Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 240207
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
607 PM AKDT Fri Oct 23 2020

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low in the southern Bering Sea is slowly
moving northeastward. The upper level ridge has been pushed to
the AlCan border leaving southwesterly flow aloft over southern
Mainland Alaska as well as the Gulf of Alaska and eastern Bering
Sea. The trough the Bering Sea low resides in digs southward to
around 35N 170W where satellite shows a developing low. This is
mentioned as this low will move along the front connecting that
area to the Gulf of Alaska (as seen on GOES-17 imagery) and is the
low that will be the main weather maker for early next week. In
the near-term, the radar over Southcentral has shown many hours of
20+ DBZ over the area yet very few reports of precipitation
though early afternoon. This is due to the very dry lower
atmosphere which is evaporating the falling precipitation before
it reaches the ground. The 00Z weather balloon launch in Anchorage
may give a good indication as to whether that lower layer will be
able to saturate and allow snow or rain to reach the surface, or
continue to evaporate it all.

&&

.MODEL DISCUSSION...
The good news is that the models are in very good synoptic
agreement into early next week and remain in very decent
agreement even into the latter parts of next week. As is often the
case, the small details are what will make or break the forecast.
This is the story for the northern Cook Inlet region Sunday into
Monday and particularly for Anchorage and surrounding areas. This
issue is partially where the surface low tracks along the north
Gulf coast, but the bigger issue is how far and fast the cold air
is able to move east of the Alaska Range. This will be the driver
in precipitation type over the area with a wide array of outcomes
possible. More on this in the short term Southcentral discussion
below.

&&

.AVIATION...
PANC...VFR conditions and northerly winds will persist into
Saturday. Ceilings may drop into the 3500 to 4500 ft range at
times but are not expected to go lower than that.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

An active weather pattern is in store for Southcentral this
weekend. A front currently over the southwestern Gulf will slowly
lift northward tonight. Some light precipitation is already
filling in along the northern Gulf Coast as the front gradually
approaches. Easterly to southeasterly winds will ramp up over the
northern Gulf waters this evening, especially over the Barren
Islands and Kamishak Bay.

The greatest challenge with this forecast has been precipitation
type. While there is some cold air in place across Southcentral,
a very warm, moist airmass associated with this front lifting
northward will bring rain across the northern Gulf Coast. A mix
of rain and snow is possible over the western Susitna Valley along
the foothills of the Alaska Range through Saturday. By Saturday
evening, a second system in the Gulf lifts northward and will
spread in more precipitation across Southcentral. Given continued
southerly flow and warmer air, the northern Gulf Coast will see
rain. However as this low gradually shifts toward the northeast
Sunday into Monday, colder air on the backside will slowly filter
down across the Susitna Valley, introducing the potential for snow
early Monday. By Monday afternoon, the low center remains in
place somewhere in the vicinity of Prince William Sound. Though
models have come into better overall agreement with the
progression of this low, they do continue to struggle with the
exact track. Stay tuned.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Friday
morning through Sunday morning)...

The longwave and shortwaves associated with the upper low will
trigger rainfall over much of the forecast period, with snow
accumulation up to 2 inches over Kuskokwim Valley tonight and
Saturday. The surface temperatures will differ slightly across the
region between tonight and Saturday night as south-southeast
winds advect warmer air from the marine area. Otherwise, colder
temperatures aloft and persistent influence of the upper low will
keep much of the region under a wet spell the next couple days.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Tonight through Monday)...

The gale force low over the central Bering will weaken as it
moves toward Pribilof Islands tonight. The strongest side of the
low lies north of Adak and Kiska, which will usher gales there
tonight. As for the rest of the region, winds and wave heights
will gradually subside through the weekend in response to the
weakening of the low pressure system. The next frontal boundary
from the northwest Pacific will brush near the Western Aleutians
on Monday morning. Depending on the parent low, the future motion
of the frontal system is unclear at this point while models
suggest it will stall over western Aleutians throughout Monday.
Overall, it will be a quiet forecast period once hazardous weather
subsides starting tonight.

&&

.MARINE (Days 3 through 5: Monday through Wednesday)...

Gulf of Alaska:

There is medium confidence that a barrier jet set up across the
northeastern Gulf coast will continue through Monday. Differences
in models exist over the strength of the winds associated with
this feature and its extent. For now, up to gale force winds are
possible from Prince William Sound east. Besides this, forecast
confidence decreases with an area of low pressure from the north
Pacific. At this time, the majority of model guidance indicates
broad southwesterly to westerly flow from the southern Gulf into
southern Alaska on Monday. However, small craft to storm force
winds are a possibility, including continuation of the barrier jet
from the day before. The winds strength and direction will depend
on the track of this low, which is currently showing a potential
path either up along Kodiak Island and Kenai Peninsula waters or
northwest towards eastern Prince William Sound. These intricacies
will be better determined with future model runs. This low looks
to remain just south of Prince William Sound and begin weakening
Tuesday before moving to the east and dissipating by Wednesday.

Bering Sea:

There is medium confidence that a front will move across the
western Aleutians through Tuesday. What models are still
struggling with is if winds will be below or above small craft
criteria. The associated low center could possibly enter the
western Bering on Wednesday, however, guidance is all over the
place with this, as well. If it does enter the Bering, it brings
a possibility of small craft winds into the central Bering
depending on the strength of the low. If it stays to the west of
the Bering Sea, winds and waves will be below small craft through
the forecast period.

&&

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

Model agreement increases for the surface low approaching the
Northern Gulf on Monday. Multiple embedded shortwaves pass through
the Gulf towards the Southeast coast early next week, with
moderate to heavy precipitation expected for the Southcentral
coasts. This is expected to be rain except for the higher
elevations with will see some snowfall accumulation. Additionally,
there is better model agreement for the arctic trough pushing
south across the Mainland mid-late next week, bringing colder
temperatures to Southcentral and Southwest Alaska. Temperatures
are likely to be below normal for both regions by mid-week.

There are still significant model discrepancies for the
North Pacific surface low approaching the Aleutians. The system
seems to be increasingly impactful, with the only similarity being
that it stays just south of the Chain. The Canadian matches well
with the ECMWF on strength but places the low much further
northeast than both the ECMWF and GFS. GFS is much deeper than
the other two, but is more cooperative with the ECMWF`s path.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning: 130.
Gale Warning: 119, 120, 131, 132, 141, 177, 351, 352, 411.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...CK/KM



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