Area Forecast Discussion
Issued by NWS Anchorage, AK

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469
FXAK68 PAFC 110154
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 PM AKST Tue Dec 10 2019

.ANALYSIS AND UPPER LEVELS...
The low that affected southern Alaska yesterday is located near
the Bering Strait this afternoon and another low can be seen well
south of the Gulf of Alaska. Therefore, most of southern Alaska
is under weak high pressure between these systems.

A large upper level low is centered over the Bering Sea and has
a trough that extends over the AK Peninsula and then south of
the Gulf of AK. The Upper level ridge is over the northern AlCan
border to British Columbia. This pattern is keeping Southern
Alaska under Southeasterly flow aloft, though not as strong as it
was yesterday. The jet stream is mainly running west-to-east along
the 35N to 40N parallels. However, it does not look like it will
amplify much over the next few days which will limit the amount
of strengthening and northward movement of the low referenced
earlier that is south of the Gulf of Alaska today.

&&

.MODEL DISCUSSION...
Model guidance continues to be in very good agreement through the
week. While small differences in track and timing can be pointed
out, they should not have significant impact on the forecast and
large scale changes to the forecast are not anticipated.

&&

.AVIATION...
PANC...Fog and stratus will be the challenges for the PANC TAF
overnight. While fog developed late this morning, it is very
shallow and mostly localized to the Anchorage area. Conditions
could worsen after sunset. However, there will be an increase in
low level flow overnight, which looks like just enough to limit
fog potential. Thus, believe conditions will generally improve
after this evening. If it turns out flow is not strong enough,
shallow fog could linger longer than advertised in the TAF.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tuesday
night through Thursday)...

Weather is calming down with exit of the low pressure system last
night. However, the break will be brief as the overall weather
pattern of broad low pressure over the state will send another
low north into the Gulf of Alaska tonight. This low will approach
the coast Wednesday morning, providing another round of rain and
wind for Southcentral AK. However, it will be substantially
weaker than the previous low and will not work as far inland. This
means winds won`t be as strong and it will be limited to just
coastal areas, with more inland locations (like the Mat-Su) not
receiving any precipitation at all. This low is already evident on
satellite tracking north in the Pacific, so the forecast looks to
be on track. There is also good model agreement with this low, so
we are confident in the forecast.

Another area of emphasis today is the clearing weather that will
allow inland temperatures to fall, especially for the Copper River
Basin. Northerly winds and cold air advection will aid temperature
decrease. Therefore, dropped temperatures in the forecast today
with teens overnight in the Copper River Basin and cooling to
single digits for lows by Friday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

An upper level disturbance is dropping some scattered showers on
parts of Southwest Alaska this afternoon. This feature comes
ahead of a north Pacific low that enters the Gulf of Alaska on
Wednesday night. A generous amount of warm air advects in with
this system and will kick off as rain over southwest Alaska, with
snow expected over the Kuskokwim River Valley. The return of cold
air at the surface from northeast winds will create the possibility
for some light snow and freezing rain on Wednesday in the
Kuskokwim Delta. Precipitation will diminish Wednesday night into
Thursday as the low pulls away in the Gulf.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

Low pressure tracked out of the northern Bering into Siberia and
left a wide band of west/northwest winds west of the Pribilof
Islands. Satellite imagery shows a broad area of scattered showers
in the western Bering. This showery regime should continue
through Thursday west of the Pribilofs. A strong low is expected
to approach the Western Aleutians on Thursday evening. A north
Pacific low will approach the Gulf of Alaska Wednesday morning and
will bring rain to the Alaska Peninsula and eastern Aleutians on
Wednesday and Thursday.

&&

.MARINE (Days 3 through 5)...

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

A rapidly deepening area of low pressure will move from the north
Pacific on Friday to near Atka by Saturday morning. This feature
will send a frontal boundary into the south central Bering Sea
during Saturday morning, with the low bottoming out near 960 mb.
Some sustained gales will likely accompany these features, with
gusts just short of storm force likely, especially out of bays and
passes. This system then moves off to the east southeast by Sunday
afternoon.

Given a long fetch length, look for seas to build into the 20 to
25 feet range south of the Aleutian chain during the forecast
period. For the remainder of the region, winds look to remain
below gales with seas less than 15 feet expected.

...Gulf of Alaska...

An area of low pressure will move into the southwestern Gulf by
Friday morning, with sustained gales developing for the eastern
Kenai Peninsula coast...eastward towards the Copper River Delta.
Sustained gales are also likely across the southern Gulf. This low
then slowly reaches Kodiak Island Saturday while weakening, before
moving into southwest Alaska on Sunday while dissipating. As a
result, winds area wide will decrease for to small craft criteria
near the coast on Saturday, and remain below headlines for Sunday.

As for your waves, the look to be highest across the eastern half
of the outer waters, where they`ll peak in the 20 to 25 feet
range Friday, dropping further for the weekend.

&&

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

Models have come into better, though not perfect agreement for the
long term. Expect a continued active pattern over the Aleutians
through the end of the weekend, while quieter weather will persist
over Southcentral. Friday, a low in the western Gulf of AK will
continue to swing a series of weak fronts toward the North Gulf
Coast, bringing periods of precipitation through the end of the
weekend. Precipitation will be concentrated on southeast facing
slopes, particularly on Kodiak Island, the Kenai Peninsula and the
Alaska/Aleutian Range. Meanwhile, surface high pressure over the
Yukon and interior AK will strengthen, and conditions will trend
colder over inland areas of the Southern Mainland. Out west, a
strong low south of the central Aleutians will swing an occluded
front across the chain and generate a healthy round of
precipitation over the Aleutians and likely into the Southwest
Mainland Saturday night into Sunday. Precipitation chances look
highest in the Bristol Bay area, with the Kuskokwim Valley/Delta
likely staying drier in offshore flow. However, it`s hard to have
much certainty this far out and with run-to-run model inconsistencies.
Another low will approach the western Aleutians Monday, bringing
another round of precipitation and active weather.

In short, expect temperatures to trend colder through the long
term, especially for interior areas. With each system, precipitation
will progress from the Aleutians toward Southcentral.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 119 120 130 131 132 136 137 138 139
                      150 155 178.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...BB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...LB
MARINE/LONG TERM...PD/MM



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