Area Forecast Discussion
Issued by NWS Anchorage, AK

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678
FXAK68 PAFC 250543 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
843 PM AKST Fri Nov 24 2017

.ANALYSIS AND UPPER LEVELS...
A strong upper level ridge over the Bering Sea, combined with a
upper level trough centered over the eastern Alaskan border has
created the perfect scenario for windy conditions across most of
the state. In the west, a 150 knot jet moving into the base of the
trough has helped provide upper level support for these gusty
winds. This jet is also pushing a couple of shortwaves through the
pattern, which is bringing some light snow to the Kuskokwim Delta
coast this morning. Further east,the strong surface winds are
mostly being supported by an influx of cold air from Canada into
Interior Alaska. The effect of this cold air should not be
underestimated, as winds in the gap areas such as Thompson Pass
have reached as high as 81 mph early this morning. This has also
been augmented by a low level jet, which has caused many aircraft
reports of turbulence and wind shear around SouthCentral this
morning.

&&

.MODEL DISCUSSION...
The run to run model consistency has improved slightly after
taking a dive in the middle of the week. Today is not without it`s
challenges though; in the east, the position center of the Gulf
low varies from model to model. For todays forecast the GFS was
chosen as the most likely solution. The NAMNest seems to have
handled the outflow winds the best and was used over the land in
the east. However, out west, the NAM is the outlier, being the
fastest and the weakest on the next low expected to move into the
Bering on Saturday. The GFS and EC have come into better
agreement, but there is still some difference on initial strength,
although by Sunday they have come into greater alignment. Still,
with all the challenges and the inconsistency of previous model
runs, the confidence in the current forecast track of the low is
moderate at best.

&&

.AVIATION...
PANC...VFR conditions will persist through the TAF period.
Northwesterly winds are expected to move slightly offshore as the
low in the Gulf moves eastward, so the gusty winds are expected to
taper off tomorrow morning.

&&

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

Much of Southcentral continues to enjoy clear skies this evening
as weak high pressure over mainland Alaska keeps the clouds
associated with an area of low pressure over the Gulf off to the
east towards eastern Prince William Sound and the Copper River
Delta. Between these features, the winds remain quite gusty
through many of the bays and passes of Southcentral. These winds
will continue gradually diminishing as both the high inland and
the low over the Gulf weaken. The winds should be peaking now
through the Matanuska Valley where there too diminishing winds are
expected overnight. Despite being from an offshore northerly
direction, the winds should help to keep temperatures up in their
areas of influence tonight.

On Saturday, despite the Gulf low expected to shift back towards
the west, its continued weakening should allow the winds to go
slack in most areas by afternoon with another mostly sunny day
expected everywhere but the Copper River Basin and Prince William
Sound, where outflow from the Gulf low makes for a mix of sun and
clouds.

For Saturday night, the primary weather story will be the bitterly
cold temperatures as much of Southcentral, including Anchorage,
likely see their coldest night of the year. This will result from
a combination of light winds, clear skies, and a robust snow pack
in most areas. South and west towards Kodiak, the next warm front
will begin to make its approach. Snow will begin there as early as
mid-morning Sunday in Kodiak City, then quickly spread northward
up along all of the Gulf Coast and Prince William Sound through
the rest of the day. Along with the front will be warmer air
which should see most areas right along the coast transitioning to
rain, including Kodiak. Down-Inlet flow through Cook Inlet along
with east to southeast winds along the Gulf Coast will keep things
cool and dry on Sunday around Anchorage and the Mat-Su Valleys.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 though 2)... Gusty
northwest winds will diminish over the course of this evening as a
brief ridge of high pressure moves over the area. This will allow
for clearing skies and cooler temperatures to spread into the
area for tonight. This ridge will provide for a brief break in the
active weather for the day Saturday. A rapidly deepening Bering
low will then begin impacting the area on Sunday, with strong
winds and snow (possibly mixed with rain) along the coast. There
is some uncertainty in the low track as it moves through the
Eastern Bering on Sunday, leading to some question in how far
inland precip will reach as the front makes landfall. This is
impacting the uncertainty not only in how much snowfall is
expected over inland areas that should experience overrunning from
the cold air currently setting up, but also for the potential for
blowing snow/blizzard conditions as the front brings gusty
easterly winds. For now the forecast in these areas reflect more
conservative accumulations, with the heaviest amounts around
Bristol Bay where the front should linger the longest and just a
mention of areas of blowing snow. The potential for blowing snow
or blizzard conditions will be revisited as new runs come in,
especially considering the new snow pack that currently exists
around Bristol Bay.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The weather in the area will be dominated by a rapidly deepening
low that will move through the Bering Sea beginning on Saturday.
Model guidance has improved significantly with this low, with now
very good agreement with widespread storm (to possibly hurricane
force) winds and seas in excess of 40 ft developing around the
Southern Bering/Aleutians. There is also a strong possibility of
high winds in the Central/Eastern Aleutians as the low continues
to deepen and a PV anomaly and upper level support approach. As a
result High Wind Watches have been posted for the Central-Eastern
Aleutians beginning late Saturday for possible wind gusts as high
as 90-100 mph. These watches are set to end on Sunday evening,
however with a deep low continuing to spin just off to the north
widespread strong winds will certainly continue into Monday.

A secondary concern from this system will be with regards to
precipitation type as strong cold air advection develops behind
the low with gusty northwesterly flow. This will bring 850mb temps
as low as -8C to most areas, which should be cold enough to switch
most areas to at least a wet snow. At this point though
significant accumulations or blowing snow conditions are not a
concern with SST`s over 40F keeping the snow too warm/moist to be
lofted from the strong northwesterlies.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Overall, expect a trend away from windier outflow conditions, to a
warmer, more active pattern with more precipitation.

The forecast begins on Sunday afternoon/evening with a big,
pattern changing system in the Bering Sea. The surface low looks
to be peaking in the 940 mb range near the Pribilof Islands with
the potential to bring high winds to the Central and Eastern
Aleutians. The front associated with the system will swing into
the Gulf of Alaska on Monday, bringing snow to the north gulf
coast.

Getting into next week the pattern looks much different than the
current one. The Bering Sea storm will shift the longwave trough
over that area, bringing the active jet into the eastern Bering,
which will result in a new strong storm system crossing the Alaska
Peninsula on Wednesday. That storm track looks to bring deeper
south/southwesterly flow to much of the mainland for the remainder
of the forecast period. With that particular storm track, also
expect the potential for an Anchorage wind event in the Wednesday
timeframe.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 131
         High Wind Watch 187 and 185
MARINE...Storm 170-178, 180-185
         Gale 119, 120, 130-165

&&

$$

SYNOPSIS AND MODEL DISCUSSION...LF
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...MTL



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