Area Forecast Discussion
Issued by NWS Anchorage, AK

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731
FXAK68 PAFC 090043
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 PM AKDT Mon Oct 8 2018

.ANALYSIS AND UPPER LEVELS...
A high amplitude upper level pattern is in place with a trough
over the Bering Sea and North Central Pacific and a ridge over
the East Pacific extending north-northwest across the Gulf of
Alaska and Southwest Alaska on up to the Bering Strait. A
vertically stacked low north of the Western Aleutian chain anchors
the trough. Multiple short-waves are rotating around this trough,
causing the trough to dig over the Pacific and the downstream
ridge to build - essentially amplifying the flow even further.
This is a perfect set-up for cyclogenesis, with rapid development
likely as surface lows reach the sweet spot between two upper
level jet streaks, just east of the base of the trough.

As far as specific features dominating today`s weather, an occluded
front is wrapped around the Bering Sea low extending northward
from the low center across the northern Bering then back due
southward across the eastern Bering on down to the eastern
Aleutians and southern Alaska Peninsula. Gale force winds and
moderate to heavy rain can be found along and ahead of this front.
A wave lifting northward along the front this morning produced
storm force wind gusts through channeled terrain of the eastern
Aleutians and Alaska Peninsula. However, the overall trend is for
weakening winds as the front continues to lift north and east.

Meanwhile, a surface low has developed on the leading edge of the
upper trough and is tracking northward toward Kodiak Island. The
upper support for this specific low is pretty weak, so do not
expect much deepening of the low center. Lastly, sitting
underneath the upper ridge, areas of stratus and fog formed over
Southcentral Alaska and Southwest Alaska overnight through this
morning. There has been some improvement along coastal Southwest
Alaska thanks to increasing winds and clouds ahead of the
approaching front. Otherwise, these areas are holding firm with
no signs of dissipating.

&&

.MODEL DISCUSSION...
Models have a good handle on large scale features over the next
few days. In particular, models have all trended toward a very
similar track inland across Southwest Alaska Tuesday night
through Wednesday. This will bring needed confidence to start
honing in on the areas of strongest winds and heaviest rain with
this system. Of course, with a lot of these local terrain driven
winds, there is a high sensitivity to storm track, so will be
looking for consistency in future model runs.

&&

.AVIATION...
PANC...Fog and stratus continue to linger near the terminal. Low
level flow will start to increase out of the southeast between
03Z and 06Z, so expect any remaining fog/stratus to be pushed off
to the west. A front will approach from the south on Tuesday, but
with strong cross-barrier flow expect downslope flow to maintain
VFR ceilings and dry conditions.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Areas of low stratus and patchy fog will persist along the Cook
Inlet and into the southern Susitna Valley through the evening
hours. After that, high and mid-level cloud cover will be on the
increase across Southcentral as the first of two surface lows
moves toward the region from the north Pacific. The first, and
weaker of the two, will approach the southern coast of Kodiak
Island tonight and continue a northward track into Cook Inlet for
Tuesday. The low and its accompanying occluded front will bring
widespread rain and gales for the western Gulf tonight and
Tuesday. The front stalls and weakens along the coast late Tuesday
with a weak barrier jet allowing for areas of gales to continue
along the coast. Rain will generally be confined to east upslope
areas. Locations in the lee of the Chugach Mountains will see
limited amounts of rainfall as a strong southeasterly flow will
aid in downsloping.

The second surface low, the remnants of former-typhoon Kong-rey
and the stronger of the two, will quickly follow on the heels of
the first as it nears the eastern AKPEN late Tuesday. By Tuesday
night, it will move onshore northeast of Kodiak Island. Another
round of rain will accompany this system; however, the brunt of
the rain will once again be along the coasts and east upslope
regions as strong cross-barrier (southeast) flow will keep areas
to the lee of the Chugach on the dry side. The bigger story with
this system will be the winds with southeasterly gap winds
increasing quickly tomorrow evening through Turnagain Arm and the
Copper River Basin, and the along the Anchorage Hillside. Gale and
storm-force winds will also develop tomorrow evening through
Shelikof Strait, the Barren Islands, and along the east coast of
Kodiak Island ahead of the front as the low continues to track
northeast. A barrier jet will also develop ahead of the front with
gales likely into Prince William Sound overnight Tuesday into
Wednesday. For Turnagain Arm and the Anchorage Hillside, the
strongest winds are expected late Tuesday night through the first
half of Wednesday just ahead of the front where gusts could
approach 70 mph. As the low tracks farther inland and the front
moves into the interior, the southeasterly winds will turn
southwest with gales continuing along Cook Inlet and over the
northern Gulf. The up-Inlet flow will also help to increase winds
over Anchorage beginning Wednesday afternoon through late
Wednesday night before diminishing as the low moves farther inland
and the pressure gradient relaxes.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Thursday night)

High pressure will continue to retreat to the east tonight as an
elongated occluded front sweeps northeast across the southwest
mainland. The front will bring rain with gusty easterly winds
overnight that will be accelerated through the terrain gaps along
the Aleutian Range. The front will shear apart as it becomes
detached from the parent low, allowing for improving conditions
later in the afternoon tomorrow, though showers will likely linger
through the entire day.

Riding rapidly behind the decaying front is a stronger surface
low enhanced with the remnants of former-typhoon Kong-Rey. The
storm system will approach the southern coast of the AKPEN early
Tuesday night at around 980 mb and track north along the spine of
the Aleutian and Alaska Ranges Wednesday morning. The tight
pressure gradient associated with the compact low will mean strong
winds across most of the southwest mainland, especially through
terrain gaps and passes on the lee side of the mountains with
easterly flow ahead of the low. Heavy rain will also accompany the
storm system, especially in upslope areas, with strong cross
barrier flow leading to some drying on the west side of the Alaska
Range. As the low pushes north of the region Wednesday afternoon,
the flow will become southwesterly up Bristol Bay and bring more
widespread gusty winds enhanced with decent cold air advection.
Winds could gust up to 40-50 mph along the southern part of the
Bristol Bay coast. Winds and rain showers will slowly diminish
Wednesday night and into Thursday before the next system moves
into the area by Thursday night.

&&

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

An area of low pressure will slowly drift east across the Western
Bering, with its associated surface front pushing eastward across
the Eastern Bering and Bristol Bay areas tonight. Gale force
southerly winds and heavy rain associated with the system will
taper over overnight, switching to southwesterly as the front
pushes through.

The next feature of interest will be the remnants of former-typhoon
Kong-Rey that will approach the Eastern Aleutians/AKPEN by
Tuesday evening. Widespread moderate to locally heavy rainfall
will develop along and ahead of this storm. A brief period of
storm force winds will develop east of the Shumagin Islands
Tuesday night with seas building to 20-25 feet.

Out over the Western Bering/Aleutians, the broad surface low will
track east acting to tighten the pressure gradient with the North
Pacific ridge to the south. This will lead to long fetch westerly
winds along most of the Aleutian chain through the whole period,
allowing seas to build up to 12 to 16 ft, especially on the Bering
side of the island chain.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The mid to long term begins Thursday with the models in overall
agreement with a low over the northern Bering Sea and a ridge of
high pressure extending from the Gulf of Alaska into the north
Gulf Coast. Moving into Friday we have a weak low, short wave that
gets caught in the strong southerly flow into the Gulf of Alaska
that rapidly moves to the north Gulf Coast by Saturday morning.
This will push the ridge of high pressure out of the Gulf of
Alaska into British Columbia. Meanwhile, out west, we have
another strong low moving into the western Bering Sea and
Aleutians late Friday night. The models are struggling to unify on
a solution as the storm track and forecast pressure have
significant spreads between the deterministic models and the
ensemble blend. Where the models are consistent is the location of
the front as it sweeps into the western Aleutians Friday night to
Saturday morning. As we move into Sunday the spread in the model
solutions continues to grow with significant differences in the
sensible weather depending on which solution is used. The ensemble
solution and WPC have a faster track to the east northeast than
the latest deterministic models. Therefore, we will maintain the
current forecast due to low confidence in any one.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 119 120 136-139 155 177 178 180-185.
Storm 130-132 150.

&&
$$

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...SA



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