Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 251251
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 AM AKDT Fri May 25 2018

.ANALYSIS AND UPPER LEVELS...
A broad upper level trough is in place over much of the state
this morning extending as far south as the North Pacific. The
trough axis is oriented from the northeast to the southwest with a
stacked low anchoring the trough over Dutch Harbor. The low is
weakening as it moves southeastward into the Gulf, bringing
showers to the eastern Aleutians and AK Pen. Behind this trough, a
ridge is building in over the Bering, bringing northerly winds to
the central and eastern Aleutians.

For Southcentral, a weak surface low in the northern Gulf is
delivering rain showers to the coastline. Surface observations
across Anchorage show that winds have generally trended down
overnight. However gusty southeasterly flow will return later in
the day as the associated front moves inland, with the areas on
the lee-side of the Chugach range experiencing drier conditions
due to downsloping.

&&

.MODEL DISCUSSION...
Guidance remains in good agreement with the position and timing
of the low over Dutch Harbor entering the Gulf tonight.
Disagreement arises later into the weekend as the low moves into
the northeastern Gulf. The NAM appears to be the outlier with a
more easterly track. As a result, forecasters relied more on the
GFS/EC solution. Out west, models are in good agreement with the
building ridge over the Bering. Forecasters trended toward higher-
resolution models for guidance on the northerly winds building
over the Aleutians, though extensive edits were made to account
for known gap-winds and other areas with terrain-induced flow.

&&

.AVIATION...
PANC...VFR conditions will persist through the TAF period. Gusty
winds along turnagain arm subsided during the early morning hours,
and will pick back up again this afternoon/evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A weather system over the northern Gulf will push inland and
dissipate later today. This will bring another round of rain and
showers to coastal locations through early Saturday, mainly over
the eastern Kenai and Kodiak Island. Some showers are expected
over the inland mountains today and Saturday, with some diurnal
enhancement. Inland valley locations will remain mostly dry,
although a shower or two could drift off the mountains. A thermal
trough dropping south from central Alaska Saturday will bring a
slight chance of thunderstorms for the Copper River basin and the
northern Susitna valley. Southeasterly gap winds will return this
afternoon and evening for Turnagain Arm, south and west Anchorage,
Knik River Valley, and perhaps the Copper River Basin. These
winds will be a little less than those experienced on Thursday,
as the gradient is not quite as strong.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The persistent easterly flow will shift more northeasterly today
as the upper level low, now south of the eastern Aleutians,
tracks east into the southwestern Gulf. For Southwest Alaska this
will mean the return of the thermal trough as it starts building
south along the western foothills of the Alaska and Aleutian
range. Instability will be fairly marginal but should be
sufficient for a few isolated thunderstorms to develop along with
the usual scattered afternoon and evening showers. The thermal
trough will become better defined on Saturday, bringing an
increased likelihood and coverage of thunderstorms to Middle
Kuskokwim Valley and northern Greater Bristol Bay area. On Sunday
the upper level ridge shifting into the eastern Bering will put
Southwest Alaska into somewhat drier northwesterly flow aloft and
shift the thunderstorm threat east, to be confined to the Alaska
Range.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Upper level ridging will continue to build over the western Bering
as the elongated upper level low stretching from the eastern
Aleutians across the Alaska Peninsula swings to the southeast
today and then tracks away to the east and into the southwestern
Gulf tonight and Saturday. The ridging will build east tonight to
cover the entire Bering by Saturday. Northerly flow will increase
across the eastern Bering and through the eastern Aleutians and
Alaska Peninsula this afternoon through Saturday before turning
more northwesterly but persisting into Sunday as the upper ridge
shifts east so that its axis is centered over the eastern Bering.
A weak frontal system will approach the western Aleutians on
Saturday, then strengthen as it continues east Saturday night and
Sunday as developing North Pacific low strengthens and tracks
northeast towards the central Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast beginning Sunday starts with a weakening
surface low over the eastern Gulf of Alaska being pushed east
into the Panhandle as an upper ridge builds in over the western
and central parts of mainland AK. By Monday, the ridge axis looks
to be centered right over Southecentral setting the stage for a
decent Memorial Day with mostly dry conditions, clearing skies,
and slightly warmer temperatures than seen over the past week.
Some diurnally driven convection will likely develop over the
higher terrain in the late afternoon hours as warmer temperatures
inland will help develop a thermal trough kicking off showers and
even some possible isolated thunderstorms.

Looking past Memorial Day, models are coming into better agreement
regarding the next system moving in from the Bering, with a front
extending out from a low center over the Eastern Bering moving
across the West Coast by Monday evening into Tuesday morning. The
front does look to weaken some as it moves further east towards
Southcentral during the day on Tuesday as it runs into an upper
ridge axis extending south from the Arctic. The GFS tries to keep
the front more intact as it moves east while the EC rapidly
weakens the system. Regardless, there is consensus in a return to
cooler and wetter weather next week as the upper low builds back
in over the Bering and Western Alaska.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...KO/TP
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JER
LONG TERM...KVP



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