Small Craft Advisory 327, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
445 PM AKDT Sat Apr 21 2018

.ANALYSIS AND UPPER LEVELS...
The two main players today are a northward moving trough over
Southwest Alaska/Southcentral and a developing surface low and
associated front approaching the Southern Gulf/Kodiak Island. The
first of the two features is producing widespread cloud cover and
shower activity across the Gulf and Southern Mainland. Surface
pressure gradients across Southcentral have increased and given
rise to gusty gap flows along the Copper River, Turnagain arm,
and over the Knik Valley. The second of features is bringing rain
along the Alaska Peninsula and over Southern Kodiak Islands.
Easterly winds are also increasing over the Southern and Western
Gulf in advance of the frontal trough and developing surface low.

Generally cloudy conditions with some areas of light rain continue
across much of the Bering Sea and Aleutians. Conditions are
becoming more showery over the eastern Bering Sea as an upper low
retrograding over the area combined with low level cold advection
increases instability over the region.

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.MODEL DISCUSSION (through Monday afternoon)...Overall, good
synoptic agreement.

&&

.AVIATION...
PANC...VFR conditions with showers this evening. LLWS potential
will develop overnight with the surface winds shift to the north
and winds above the surface remain southeasterly.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Sunday and
Monday)...

The scattered shower activity over much of the Cook Inlet region
this afternoon will diminish tonight with the loss of solar
heating. Broad southeasterly flow will keep the precipitation
ongoing through the southern Kenai Peninsula and Prince William
Sound as the winds are upsloped over the Chugach and Kenai
Mountains. A front currently entering the southern Gulf will push
northward, resulting in gale force easterly winds and locally
heavy rain through Kodiak, the Barren Islands, and across much of
the rest of the Gulf tonight. The Knik River Valley and Matanuska
Valley winds will continue overnight, moving southward along the
Chugach down Knik Arm.

On Sunday, the front will move into the northern Gulf and the
north Gulf Coast, reinvigorating the rainfall and higher elevation
snow throughout the day. The front's presence will locally diminish
the off-and-on showers during the day in favor of a steady rain.
With the increased southeasterly flow, the Cook Inlet region will
return to downsloped conditions with mostly cloudy skies and
channeled gap winds. As the front moves into the coast, stronger
northerly down- Valley flow will push the Turnagain Arm winds to
the Kenai Peninsula side of the Arm. However, the Matanuska Valley
wind will contribute to the northerly winds blowing through much
of Anchorage during the day, especially along the Knik Arm. The
next low moving into the southwestern Gulf will restart the
steadier rainfall across Kodiak Island late in the day. A 40 kt
barrier jet developing ahead of the front early in the day will
persist along the north Gulf Coast into Prince William Sound
throughout the day.

By Sunday night, the low over the southwestern Gulf will weaken as
it moves northward off the Gulf coast side of Kodiak Island. The
barrier jet will persist throughout the night, along with heavy
driving rain through Prince William Sound. This will also support
the continuation of the Turnagain Arm and Knik/Matanuska valley
winds. The low rapidly weakens during the day Monday, resulting in
the weakening of the barrier jet. However, because the low turned
remnant trough moves into the southern Kenai Peninsula during the
day, the weakened downsloping will result in an increased chance
of some scattered shower development across the Cook Inlet region
and Susitna Valley, especially Monday afternoon and evening.
The remnant Turnagain Arm and other downsloped channeled wind may
keep Anchorage and immediate leeside communities dry.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Sat and Sun)...
The longwave pattern features a dome of high pressure over NE
Russia with a deep vortex over Western and NW Alaska. Some very
cold temperatures aloft are associated with this vortex over
Hooper Bay. Today both the radars at Bethel and King Salmon have
been very unstable with a report of small hail in Levelock. More
isolated to possibly scattered convection is anticipated this
evening. Sunday, this vortex move into the Eastern Bering and then
the path will recurve. This recurvature will then bring a more
organized band of precipitation into southwest. Additionally,
expect gale force winds in Bristol Bay as the pressure gradient
force intensifies.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

The latest Himawari satellite image shows a mature low near
Kamchatka and GOES-West has a mature system with an elongated
occlusion south of the Aleutians. There is a vast swath of cold
air strato-cu over the Bering along with some mid-level clouds.
The latest ASCAT (advanced scatterometer) pass depicts gales with
the storm south of the chain. For the low south of the chain, the
models propagate the main low eastward, but an inverted trough
develops tonight. A second piece of energy will move into the
region virtually marrying up with remnant energy from the southern
low. And by Sunday morning there is a new surface low near the
Pribilof Islands also producing gale force winds. Meanwhile,
upstream the Kamchatka low will continue to deepen as it pushes
towards Attu Island producing gale force winds for the Western
Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning Monday night a broad area of low pressure will continue
to dominate the Bering with its front having pushed through the
southern mainland bringing rain to the north Gulf coast and
southwest Alaska. Current model runs are in better agreement than
previous runs with a triple point low forming in the eastern
Bering Monday night, but there does remain some discrepancies with
placement of this feature. These discrepancies grow through
Tuesday as the triple point low moves into the northern Bering
while the parent low tracks east. Impacts will remain fairly
similar for the southern mainland and Aleutians even though
guidance is struggling with where exactly to put these two lows.
On Wednesday high pressure will build in from the west and the
models fall apart with how to handle low pressure over the Bering
with widely varying solutions. This resulted in ensembles being
preferred for the remainder of the week into the weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale warning 119 120 130 131 132 136 137 138 139 150 160
176 177 178 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
LONG TERM...DK