Small Craft Advisory 395, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
547 PM AKDT Tue May 15 2018

.ANALYSIS AND UPPER LEVELS...
The upper level low that has brought cold air and mixed
precipitation to southwest Alaska is pulling in southerly flow at
the surface as it moves northward. This southerly flow is bringing
a long fetch of moisture into Kodiak and the eastern Kenai
Peninsula. Colder temperatures aloft under the upper level low
are producing an unstable environment over the mainland, as seen
in many of the morning soundings. Accordingly, radar indicates
that precip around the region is falling as showers this morning.
Out west, a quick moving shortwave over the central Bering is
heading south causing a rain/snow mix for the Pribilof Islands and
eastern Aleutians. Ridging over the western Bering is continuing
to build in with warmer air.

&&

.MODEL DISCUSSION...
Models are in good agreement in the short term, although there are
differences in timing as shortwaves move around the upper low. A
North Pacific low is approaching the Alaska Peninsula on Friday is
where models begin to differ. The GFS and GEM are in good
agreement for placement with the NAM to the west, while the EC
struggles with the position of the low. Models are also in less
agreement with a low nearing the western Aleutians Friday. The GFS
and NAM are in consensus with the GEM lagging and the EC between
these other models. The GFS has also shown better run to run
consistency with this system leading to it being preferred out
west.

&&
.AVIATION...
PANC...Turnagain Arm winds are expected to pick up this afternoon
as a trough approaches from the southwest. Although showers may
move through at times with various shortwaves, predominantly
easterly flow aloft should cause VFR cigs/vis to prevail for most
of today. The possibility of MVFR conditions increases toward the
end of the TAF period as flow aloft becomes more southerly and the
precipitation moves more towards steady rain, but this was left
out of the TAF for now with differences in how much rainfall reaches
the Anchorage Bowl.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
Expect a continuation of recent weather as we head through the
next couple days, with fairly cool temperatures, extensive cloud
cover and fairly widespread rain from Kodiak/western Gulf on up
to the Kenai Peninsula, Anchorage and the Mat-Su. First, for this
evening, a short-wave rounding the upper low over Southwest
Alaska will combine with the weakly unstable environment to keep
isolated to scattered showers going for much of the region through
this evening.

The air mass will then stabilize tonight through Wednesday as a
warm front over the Gulf lifts northward to Southcentral. Rain
will spread across the eastern half of Kodiak island this evening
and on up to the Kenai Peninsula overnight. The rain will then
try and advect northward up Cook Inlet on Wednesday. Low level
flow on Wednesday will be out of the southeast across the Kenai
and western Chugach Mountains, though flow from around 7000 feet
on up will be out of the south to south-southwest. With primarily
low level processes producing the rain it seems the downslope
flow will tend to keep things mostly dry for Anchorage and the
Mat Valley, though a few sprinkles or a brief period of very
light rain is certainly possible.

An mature low pressure system over the North Pacific will then
send a frontal system northward Wednesday night and Thursday,
with a good southerly fetch of moisture. While the low level
southeasterly flow will continue across the Kenai and Chugach
Mountains, this system looks more impressive, so expect all the
most populous cities to see a period of rain, though it will
remain on the light side. The heaviest rain will fall across
Kodiak Island and the Gulf coast.

Looking out to Friday, an upper ridge will nudge in from the
east, pushing the frontal system and associated rain westward
toward Southwest Alaska and the eastern Bering Sea. Expect
decreasing clouds in the Copper Basin, extending westward toward
Anchorage and the Mat-Su by late in the day. With some sunshine
and a warming air mass this likely will be one of the warmer days
so far this spring. In particular, it looks like the Copper River
Basin will see high temperatures in the 60s, with potential for
the typically warmer areas in the heart of the basin to approach
70 degrees.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
The upper level low that has brought cold air and showers to
southwest Alaska continues its track to the north. As it exits,
weak ridging will move over the area ahead of a warm front that
will bring more rain to southwest Alaska Thursday. After the
front passes light rain and weak easterly winds will linger
through Friday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
The fast moving shortwave that is pushing through the central
Bering will bring rain and snow showers to the eastern Aleutians
into early tonight before tracking off into the North Pacific.
Northerly flow will continue behind this feature due to ridging
that has built in over the western Bering. The ridging will also
block a front that is tracking toward the western Aleutians from
bringing anything more than a light rain to the islands Wednesday.
The ridging will break down on Friday allowing for a new low to
track into the central Bering from the east.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast begins Friday as a persistent upper level ridge
over Western Canada and the Yukon becomes undercut by an upper
level trough moving from the North Pacific into the Gulf of
Alaska. This will act to tilt the ridge NW into Southcentral
leading to decreasing cloud cover and warming temperatures during
the day. High temps on Friday look to top out in the upper 50's to
low 60's in Anchorage and the Mat-Su while portions of the Copper
River Basin should see temperatures approaching the 70 degree
mark as mostly sunny skies are expected there.

This first taste of summer will be short lived as the trough
continues to under cut the ridge acting to push the ridge axis
further east into Canada allowing an upper level low to build back
over the southeastern Bering and Aleutians. This will bring a
return of cooler, cloudier, and wetter weather to the southern
half of the state this weekend into early next week as several
systems move through the region. Past mid-week, models become
muddled regarding the placement of specific features but most show
a persistent upper level low over the Gulf and Northern Pacific. There
are some hints of this feature weakening and breaking down
allowing more seasonable weather to take over by the end of next
week, just in time for the Memorial Day weekend. Stay tuned for
details!

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 178.

&&

SYNOPSIS AND MODEL DISCUSSION...ALH
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...KVP