Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 171238

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 AM AKDT Thu May 17 2018

The pattern aloft is beginning to look a little different now that
the previous upper low has weakened and moved northward. Southern
Alaska now lies on the edge of two different long-wave features.
The first is a north Pacific upper low, and the second is a
Pacific/North American ridge. Out west, the long-wave pattern is
looking a little blocky as a closed high is centered over the
western Bering. The southern stream jet remains active with a
progressive upper low skirting just south of the central Aleutians.

The southerly flow aloft has brought a couple of weak fronts over
and near the southern mainland. The first is responsible for
sprinkles or very light rain over the Cook Inlet region and is
rapidly falling apart, while the second is a little bit stronger,
making landfall on the southern Kenai early this morning. Winds
across the southwest have turned northerly in response to the
approaching low/front, while Southcentral AK remains in fairly
weak diurnal gap flow.


Lots of moving parts with this forecast. Early this morning,
guidance shows five distinct upper low centers, three along the
jet in the Pacific, and two in the northern stream from the Bering
Strait through the AKpen. By the weekend, all guidance combines
all five low centers into one complex, sprawling low just south of
the AKpen. Despite the complexity, there`s great agreement with
the evolution of the pattern.


PANC...VFR conditions and light winds will persist.


A weak deformation zone continues across Southcentral today. It
will likely get a little juicier with the intrusion of moisture
from a weakening frontal trough heading north into the Gulf
today. The brunt of the rain will continue along the coasts, but
will likely spread inland this afternoon and early evening and
with it with the frontal trough. Rain will diminish significantly
inland this evening with trough passage as weak upper ridge moves
in. Though, broad southeasterly flow in the lower elevations will
likely keep chances of light rain along the coasts through the
overnight hours. Some brisk winds will be likely across Turnagain
arm and through the Knik River Valley this afternoon and evening.

The next, in what seems like an endless series of systems, heads
northwest across the Gulf toward Kodiak Friday afternoon through Friday
night. It will likely bring renewed rains to the the western and
northern Gulf. Inland areas will be largely downsloped, so
precipitation potential will be fairly slight. Winds are expected
to increase across much of the Gulf. Should see a significant
increase in gap flow across Turnagain Arm and through the Knik
River Valley Friday night.



The main event this forecast period will be the low from the North
Pacific moving northward and widespread moisture advection across
southwest Alaska. On the latest scan from the King Salmon radar
(PAKC) there are returns from Port Heiden to Levelock. Expect this
moisture to continue to push northward. The thermal profiles
suggest this will be an all rain event. The trough axis aloft
doesn`t even reach the AKPEN until Saturday morning. So expect two
solid days of wet and cloudy weather for most of the region.



The latest ASCAT/Advanced scatterometer pass has a dome of high
pressure west of St Matthew Island and the Pribilofs. There are a
pair of surface lows in the north Pacific just south of the
chain. One is south of the Shumagin Islands and the other is south
of Attu. The storm near the AKPEN will track north this forecast
period bringing a warm front into the Eastern Aleutians. A deep
fetch of moisture will glide across the eastern portion of the
chain today and tomorrow bringing rains. A third storm will
develop near Kamchatka and rapidly intensify. This storm is
projected to bring possible gale-force winds south of the
Aleutians Friday afternoon and Saturday.


.LONG TERM FORECAST (Days 3 through 7)...
Beginning Saturday, there is increasing confidence that our
overall pattern will transition to a more progressive one as the
persistent upper level ridge over Western Canada and Southeastern
Alaska will move east allowing an upper level low to reposition
itself over the Eastern Bering. This will allow several systems to
rotate into the southern mainland over the weekend and into next
week. Strong southeasterly onshore flow associated with these
systems will favor coastal areas for precipitation, while the
typical lee-side locations will downslope out and remain mostly
dry, though seeing continued cloudy and cooler conditions.

By mid week, a ridge will amplify over the Eastern Aleutians and
Bering and creep east. Though model solutions are widely varying
at this point, there is some hint that the ridge may displace the
upper level low far enough east to push the main area of on-shore
flow towards SE Alaska, bringing a bit of a break to Southcentral
and the SW mainland. Some model solutions, though, show this
ridge quickly breaking down as another strong low barrels into the
the area from the west. Either way, it appears the persistent
pattern of cooler, cloudy, and unsettled weather that has plagued
the southern mainland for nearly the past month will remain for
the foreseeable future.


MARINE...GALE 130 131 178.



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