Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 291259

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 AM AKDT Mon May 29 2017

There is an upper level low centered over the Bristol Bay zone and
the western Bering sea, with a ridge of high pressure centered
between them over the Eastern Bering and Alaska Peninsula. There
is a rather strong and large ridge over northwest Canada extending
southward into the Pacific Northwest. At the surface, a negatively
tilted front with plenty of moisture remains situated over the
northern Gulf and continues to track northward. This feature is
evident in both satellite and radar imagery, with observations
continuing to report rain over the Northern gulf coast and Prince
William Sound areas and showery conditions around the Cook inlet
region and Susitna Valley. However, most of the rain for this
system is falling along the coastal Mountains, with the inland
areas remaining more "down-sloped" and thus fairly dry. There is a
Large vertically stacked low over the western Bering Sea, centered
just north of the western Aleutians, with an associated front
tracking through the central Aleutians/Bering.


The numerical models are still struggling a bit with the pattern
over the Gulf and mainland Alaska. There is decent agreement on
the surface pressure pattern, but there are still some decent
differences in the timing and amounts of precipitation expected in
the next 24 hours. A middle ground (a "poor mans ensemble")
approach was used to handle the uncertainty.


PANC...VFR conditions will likely persist through Tuesday morning.
Some gusty Turnagain arm winds will briefly make their way into
west Anchorage this morning before the gradient switches to up-
inlet flow and takes over by the afternoon.


There will be one more day of cool and wet conditions across
Southcentral before a drier, sunnier, and warmer pattern quickly
settles in over the next couple days. First off, an upper level
low and negatively tilted trough over the northwest Gulf of Alaska
this morning will continue to slowly progress northward. Steady
rain out ahead of it, which is widespread across all but the
Copper River Basin this morning, will transition to showers in
most areas by afternoon. The most unstable air will be near the
upper low center, from the western Kenai northward to Anchorage
and the Mat-Su, where there could be heavy showers. There may be
just enough instability to support isolated thunderstorms inland,
but it is very marginal.

Showers will quickly diminish from south to north this evening,
with clouds beginning to break up in the Cook Inlet/Kenai
Peninsula region. A lingering upper trough will keep at least a
few mid to high clouds in the region overnight. For areas which
do clear out it will be another very chilly night, with potential
for the typically colder spots to bottom out near freezing.

Low level offshore flow will develop Tuesday and persists through
wednesday. With abundant sunshine for all but perhaps the eastern
Copper River Basin, this will lead to rapidly warming temperatures,
with widespread temperatures in the 60s on Wednesday. The one big
exception will be Kodiak Island, which will likely see a marine
layer move in from the east tonight or tomorrow, leading to
cloudier and cooler conditions.


A pattern change will unfold over the next few days. To start off
though, showers and afternoon/evening thunderstorms will continue
through today. But ridging will start to build in from the south
by late in the day, so showers and thunderstorms will focus over
the higher terrain near the Kuskokwim Valley and the Alaska Range.
Thunderstorms are once again expected to be very spotty in nature.

Model agreement has increased that a fairly stout ridge will build
in for the middle of the week. This will bring widespread sunshine
and temperatures much more typical for late May. While a shower or
two can`t be completely ruled out over the mountains Tue
afternoon, with temperatures warming aloft (and heights rising),
convection will be very limited.



The closed low over the Western Aleutians will sit and spin
to start the week while the ridging over the Eastern Bering will
yield to some wet weather from the North Pacific. The low
pressure system north of Shemya will continue to bring some
small-craft winds and steady rain to much of the area. A
reinforcing shot of energy/moisture from the North Pacific on Tue
will bring more rain associated with this system to the Central

Over the Eastern Aleutians and Alaska Peninsula, ridging will
slide off to the north allowing a weak system to bring some rain
to places like Cold Bay and Sand Point by early Tue. This will
open the door for the front out west to slide eastward into the
area. As it does so, it will bring increasing southeast winds and
chances of rain.


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

The long range forecast beginning Tuesday night through Thursday
is expected to trend toward dry conditions inland with periodic
breaks in the clouds as a transitional ridge pattern builds behind
the exiting upper low. Meanwhile, a weakly organized low over the
southern Bering tracks along the Aleutians to the Alaska
Peninsula bringing light rain as it moves through. Northerly flow
along the Bering on the backside of this low cools the air mass in
its wake.

At the end of the week, there is high uncertainty in how models
evolve the low pressure system moving near the Alaska Peninsula.
This is largely due to the models currently struggling with how
to handle energy that races ahead of the low center, rounding the
base of the trough near the eastern Gulf. Models typically
struggle with how fast they can break down a ridge, which in this
case, will be stretched along and east to west axis from the Yukon
territory to Southcentral. The inherited forecast was updated
with WPCs ensemble blend approach with a higher percentage of the
ECMWF used mid week as it had better consistency during the
latest runs.





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