Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 240122
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
522 PM AKDT Mon Oct 23 2017

.ANALYSIS AND UPPER LEVELS...
An elongated upper level low centered along the Kuskokwim Delta
has several short waves propagating about its center. One wave is
currently moving north across the Kenai with another downstream
approximately 300 miles south of Kodiak Island. Another low is
wrapping along the backside of this system diving south along the
eastern Bering. Ridging is separating this parent low from
another parent system spinning along the Kamchatka Peninsula.

At the surface, rain and snow is developing along a weak frontal
boundary swinging north along the Kenai. A few isolated
thunderstorms developed along this front as is pushed north across
the peninsula. This front produced a significant amount of snow as
it moved along the Aleutian Range with 9" measured at Iliamna
airport. Offshore flow continues across Southcentral with
southeasterly flow returning along the eastern Kenai. To the west,
extratropical system Lan is making its transition to the Bering
with heavy rain and gusty southeasterly flow gusting up to 60 mph
at Shemya this afternoon.

&&

.MODEL DISCUSSION...
Models are generally in good agreement with the overall synoptic
pattern, however they are struggling with handling the important
weak dynamic features for evolving the precipitation forecast
across Southcentral heading into mid week. A blend was used from
the operational models to update the afternoon package.

&&

.AVIATION...
PANC...A brief period of MVFR conditions is expected this evening
as the front along the Kenai moves north with snow along the
boundary. Another front moves through on Tuesday, however kept low
end VFR conditions at the end of the TAF package as the timing of
this next system remains uncertain.

&&

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

An upper-level wave/surface trough is progressing northeastward
up Cook Inlet and the Kenai Peninsula this afternoon. The compact
system will weaken as it moves to Anchorage this evening,
spreading some light snowfall over the Anchorage and Mat- Su
Valleys area.  Expect a quick shot of snow this evening that will
taper off overnight.

Attention then turns to the next upper-wave moving into Kodiak
Island tonight. This feature will bring snow to the Kenai
Peninsula by early Tuesday morning, moving north to the Anchorage
area late Tuesday morning/early afternoon.  At this point, there
is quite a bit of uncertainty as to how much snow will make it
into Anchorage and the Matanuska Valley Tuesday. Recent model
runs have shifted the track of the surface low further west,
taking the low inland over the Kenai Peninsula or tracking it
northeastward up Cook Inlet. This westward shift results in
stronger southeasterly flow over the Chugach Mountains and
Turnagain Arm, which would favor more of a downslope scenario over
Anchorage. The amount of snow that lower elevations of Anchorage
and the Matanuska Valley will see Tuesday is highly dependent on
the timing and magnitude of these southeasterly winds. With the
trend towards stronger southeasterly flow, keeping snowfall
accumulations fairly low in lower elevations.

The active pattern continues as a gale force front moves into the
southwest Gulf Tuesday night, then pushes to the central Gulf and
northern Gulf coast by Wednesday afternoon. Behind this front,
deep southwesterly flow will transport a significantly
warmer/moister airmass northeastward. Expect snowfall to
transition to rain Wednesday morning over the bulk of
Southcentral, with high temperatures approaching the low 40s by
Wednesday afternoon.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A weak upper level low to the north of SW AK will continue to pump
moisture into the area leading to scattered snow showers along
the Bristol Bay Coast tonight. Accumulations will be no more than
a few inches in most locations due to the spotty nature of the
precipitation. Showers will gradually taper off late Tuesday
morning before the next system moves in. A strong warm front
associated with a deep low out in the Bering will push ashore by
early Tuesday evening brining copious amounts of moisture and much
warmer air. Initially, precipitation will fall as snow but will
quickly transition to rain along the coast before midnight. Warmer
air will take a bit longer to reach inland locations leading to a
more prolonged period of snowfall before changing over to rain by
early Wednesday morning. Depending on the speed of the warm air
moving in, there could be some noticeable snowfall accumulations
along the upper Kuskokwim River Valley Tuesday night.
Precipitation should taper off by Wednesday afternoon though
persistent southerly flow behind the front will keep temperatures
much warmer than what was seen across the area last week.


&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A very active weather pattern is in store for the Bering and
Aleutians over the next couple of days. The remains of Tropical
Cyclone Lan over the Kamchatka Peninsula have combined with a
strong North Pacific jet to form the strongest low of season so
far in the Bering, with models deepening the central pressure to
935 mb as it approaches the Western Bering. Winds are already
gusting to hurricane force at Shemya and these winds are expected
to spread east across the Aleutians tonight and tomorrow. Though
the low will be weakening as it traverses east, strong southerly
winds associated with the warm front out ahead of the low will be
channeled through terrain gaps, keeping the potential for
hurricane force gusts along the northern side of the Eastern
Aleutians and AKPEN. Thus, high wind warnings have been issued for
the Western Aleutians through tomorrow morning and for the
Eastern Aleutians/AKPEN tomorrow morning through the evening.
Winds will remain strong on the backside of the low as the system
progresses east, though not as strong as with the warm frontal
passage. Colder air aloft will keep the threat for rain/snow
showers over much of the Bering through Wednesday.

.LONG TERM FORECAST (Days 3 through 7)...
An active pattern is expected to continue during the long range
forecast through the end of next week. The focus remains on an
upper level disturbance tracking over the northern Gulf and
Southcentral Alaska Tuesday evening into Wednesday, as well as
the remnants of Typhoon Lan tracking through the Bering and
Southern Mainland through the end of next week.

An upper level wave tracking further inland over Southcentral
Tuesday night, along with a surface low over the northwestern
Gulf, can bring another shot of snow for the southcentral area
through Wednesday. Highest snowfall totals are currently expected
along the Eastern Kenai and areas surrounding Prince William sound
Tuesday afternoon though Wednesday morning. Models are still
struggling with the overall timing and track of the low, but have
started to show a trend toward tracking it over the western
Gulf/eastern Kenai. Confidence is still not high in this scenario,
but if models continue to trend in this direction it will increase
the potential for the above mentioned snowfall to occur.

The leading warm front associated with the remnants of Typhoon
Lan will track eastward through the eastern Aleutians, Alaska
Peninsula, and Southwest Mainland Tuesday into Wednesday. This
will begin to usher in warmer temperatures over the eastern Bering
and southwest mainland by Wednesday morning, and build over much
of the southern Mainland through the end of the week. This system
will also bring some moderate to heavy precipitation to these
areas, as well as strong gusty winds. A mixture of rain and snow
is expected as the dominant precipitation type along the Aleutians
and Alaska Peninsula given the warming southerly flow and heavy
marine influence. However, the southern Mainland will start off
as snow Tuesday afternoon and transition to rain or a rain snow
mix along the Gulf and Southwest coast Thursday. Models remain in
good agreement through the later half of the week regarding this
system, but confidence begins to wane as the models begin
struggling more with the track and timing as the low center
tracks into the eastern Bering on Thursday.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 181 185 187 191.
         High Wind Watch 195.
&&

$$

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...RD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...TP


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