Small Craft Advisory 334, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
510 AM AKDT Mon Apr 13 2020

.ANALYSIS AND UPPER LEVELS...

The longwave pattern features a ridge axis over Southeast Alaska
and extending into the Yukon Territory. There is a broad area of
troughing from the Pribilof Islands to Kamchatka. There is some
shortwave energy propagating through the ridge. However, the
ridge axis is robust and high amplitude. The top of the ridge is
actually north of 79 degrees north latitude. The radars at Kenai
and Middleton Island have been active overnight with the shortwave
energy that is approaching the Anchorage Bowl. Gale force winds
were detected south of Shemya with ASCAT (Advanced Scatterometer)
data early this morning. There is a 100 knot jet over Anchorage
and a 150+ knot jet core south of Attu Island.

&&

.MODEL DISCUSSION...
Models are in decent agreement through the middle of the week.
The timing of the precipitation shield up Cook Inlet and across
Prince William Sound looks reasonable with the GFS, NAM and
Canadian. However, there are some minor differences in the exact
timing of the precipitation. After this system passes the models
show a warming trend and many locations in southcentral can
anticipate temperatures near or at or slightly above 50 degrees
Fahrenheit.

&&

.AVIATION...
PANC...As of 9z, light rain and fog in the vicinity were observed
at PANC. The cigs will remain below 5,000 feet this morning with
improved anticipated later in the TAF package.

&&

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

The precipitation over Southcentral will be tapering off this
morning as the disturbance bringing in the rain and snow moves to
the northeast and out of the area. There has been some fog that
has developed in the Kodiak area and the northern part of Cook
Inlet this morning. The fog in these areas may be slow to
completely dissipate with the extra moisture at the surface from
rainfall yesterday and last night. However, temperatures will rise
enough today along with mixing from the winds to make it likely
for it to be gone by afternoon and went with that solution in the
forecasts.

A change in the morning forecast is that the precipitation from
the next system will probably be slower to arrive in Southcentral
than previous forecasts. Whereas it looked as though
precipitation would move up Cook Inlet late Tuesday into Tuesday
night. Due to a slower progress of the front, the north Gulf coast
will see the rain move in there late Tuesday or possibly even
Tuesday night for the eastern Prince William Sound area. For
Anchorage, some downsloping, will also keep the rain at bay a
little longer so it looks like it may be Wednesday or Wednesday
afternoon before it reaches the Anchorage area. This system is
even warmer than the one that just went through so even the
Susitna Valley should expect to see rain instead of any snow, at
least until one gets well into the Alaska Range.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Showers will continue to taper off this morning along the western
Alaska Range as low pressure exits the region. Upper level ridging
that moved to interior Bristol Bay has allowed the development of
fog from Cape Newenham to Dillingham, which should burn off later
this morning. The high pressure across Southwest Alaska will keep
a pause in any new precipitation before the next system moves in
later today.

The next front associated with a low lifting northward in the
eastern Bering will bring the next round of precipitation and
southerly flow to the Alaska Peninsula by this afternoon.
Temperatures above freezing will keep rainfall the dominant
precipitation type across Bristol Bay as the front continues to
lift northward this evening. This front will then bring the
warming temperatures and a mix of rain of snow, transitioning to
rain, to the Kuskokwim Delta and Valley by Tuesday morning. Expect
the wet pattern to continue through Wednesday as the subtropical
jet remains situated along the Southwest coast.

&&

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

The eastern Bering and Aleutians is seeing a brief break in the
active weather pattern as an upper level shortwave ridge persists,
while a front over the western and central Aleutians continues to
weaken early this morning. The next surface low will cross over
the Aleutian Chain into the Bering Sea late this morning, which
will bring the next round of strong winds and rainfall stretching
from Adak to the Alaska Peninsula as the front moves across the
region. The active pattern will persists through Tuesday as these
waves of energy keep moving northward from the North Pacific to
these areas. The next potential gale force system approaches the
western Aleutians and Bering by Wednesday morning.

&&

.MARINE (Days 3 through 5)...
(Tuesday through Thursday)

Monday

Bering Sea/Aleutians…

The well established area of high pressure residing over the eastern
Pacific will continue to influence the Bering with consistent
southerly winds and a tap of warmer, moist air. Tuesday morning a
1000 mb low over the central Bering looks to be on its way to
degrading into an open wave under an upper level trough. Wednesday
looks to be more of the same with the ridge holding its ground
and a series of minor disturbances moving across the eastern
Aleutians and eastern Bering. However, a bigger pattern shift
looks to be advertised for Thursday as a stronger low pressure
system materializes in the northwest Pacific and pushes through
the Bering. The first impacts look to be a strong push of
southerly moisture into the Aleutians Thursday morning and
spreading across the Pribilofs by Thursday afternoon before the
parent low pushes deeper into the Bering Thursday night.

Gulf of Alaska…

The resident high pressure in the eastern Pacific will continue to
push strong southerly flow into the Gulf of Alaska. The frontal
boundary stretched across the north Gulf coast Tuesday morning
will slowly erode as the low pressure in the Bering moves beyond
60N. This will usher in a relatively quiet Tuesday afternoon for
the Gulf of Alaska under the upper ridge that remains in place.
Late Tuesday night another upper level disturbance pushes into the
southwest Gulf and will extend its impact to the north Gulf coast
and the remainder of the Gulf Wednesday morning into mid-morning
Thursday. The remainder of Thursday looks to be quiet again as
high pressure regains a foothold in the wake of this disturbance.

&&

.LONG TERM FORECAST (Days 4 through 7)...
(Wednesday through next Saturday)

Beginning Monday night, the entire forecast area will come under
the influence of the trough currently established over the Bering
Sea. While the main low center will remain to the south of the
Aleutian Islands, several waves will develop on the eastern side,
moving northward over Kodiak and into Southcentral. This will
bring periods of precipitation for both Southwest Alaska and
Southcentral as these waves move northward. With a southerly fetch
of warm air, for the most part this looks like precipitation will
mix across the mainland. However, with winter still hanging on by
its teeth in some of the more northern regions, this could mean
another round of snowfall for the Lower Kuskokwim Valley and the
northern Susitna Valley.

Beginning Thursday, the operational model runs are in fairly good
agreement on an upper level ridge building in over eastern AK,
with a broad longwave trough settling in over the Bering Sea.
However, as the models progress, there is some difference in the
orientation of the ridge axis over eastern AK. GFS indicates a
broader, flatter ridge, which EC shows a much sharper ridge with
the axis slightly further west. This will make a difference in the
sensible weather at the surface, in particular for Southcentral
and areas eastward. Should the GFS solution set up, the broader
ridge will allow waves to move across the forecast area, thus
bringing the possibility of rain to the eastern half of the state.
However, should the EC solution eventuate, the sharper ridge will
keep weather systems further west and the eastern half of the
state will remain dry.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ALH
MARINE/LONG TERM...LB/MV