Area Forecast Discussion
Issued by NWS Anchorage, AK

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
981
FXAK68 PAFC 240337
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)...
The extended forecast begins Wednesday evening with the same
longwave pattern in place. A cold upper trough sits over the Bering
Sea with upper ridging to the east toward the Panhandle.
Between these two features lies a strong jet stream with a
tropical connection, currently preciptable water is near 2.3
inches in the northwest Pacific but will moderate as it moves east
and north.

As the associated front moves into the north Gulf coast late
Wednesday into Thursday, the surface pressure gradient will
respond, making for potentially strong winds through Portage
Valley/Turnagain Arm and the upper elevations of the Chugach.
Warmer conditions will accompany the system as strong cross-
barrier flow bring downslope conditions to the area. However, the
strong moisture tap could lead to some rain making it through the
dry low-levels. Strong winds will remain a threat through
Thursday.

Heading into the long-term, a sub-tropical system pushes up into
the Gulf, returning warmer temperatures and rain to much of the
area on Friday. Into the weekend and beyond...guidance really
struggles with the evolution of these sub-tropical waves phasing
into the longwave trough. Individual systems are not timed well
or agreed upon in the surface pressure fields. Uncertainty remains
high, but the active and warmer pattern will continue.
&&


.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...MTL



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.