Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 240033
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
433 PM AKDT Mon Apr 23 2018

.ANALYSIS AND UPPER LEVELS...
While a couple upper level lows Fujiwara around each other in the
Bering Sea, a short-wave trough is digging across the North
Pacific, with arctic air advecting in behind it. This is leading
to downstream amplification, with what had been a zonal jet streak
(west to east) rapidly on its way to becoming meridional (south
to north). The Blended Total Precipitable Water product shows much
moister air beginning to surge northward toward the Alaska Peninsula.
The amplified pattern will lead to widespread impacts from wind to
heavy precipitation across the southern mainland of Alaska over
the next couple days.

Currently over the southern mainland a series of upper level waves
along with a dying occluded front moving onshore of Prince William
Sound is producing widespread cloud cover with areas of rain (and
some higher elevation snow).

&&

.MODEL DISCUSSION...
Models remain in very good agreement with large scale features,
with excellent run-to-run consistency with many of the details
of the storm system about to affect the southern mainland. The
primary forecast challenge is determining strength of winds on
Tuesday in Anchorage and vicinity. The NamNest is notably
stronger with winds than any other model and is largely being
disregarded. Forecast soundings and model cross-sections show
the NamNest initiates strong mountain wave activity and ducts
very strong winds down to the surface. The atmosphere generally
supports unstable lapse rates in which the highest winds would
escape upward, but some stronger winds would mix down to the
surface. We just don`t think it will be nearly as strong as
NamNest indicates due to the lack of a critical trapping level in
this unstable environment. The latest forecast seems like a nice
middle ground (and reasonable solution) among the guidance.

One other notable forecast challenge is precipitation-type over
the Susitna Valley Tuesday through Tuesday night. It`s always a
challenge this time of year because of the affect of solar
radiation on warming surface temperatures. Models are actually
pretty close with the thermal profile above the boundary layer,
but are all over the place in the boundary layer. Based on the
strength of upslope flow and resultant heavy precipitation will
go heavily toward colder guidance, particularly for the higher
elevations of the northern Susitna Valley (north of Talkeetna).

&&

.AVIATION...
PANC...Southeasterly winds will be the big forecast challenge over
the next 30 hours. Turnagain arm winds will continue to bend into
the airport through early this evening. After that the core of
the Turnagain Arm jet will shift back southward a bit, so winds
may briefly trend down during the late evening. However, overall
tightening pressure gradients combined with low level instability
suggest winds will continue to steadily strengthen overnight
tonight through Tuesday morning ahead of frontal system moving in
from the west. More uncertainty exists during the day Tuesday with
potential for mountain wave activity in the Anchorage area. It
does look like the winds will peak Tuesday afternoon just prior to
the cold front moving through with gusts to around 40 kt with LLWS.
Unstable air and deep southerly flow behind the cold front will
maintain gusty south winds, though they will be weaker than the
peak winds observed Tuesday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An active week is in store for Southcentral Alaska with a number
of unusually strong systems for this time of year rumbling
through. The first one is bringing rain to the north Gulf coast
today with just scattered showers inland. the exception could be
the Copper River Basin where satellite shows a bit more lifting
in the eastern half of the Basin. Winds will increase again
through the typical gaps this evening and then crank up a few
notches tonight through Tuesday. A strong front will push into the
western Gulf of Alaska and up Cook Inlet tonight which is
associated with a large low in the eastern Bering Sea. A
significant warm push aloft will be ahead of this front with some
colder air moving in behind it Tuesday night. This front will
bring strong southeast winds into the Anchorage area and Matanuska
Valley and strong south winds along the Copper River. Winds will
make it into the Anchorage bowl as they bend northwestward out of
Turnagain Arm. It has been a while since winds this strong have
made it into town, and especially at this time of year. The
strong winds will mostly keep the area dry with the best shot of
rain for Anchorage and the Matanuska Valley late Tuesday afternoon
through mid evening as an upper level wave moves through. There
is also some questions as for snow amounts Tuesday afternoon into
Tuesday night for areas north of Talkeetna. Talkeetna will be on
the borderline for snow, but areas north of there should be able
to get mostly snow by Tuesday night with perhaps a number of
inches of accumulation.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An upper level low over the Bering Sea will continue to fuel an
active weather pattern over southwest Alaska. A series of
shortwaves will get kicked out of the low and push into southwest
Alaska ushering moisture into the region. The vertical temperature
profiles are sufficiently complex, some areas will see rain, some
snow and others will see a mix. Additionally, the jet core will
nose its way into southwest tonight and tomorrow which will result
in strong winds at the surface and in the boundary layer. Winds
will be strongest through channeled terrain including through the
Kamishak Gap and out across Lake Iliamna. As the storm careens
into southwest, long gale force fetch will materialize over
Kuskokwim and Bristol Bays. High surf with water levels at or
slightly above highest astronomical tide levels is likely around
the time of high tide Tuesday afternoon and early Wednesday
morning.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
There is a large upper level low centered over the entire Bering
Sea. There are multiple shortwaves embedded in this low. The
latest ASCAT/advanced scatterometer pass has detected gale force
winds near Amchitka, north of Shemya and also west of St. Matthew
Islands. Broad cyclonic flow will dominate the region the ensuing
36 hr period resulting in more blustery and wet weather. As the
surface low slowly pushes eastward, a dome of high pressure south
of Adak will bring weak ridging into the Western Aleutians later
Wednesday. However, the latest Himawari satellite image has
another low in Eastern Russia which will keep that ridge from
amplifying too much.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long range forecast beginning Tuesday night will be in an
active pattern as a broad Bering low spins multiple shortwaves
around its perimeter which spans across the state and surrounding
waters. Onshore flow supplies abundant moisture for showers to
develop with the embedded shortwaves as they move around the low
center which brings higher precipitation chances for the southern
mainland through mid week.

Late Thursday, the Bering low is kicked eastward as the next low
races up from the North Pacific to the western Bering. Models are
in good agreement on the timing of this storm reaching the
central Aleutians early Friday morning; however, they diverge on
the timing and track of the system thereafter. The main challenge
for models is resolving a ridge that builds into the southern
gulf ahead of the next approaching low. The ECMWF had the best run
to run consistency and also had good continuity between its
operation and ensemble solutions for the extended range forecasts.
Overall, only minor updates were made for second half of the
forecast as there is low confidence concerning the track the track
of the Bering low.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 101.
MARINE...Gale 119 120 130-132 139 150 155 160 165 170-178 180.

&&

$$

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


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