Bristol Bay Sunday into Monday. The GFS and
NAM are further west, keeping the low on the western periphery of
Bristol Bay, while the ECMWF and Canadian GEM put it into the
eastern portion of the bay. The GFS/NAM solution was preferred.
Forecast confidence is near normal today.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist with the
downslope on the lee side of the Chugach Mountains keeping
PANC out of most of the rain. While some wind shear is possible
this afternoon, it will likely be above the 2000 ft threshold for
low level wind shear being mentioned in the TAF. Any shear that
is there should dissipate by evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The remnants of a barrier jet remain along the northern Gulf
coastline this evening and will taper off by Sunday morning with
weak onshore flow. Meanwhile, an upper level closed low is just
east of Kodiak Island, and looks to track through the Prince
William Sound region by late Sunday afternoon. This feature is
reflected at the surface with a weak 994 mb low just offshore of
Chiniak Bay late Saturday evening, and tracks northward into the
Prince William Sound area by Sunday before dissipating by Monday.
This is a little different track for the low than was expected
yesterday as it is now farther east in Prince William Sound as
opposed to heading over the western Kenai Peninsula.
Look for the northern coastal communities to receive the majority
of the precipitation. The Anchorage Bowl and the upper portions
of the western Kenai Peninsula will have drier conditions with
downsloping occurring due to the cross-barrier flow through rest
of the weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
There are two upper level lows helping to bring rain to southwest
Alaska. One is near Kodiak Island while the other is near the
Kuskokwim Delta coast. This setup has brought the heaviest rain
to the Bristol Bay area. The heaviest rains will move northwest
towards the Kuskokwim Delta area this evening as the low near
Kodiak pushes off to the northeast and the one near the Kuskokwim
Delta tracks south along the coast. As this low tracks south there
is uncertainty as to its exact track. These differences will
determine how strong winds get in Bristol Bay on Sunday. Sunday
night the low will continue its track south into the North Pacific
leaving behind light rain over southwest Alaska.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
High pressure continues to dominate the majority of the Bering
and Aleutians and will continue to do so through Monday. This
pattern will keep northerly winds with minimal rain over the
region. The exception will be the eastern Bering Sea and Alaska
Peninsula as an upper level low near the Kuskokwim Delta tracks
along the coast into the North Pacific through Monday.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Confidence continues to increase in regards to an area of low
pressure quickly moving into the Gulf of Alaska from the North
Pacific on Tuesday. The ECMWF is the fast outlier currently but it
now looks like gale force gusts could be expected near Kodiak
Island and the northern Gulf Tuesday afternoon through Wednesday
afternoon as a front associated with the low pressure quickly
moves through the region. Rainfall with this system will upslope
along the coastal mountains but there is a bigger question for
interior locations and it really depends on the low track.
Currently it looks like inland areas like Anchorage and Palmer may
miss out on much of the precip on Wednesday.

Looking at the big picture from late next week through the
weekend, it appears the wet pattern across Southwest and
Southcentral Alaska will remain the norm. A jet streak on the
backside of an upper level low over Southwest Alaska on Tuesday
will help amplify the longwave trough across the state of Alaska.
This will keep cyclonic flow across the region with colder air
being advected south which should keep increased instability and
rain showers in the forecast for much of the state. There could
be a few breaks with some shortwave ridging across Southwest
Alaska over the weekend but zonal flow will return to the state by
late Sunday and allow for another unstable pattern to develop at
that time. Afternoon high temperatures look to remain below
normal with low temperatures across the region remaining near
normal. The majority of the Bering Sea will be under the influence
of high pressure come Tuesday with low stratus and fog. By late
Friday, a strong low will push off of Russia and into the Bering
Sea which will force the high southward into the North Pacific
and a rather wet and windy period will be observed across the
Bering next weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Warning 121 125.
MARINE...Gale 119.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...MC



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