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fxak68 pafc 270040 

Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
440 PM akdt Fri may 26 2017

Analysis and upper levels...
an upper level low centered across Norton Sound, with a deepening
trough to the south will continue to be the key player in weather
for much of the state. The strong upper level jet across the
western Aleutian chain continues to slowly weaken and dive
farther south in the Pacific. At the surface below the upper
level low is a weak low that will move inland across Norton
sounds. Farther south across the western akpen, is a surface low
that deepens as the upper level trough deepens and stretches
south. Otherwise, a north to south oriented ridge of high pressure
spans much of the eastern Pacific and Gulf, nosing into the
eastern Copper River basin.


Model discussion...
models in the short term appear to be a good agreement.
Differences start to show after about the f30 hour (or 00z
sunday). In the upper levels, guidance struggles with how to
merge energy dropping south along the upper level trough, across
the Kuskokwim Delta and merging at the base of the trough that is
south of Cold Bay. It's as this energy maxima joins the secondary
maxima in the trough that models start to diverge. That then plays
into the handling of surface related elements moving forward. The
NAM, Euro and Canadian models have all merged on to a solution of
closing the upper level low out. The GFS fails to form any
consolidated low and is the faster solution moving it into a
weakening ridge across the eastern Mainland. The importance in
this difference is in how quickly the front moving with the
surface low pushes across southcentral, impacting timing/longevity
of strong winds and heavy (coastal) rainfall.


panc...gusty southerly winds will continue through this afternoon.
Some showers are possible again this afternoon/evening as well,
but VFR conditions should prevail.


Short term forecast southcentral Alaska (days 1 and 2)...

The very active weather pattern across southcentral continues this
afternoon. Widespread shower activity has developed in many areas,
including portions of the western Kenai Peninsula that did not see
much of anything yesterday. In contrast, while the showers are
still plentiful over the mountains, less instability, a slightly
drier atmosphere, and a different steering level airflow
direction have all contributed to the showers being much less
robust in their ability to generate precipitation. Isolated
thunderstorms remain possible today with the strongest showers,
but they aren't expected to be nearly as plentiful as yesterday.

Precipitation should quiet down for inland locations tonight, as
the focus shifts southward to a low moving into the Gulf from
near the Alaska Peninsula. An associated front will begin
spreading precipitation into Kodiak this evening, with it reaching
the coastal Kenai later tonight. The front will turn the winds out
of the southeast for all of southcentral, leading to strong
downsloping conditions in interior portions of the area, which
enhanced precipitation along the coast.

Heavy rain and mountain snow will intensify through the day
Saturday and continue straight through Sunday night for the north
Gulf Coast. Event total rainfall will approach 3 inches in
Seward, and more than 5 inches in the western Prince William
Sound. Further inland, downsloping will dry out almost all
attempts for any precipitation to reach the Matanuska Valley,
Anchorage, and the western Kenai Peninsula, though an isolated
shower or two can't be ruled out. Regardless of whether or not
there is any precipitation, extensive cloud cover through the
weekend will make for continued below normal temperatures. Those
same downslope areas will have to contend with an extended period
of gusty winds through Turnagain Arm and the Knik River Valley.
The winds will affect south and west Anchorage both this afternoon
and then possibly again on Sunday afternoon. However, higher
elevations on the hillside will see gusty winds through the


Short term forecast southwest Alaska (days 1 and 2)...
the Mainland will continue to see cloudy conditions with showery
rain for the weekend. Two upper level systems are combining; one
trough in the northern stream, another in the southern stream. The
end result is continued cold air aloft, which will promote shower
development, and a steady steam of moisture from the south. Most
of the showers will be diurnal in nature, the most coverage during
the daylight hours. However, as the low pressure from the south
gets captured by the northern system tonight, some steadier rain
can be expected from Dillingham westward along the mountains.
Winds will be generally light, except for some gusty easterly
winds coming through the gaps of the Aleutian Range. Expect
temperatures to top out in the low 50s with lows in the upper 30s
to low 40s.


Short term forecast Bering Sea/Aleutians (days 1 and 2)...
high pressure will encompass most of the Bering through the
period, promoting widespread low stratus. Two system are on the
periphery of the Bering, a low near Bristol Bay, mainly affecting
the Mainland, and a front toward the western Aleutians. The front
will slowly move eastward and dissipate through the weekend.
Little impacts expected as far as wind/rain with this one.


Long term forecast (days 3 through 7)...
beginning the extended forecast period Sunday evening, there will
be a dynamic shortwave trough lifting north from Kodiak Island
with an extensive upper ridge extending from the southwest United
States Mainland to the Yukon. Another, much lower magnitude
shortwave ridge will be across the Bering Sea. As is typical in
this pattern, there will be a frontal zone moving into the north
Gulf Coast and Prince William Sound along with extensive rain
along the coast. With a negative tilt to the upper trough, there
will be rather efficient cross-barrier downslope drying across the
coastal mountains and the Chugach Mountains. By Monday morning,
this shortwave trough will be lifting into the interior part of
Alaska, with a south to north drying trend through the day as
showers become more scattered.

Uncertainty develops after Monday as all global numerical models
simulate the development of a broad low developing over the North
Pacific then moving into the southern Gulf of Alaska. Here, the
details matter, and the European model (ecmwf) and Canadian gdps are depicting a
slightly farther south track than the more north GFS. The slower
solutions delay the progression of a front moving toward the coast
by an entire day and a half compared to the more bullish GFS.
However, the pattern by mid week is rather similar in all models
with extensive coastal precipitation with extensive clouds and
mountain precipitation inland. For now, the wpc approach was
favored (i.E., A blend of ensemble means which are better at
dealing with uncertainty and will take a more middle ground
approach). This means southern Alaska should anticipate a return
to cloudy weather by Tuesday, although with this downslope
pattern, expect temps to return back to normal if not slightly
above normal. For inland areas, while uncertainty exists, these
patterns tend to be rather dry and strong east to southeast upper
flow results in efficient downslope drying.


Afc watches/warnings/advisories...
Marine...gales...120 119 125. Fire weather...none.



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