Area Forecast Discussion
Issued by NWS Anchorage, AK

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110
FXAK68 PAFC 131244
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
444 AM AKDT Sun Oct 13 2019

.ANALYSIS AND UPPER LEVELS...

High pressure over interior Alaska with relative low pressure over
the Bering and the Gulf has resulted in nearly clear skies over
almost all of southern Mainland Alaska this morning. The clear
skies and long nights are resulting in falling temperatures
across the area. Few areas away from the immediate coast are still
above freezing, with 20s widespread over Southcentral and interior
Southwest Alaska. An approaching North Pacific low is spreading
high cloud cover over Bristol Bay, Kodiak Island, and is slowly
encroaching on the southern Kenai Peninsula. These high clouds as
they are, are not able to even threaten any precipitation. Thus,
dry weather continues across the area.

Closer to the North Pacific low, centered well south of Sand
Point, rain is being drawn northwestward from the parent low due
to favorable upper level winds. Thus, from the Alaska Peninsula
over to Unalaska Island, light rain is moving into the area. A
separate front is moving across the western Aleutians, with rain
overtaking the islands around Shemya. This is in advance of the
extratropical low that was Typhoon Hagibis. Associated winds will
begin impacting the western Aleutians around midday today.

In the upper levels, a weak rex block has developed with a
relative upper level high over the Y-K delta, with an upper level
low near Dutch Harbor. A very strong 150 kt jet streak over
Kamchatka is beginning to impinge upon the western Bering.

&&

.MODEL DISCUSSION...

Some uncertainty as to the track of former Typhoon Hagibis
persists in the models this morning. While its initial track has
been well agreed upon in the models for a couple days now, where
the center of the low ends up beyond Monday has been in constant
flux. The center of the rapidly strengthening low crosses the
western Aleutians late this afternoon, entering the Bering. Most
of the models continue to show the center turning towards the
north before it reaches 170W. How quickly that turn occurs is very
much in doubt, and is adding significant uncertainty to the
forecast. The models order themselves by Monday night by how
strong the low is at that time. The NAM is strongest and has the
low center near Kamchatka. Meanwhile the Canadian is weakest with
almost no turn depicted by the model. Thus, the Canadian has the
same low a bit north of St. Paul Island at the same time as the
NAM has the low on the coast of the Kamchatka Peninsula. The two
global models are about in the middle. Fortunately, regardless of
which of these tracks the low ends up following, the impacts over
land will be minimal with any of them.

Elsewhere, another area where there is some uncertainty in the
forecast is the depth of cold air over the Copper River Basin by
Monday night. Not only would that have an obvious impact on the
temperatures in that area, but more importantly, how strong the
winds are through the Matanuska Valley, and through
Valdez/Thompson Pass. Otherwise, quiet weather is highly likely to
persist over nearly all of mainland southern Alaska. Thus,
forecast confidence is dependent on ones location. For the Bering,
confidence is lower than normal, while for most of mainland
Alaska the confidence is very high.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

High pressure remains over Southcentral and the Gulf of Alaska
and should persist over Southcentral the next few days. Over the
Gulf, a front will push into this ridge and bring gale force
winds into the Gulf tonight into Monday. Due to the strength of
this blocking ridge over the mainland, the winds along the coast
will increase with the tightening presure gradient, but the
precipitation should stay over the Gulf (and Kodiak) until at
least Tuesday.

This ridge will also keep clear skies over the inland areas of Southcentral
until some higher clouds move inland Tuesday. This will bring in
colder temperatures the next two nights with numerous areas
reaching into the teens and perhaps some single digit lows in the
Copper River Basin.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
The forecast pattern commences with ridging over western Alaska.
Mild weather expected today and most of Monday for southwest
Alaska. A strong system will be pushing through the Bering on
Monday. The leading edge of the precipitation shield will move
into the west coast Monday afternoon. Expect moisture to increase
with gusty southerly or southeasterly winds.

&&

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

The main event will be the extratropical transition of Hagibis
and the ensuing re-strengthening of the surface low over the
Bering as cold air gets entrained. By 00z Monday, both the GFS
and ECMWF have a 985mb/984mb low south of Attu Island and rapidly
moving into the Bering by 06z Monday and deepening to 975mb/976mb
respectively. By 18z Monday, the GFS and ECMWF have the surface
low west of the Pribilof Islands with surfaces pressures of
956mb/959mb. The pressure gradient forces over the Bering will be
formidable most of the forecast period. Storm force winds and
hurricane force winds will materialize over the Bering. Rough seas
and widespread precipitation anticipated Monday and Tuesday.

&&

.MARINE (Days 3 through 5)...

Bering Sea/Aleutians: Post-tropical cyclone Hagibis will begin to
fill and exit to the northern Bering Tuesday. Thus, while storm
force winds will initially be widespread in the Bering (with a
broad swath of gales elsewhere), winds will diminish during the
day. While models have come into better agreement with the track
of former Hagibis, there is still a lot of uncertainty with what
comes after. The main theme is that either a short wave or a
closed low will develop near the western Aleutians late Tuesday.
This feature will track eastward across the Bering just north of
the Aleutians late Tuesday into Wednesday. Increased winds are
expected, but the strength will depend whether a closed low
forms...so for now, the range of possibilities spans from low end
gales to storm force winds. As the cold air mass behind the
associated front moves across the warmer SSTs of the Bering,
instability will increase thus isolated thunderstorms look
possible over the southern Bering Sea Wednesday. Winds will
diminish Thursday and general cyclonic flow will prevail in the
Bering.

Gulf of Alaska: Easterly winds in the northern Gulf will diminish
Monday into Tuesday. The forecast in the Gulf becomes murky by
Wednesday. General low pressure is expected with winds mostly
below advisory level... however small craft advisory level swell-
driven seas will likely persist. Late Wednesday, an upper low
will cross the AKPEN then redevelop a surface low in the Gulf
early Thursday. High uncertainty continues with this setup, but
assuming this low comes to fruition, expect increased winds and
precipitation in the periphery of the low.

&&

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

The excitement in the long term continues to be focused in the
Bering Sea and Aleutians. As post-tropical cyclone Hagibis exits
to the northern Bering Tuesday, focus turns to what happens in its
wake. Model consistency remains poor, but the general theme is
that either a short wave or a deeper closed low will develop in
the baroclinic zone under a 160 to 170kt jet. It will traverse the
southern Bering (just north of the Aleutians) from west to east
and likely bring a brief return of gales to the Aleutians/
southern Bering. Isolated thunderstorms are possible behind the
associated front Wednesday as modified Arctic air flows over
relatively warm water.

Meanwhile, due to the track of the former typhoon, impacts over
the Southwest mainland look minimal. As the system shifts
northward, it will push a cold front across the Aleutians into the
west coast in the rough Tuesday to Wednesday time frame.
Precipitation will be possible along the west coast, and increased
southerly winds are expected. However, it`s worth noting that the
front will have significantly weakened by the time it reaches the
mainland. Thus, while the Kuskokwim Delta Coast could see gusts
above 40kt, warning level winds are not expected. In the Wednesday
to Thursday timeframe, precipitation in the Bristol Bay area will
be possible as the other low/short wave moves into the coast...
but confidence is poor given the inconsistency in that feature.

Friday into Saturday, longwave troughing will set in from the
central Pacific into the Bering. Expect cloudier weather and
general southerly flow over Southwest. Models indicate shortwaves
will likely rotate around the low bringing periods of unsettled
weather, but it`s way too soon to get into timing/location
specifics.

Over Southcentral, generally dry weather is expected through
midweek. High pressure will persist over northern Alaska, and
cold conditions inland with offshore winds will dominate through
midweek. With this scenario, increased winds are likely in the
usual gaps, including Thompson Pass and the Matanuska Valley. A
series of fronts will move from the Gulf northward. This will
potentially bring precipitation to the North Gulf Coast, but
generally expect precipitation to stay over the marine area.
During the later part of the week, the ridge will retrograde to
the north. A wetter, or at least cloudier pattern is possible and
overnight lows won`t be quite as cold as earlier in the week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Hurricane Force Wind Warning: 177, 411, 413.
Storm Warning: 173-176, 178, 179, 412.
Gale Warning: 120, 131, 132, 138, 150, 155, 165, 170, 172, 180,
  181, 185, 351, 352, 414.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
MARINE/LONG TERM...MM



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