Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 281257

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 AM AKDT Sun May 28 2017

Water Vapor and IR satellite imagery continues to show a deep
stream of moisture advecting north across the Gulf of Alaska
between an upper low center southwest of Kodiak Island and high
pressure along the northeast Pacific. There is a trough extending
over the western Bering/Aleutians from eastern Russia, with an
upper low centered over the Western Bering. Between the two
troughs is some weak upper level ridging over the central and
eastern Bering/Aleutians.

At the surface, cloudy conditions persist during the overnight
and morning hours as the above mentioned moisture stream continues
to flow over southeast Alaska. Radar imagery and observations
show persistent rainfall along the northern Gulf coast/Prince
William Sound area, western parts of Cook Inlet, and over the
southern Kenai. Whereas areas along the lee side of the Chugach
mountains remain downsloped and on the drier side. Gusty southeast
flow during the overnight hours was observed through turnagain
arm and the Knik River Valley, as well as down-inlet small craft
gusts over Cook Inlet and a small craft winds over Prince William
Sound and the northern Gulf. Some showery conditions over the
southwest Mainland have persisted overnight and into the morning
as the upper low near Kodiak shifted northward slightly. Further
west, a low over the western Bering and associated front
transitioning from the western Aleutians/Bering to the Central
Aleutians/Bering continue to bring rain and gusty winds to the
area this morning.


The main challenge in the forecast through Monday remains the
timing of the front moving from Kodiak Island and into the
southcentral Mainland. Models are in good agreement with the
general synoptic pattern through the short term. However there
are some minor differences still on how the low pressure system
near Kodiak lifts north today through Monday, and when the
precipitation will begin to taper off over parts of southcentral
Alaska. Only minor changes were made as trends continue to bring
the main frontal passage across Southcentral late Sunday night
into Monday morning.


PANC...VFR conditions and light winds will likely persist
throughout the day today. There is some uncertainty regarding the
turnagain arm gusts making it into west Anchorage again today.
Conceptually chances seem less likely with the stronger down-
inlet gradient, however a small increase to the instability over
the area can cause them to pull into west Anchorage again today.


An upper level low and negatively tilted trough will lift northward
across the western Gulf today, keeping the area of best upward
vertical motion, moisture advection, and low level upslope flow
centered over the eastern Kenai Peninsula/western Prince William
Sound. Thus, expect the steady rain (heavy at times) to continue
from Seward up to Whittier. The northward progression of the low
will also spread heavier rain into eastern Prince William Sound
and keep rain going in the Homer/Kachemak Bay area. Snow levels
remain quite low, generally 1500 to 2000 feet where precipitation
has been falling. Thus, could see some wet snow mixed in in the
hills near Homer this morning along with some light snow
accumulation through the top of Thompson Pass near Valdez.

Strong southeast flow and stabilizing conditions (as temps aloft
warm) will keep most inland locations dry. The exception will be
the western Susitna Valley where upslope along the Alaska Range
will produce some light precip. This will change tonight through
Monday as the low continues northward, passing west of Cook
Inlet. This will shift flow to a more southerly flow, spread deep
moisture inland, and destabilize the mid/upper levels once again.
The sweet spot for precip will be along the eastern periphery of
the low center from Cook Inlet to the Susitna Valley. Rain and
showers will spread northward overnight tonight into Sunday
morning, with the most likely location of steady rain in the
Susitna Valley. Expect some breaks of sun to develop in the
afternoon as the low exits northward, but with a moist and
unstable atmosphere scattered showers will reform and persist
into the evening.

Meanwhile, rainfall will diminish or taper off completely through
the day Monday for the eastern Kenai and Prince William Sound
regions. The widespread small craft winds across the northern
Gulf and Prince William Sound will rapidly diminish tonight
through Monday morning.

In the wake of the upper level low Monday night, there will be
partial clearing (at a minimum). With light winds this will be
another cold night. For areas that clear out completely
temperatures will likely drop to near freezing.


Cloudy, cool, and showery conditions will persist today before
some clearing and drying starts late Monday. A cold upper level
low just west of Kodiak this morning will track through the area
today. Ongoing showers over inland portions of Bristol Bay will
spread north and increase in coverage through the day as the low
moves overhead. The upper level low will help to increase
instability and provide a forcing mechanism for some more isolated
thunderstorms this afternoon and evening. This low will also help
to keep a southeast to northwest gradient over much of the area
(as has been the case the past few days). This will allow the
favored gaps to remain gusty once again today.
By Mon, the upper level low will lift to the north of the area by
late in the day. As it moves into the Lower Kuskokwim Valley and
the Delta, it will center the thunderstorm potential over these
areas, especially along higher terrain. By Mon evening, skies
will start to clear over Bristol Bay as winds diminish. This will
set the stage for Tue to be one of the sunnier, warmer, and drier
days than the area has seen for some time.


A tale of two weather pattens persists over the Bering. In the
east, a sharp ridge of high pressure is keeping conditions calm
with some marine stratus. The marine stratus has been compressed
overnight and some places from St Paul over to the Alaska
Peninsula will see some patchy fog and light drizzle today and
tomorrow morning. Out west, a tightly wrapped low and associated
warm occluded front dominate the scene. The low itself has some
gale- force wind gusts around its center. Along the front,
southeasterly small-craft winds become westerly as you get near
Kiska. Light to moderate rain will continue to accompany the
entire system as it more or less stalls over the area. The ridge
to the east is halting any real progression to the pattern. A new
piece of North Pacific energy will move quickly from south to
north on Monday. This will bring rain to Dutch Harbor and help to
displace the ridge.


.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 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.


MARINE...Gale Warning...130.



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