Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 190048

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
448 PM AKDT Sat May 18 2019

The previous discussion noted closed upper level lows near Adak,
the Shumagin Islands, and Ketchikan. This afternoon the Adak and
Ketchikan lows have weakened as they get absorbed into the
Shumagin Island low which is now the main upper level low. This
is still an elongated feature putting southern Mainland Alaska
into easterly to southeasterly flow aloft.

An easterly wave is propagating across Southcentral in this
easterly flow this afternoon. While this wave is a lifting
mechanism, it is not creating as much lift as the unstable
atmospheric conditions from the last few days. The atmosphere over
Southwest Mainland Alaska is also more stable this afternoon.


Models are coming into much better alignment with the 12Z runs
from this morning. The main model issue in the short term is the
timing of easterly waves across Southcentral Alaska.

The synoptic pattern is all very similar into Tuesday and then
models diverge with the track of a fairly strong low approaching
the Gulf of Alaska from the southwest. The good news is that
models all have this feature coming from the same source region.
The issue is that on Wednesday afternoon there is a spread of over
400 miles between the ECMWF solution and Canadian solution with
the GFS sitting right in between. For now that GFS position and
strength looks very reasonable.


PANC...VFR conditions will persist. Southeast winds are expected
to kick in late this afternoon through the evening but are not
expected to be as strong as yesterday.

A frontal system associated with a low south of the Gulf will
rotate from the central Gulf north and west this evening to
stretch over the northern and western Gulf coasts. The front will
remain stalled in this general location as the surface low tracks
to the west to swings past southern Kodiak Island on Sunday.
Another easterly wave and associated surface trough rotating
through the eastern Gulf on Sunday will push into the northern
Gulf Sunday night followed by a building surface ridge on Monday.
This combination will bring a moderate surge of easterly winds
into Prince William Sound and through Turnagain arm late Sunday
night through Monday morning as well as southeasterly gap winds to
the Copper River Basin. The onshore flow will then diminish fairly
quickly as a weakening surface low track north through the western
Gulf Monday night and Tuesday. The next easterly wave will take a
more inland track, pushing from the Yukon Territory into the
Copper River Basin and Interior Alaska Tuesday afternoon or

afternoon through Tuesday afternoon)...

Showery conditions will continue across Southwest through early
next week. A Gulf low will move in from the east Sun morning,
bringing more widespread rain showers. Isolated thunderstorms
cannot be ruled out over the inland areas of the Kuskokwim Delta
and Middle Kuskokwim Valley where conditions would be most
favorable on Sunday afternoon. This showery and easterly flow
pattern will continue through Tuesday.


3/Saturday afternoon through Tuesday afternoon)...

Generally quiet conditions are expected across the Bering through
early next week. A weak area of low pressure will move into the
central Bering from the north Sunday bringing showers to the
Pribilofs. A second low moves into the western Bering Monday
afternoon, spreading in a warm front over the western and central
Aleutians. By Tuesday afternoon, a North Pacific low begins to
approach the eastern Aleutians.


.MARINE (Days 3 through 5)...
(Tuesday through Thursday)

...Bering Sea and the Aleutian Coastal Waters...

A Pacific low (currently forecast to be in the 980 to 990 mb) will
approach the east central Aleutians by Tuesday afternoon, bringing
increasing winds and seas potentially to the region. However, by
Wednesday, some guidance takes this low north into the eastern
Bering, while others eject it towards Kodiak Island.

Thus, while the potential is certainly there for small craft or
even sustained gales to be added to the forecast at some point,
uncertainty at this time precludes any of this being included in
the forecast. Otherwise, look for nominal conditions to persist
across much of the region through the period.

...Gulf of Alaska...

The forecast has changed some from yesterday, as the models have
come into better agreement. It now looks like a front will move
into the Gulf on Wednesday, with an associated wind shift.
However, winds look to remain below small craft criteria, with the
net effect/concern being offshore seas increasing towards 15 feet
Thursday. High pressure then regains control of the weather by
Thursday evening.

Forecast Confidence: Moderate


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

The forecast picture is still becoming a little less murky for
the long range portion, although it`s still pretty cloudy in terms
of details. The overall theme is for the guidance to have come
into decent agreement with regards to a Pacific low approaching
the eastern Aleutians/AKPEN by Wednesday morning, with an
attendant surface front swinging north and east. That said, the
agreement ends right there, as the ECMWF (European) wants to eject
the low towards Nunivak Island on Thursday, where as the GFS
(American) has it just east of Kodiak Island, with the Canadian
Global following the GFS ides albeit further southeast.

Which track this storm takes will have obvious impacts on
temperatures and precipitation potential/amounts. One example of
this is the forecasted 500 mb heights, and the more western
European track would allow for heights being a good 60 to 80 dkm
(decameters) higher, with a corresponding increase in the 850 mb
temperatures. Heading into the weekend, the GFS (with support
from the Canadian) and European models disagree even more with
regards to where the ridge and trough axis set up, with one
showing the almost exact opposite of the other.

The net effect is it does look like precipitation potential will
be on the increase for Southwest and Southcentral Alaska,
temperatures will be highly dependent on which of the patterns
develop, as upstream effects over the western Bering and Aleutians
having a big effect and similar temperature concerns (although the
maritime effects help reduce this variance greatly).





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