Small Craft Advisory 314, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
519 AM AKDT Tue Apr 17 2018

.ANALYSIS AND UPPER LEVELS...
Two dominant features stand out on water vapor satellite this
morning. The first is the remnants of a front stretching from the
Seward Peninsula southeastward through the Gulf of Alaska,
ultimately connecting to a compact upper-level low pressure in the
north Pacific. The other is a mature, stacked low pressure system
over the central Bering Sea. The jet stream is running parallel to
the Aleutians and is beginning to shift southward as the Bering
system moves eastward, we can expect no further development of
this front and low. Those two features will control the active
weather in the short term, but another feature will come into play
as well. A shortwave trough is dropping southward along the Al-Can
border, the net effect of this feature should be to stop the
northward progression of the two fronts approaching the north
Gulf coast.

From a sensible weather perspective, the first front over the
southwest mainland is rapidly falling apart this morning, only
leaving an area of light rain from Kodiak through Aniak. Middle
and high clouds in advance of the front are draped over
Southcentral. The front associated with the Bering low is
currently over the Alaska Peninsula, bringing moderate southerly
winds to the area along with rain.

&&

.MODEL DISCUSSION...
Synoptically, guidance is in pretty good agreement through
Thursday, or longer for that matter. High resolution guidance was
utilized for this forecast package. The general theme between
guidance is developing a basal shortwave trough as it interacts
with the front approaching the Gulf of Alaska.

&&

.AVIATION...
PANC...VFR conditions and generally light winds will persist. Turnagain
Arm wind should make it into the airport this afternoon as
warming in the Susitna Valley pulls it northward. However, a
down-Inlet gradient will simultaneously be developing, which will
try to pull the wind the other way. Either way, some light to
moderate southeasterly winds can be expected today.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA...
The remnants of the front that has been slowly weakening over the
Western Gulf over the past few days will become reinvigorated by
an upper trough moving into the Eastern Bering. This will not
cause winds to increase to gale force as it develops a barrier jet
along the North Gulf Coast, but also will drag precipitation that
for the most part has been limited to Kodiak and the Southern
Kenai Peninsula lately northward into Prince William Sound. It
still appears that temperatures will remain just cool enough to
support snow down to the road system of the Kenai Peninsula but
with just a few inches of accumulations expected, while the coast
should remain as a rain/snow mix and minimal accumulation. For the
lee side of the Chugach from the Kenai Peninsula northward into
the Anchorage Bowl/Matanuska Valley, widespread and strong cross-
barrier flow and very dry low levels will provide for widespread
downsloping, and instead will bring gusty winds through Turnagain
Arm. For the most part these winds should remain away from the
lower elevations of the Anchorage Bowl, with the exception of a
few hours this afternoon/early evening when gusty southeasterly
winds will briefly move into town.

This front will then stall out over the North Gulf Coast into
Wednesday ahead of yet another Gulf low that will develop under
broad cyclonic flow aloft, leaving much of the area under a
persistent weather pattern that looks to last through the day
Wednesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Today through Thursday)

Rain chances will be on the increase over the next 24 hours.
Currently, an area of showers persists of an Egigik to Aniak
line, in an area of weak left courtesy of a leftover inverted
trough (the remnant cold front that stalled and washed out 2 days
ago). However, another low slowly churning towards the Pribilof
Islands is sending another front towards the coast of Southwest
Alaska. This front will make landfall tonight while slowing its
progression on Wednesday and occluding as frontolysis begins on
Wednesday along a Nunivak Island to just northeast of King Salmon
line. Widespread precipitation will accompany it for this evening,
becoming more showery in nature on Wednesday. Relatively warm
temperatures in the lowest layers of the atmosphere will mitigate
any snowfall concerns, with the best potential being at the onset
near the coast tonight, and at the higher elevations overnight.

As the low continues to fill on Thursday, the remainder of whats
left of the Bering low will get absorbed by a developing Kodiak
low. With the decrease in ascent and the region becoming further
removed from the best upper-level dynamics, showers coverage will
dwindle, with perhaps some lingering activity possible through the
evening for locations east of a King Salmon to lime Village line.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days through 3)...
(Today through Thursday)

A slow-moving area of low pressure continues its trek towards the
Pribilof Islands, with the associated surface front on the
doorstep of False Pass and Cold Bay. The parent low is about to
begin a filling process, as the system is expected to occlude/fill
over the next 24 hours. As such, decreasing dynamics will allow
for precipitation to become less in coverage and intensity, with
the same holding true for the winds. Still, some gale force winds
are likely near the front through the evening, and along the
southern/southwestern periphery of the low through Wednesday, as
additional vort maxes drop southeast across central portions of
the chain and its adjacent waters.

Heading into Thursday, the weakening low will move southeast of
the region which will really put the ki-bosh to shower potential
for much of the area, especially by evening as weak ridging builds
across the region. Seas and winds will remain near small craft
criteria through the evening near the central Aleutians, with no
headline criteria expected for the remainder of the chain.

&&

.LONG TERM FORECAST (Thursday through Monday)...
Southeast flow and wet weather will continue on Thursday across
the Gulf coastal areas, with persistent cloudcover across interior
areas and parts of Southwest Alaska. On Friday and Saturday, an
upper low will start to dig down from the northeast across the
mainland, which will pull shower activity inland across Cook Inlet
and perhaps even into Southwest Alaska as the upper low drops
into the longwave trough that will be persisting over the eastern
Bering. A storm on Sunday will approach the state and, depending
on the strength of the aforementioned upper low, will start to be
pulled northward toward southern Alaska. There is still quite a
bit of uncertainty with where this low will end up, but its likely
the Gulf Coastal areas will see another round of steady rain
develop on Sunday and continue through the start of next week.
Overall, the forecast for most of the state is fairly cloudy with
near-normal temperatures, which will be a change from the endless
sunshine Southcentral has had recently.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 155 165 170 172 173 174 175 176 177 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...AD