Small Craft Advisory 414, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
549 PM AKDT Tue May 22 2018

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low continues to spin over the Gulf, bringing
showers primarily to the Gulf coast. Inland areas of Southcentral
Alaska are mostly dry, with scattered showers advecting across
the northwest Copper River Basin right ahead of a weak upper level
wave. Out west, a gale force low is tracking eastward across the
western Aleutians. Satellite imagery shows a frontal system
associated with this deep low spreading north and east across the
Bering Sea and Aleutians.

An upper level col is sitting over the Southwest Alaska coast.
As is typical with this feature, a stable air mass combined with
weak winds has resulted in a large area of stratus and fog over
the eastern bering and along portions of the southwest coast.
Lastly, a surface thermal trough extends from inland areas of
Southwest Alaska northeastward to Interior Alaska. This will act
as the focus for shower and thunderstorm development as surface
temperatures climb this afternoon.

&&

.MODEL DISCUSSION...

Models are in good agreement with the placement of the low in the
Gulf and timing on when it will dissipate. Models are also in
agreement with the center of low tracking eastward across the
Aleutians in the first 60 hours. However, there is disagreement
in the placement of a weak developing triple point low southeast
of Kodiak Thursday into Friday, though this will have little
impact on the overall forecast for the region. The primary
forecast challenge for the next couple days is location and
coverage of convection over the southern Alaska mainland as
weak upper level features move overhead.

&&

.AVIATION...

PANC...VFR conditions will persist. The diurnal Turnagain Arm wind
will continue as thermal trough strengthens over Interior Alaska
and ridge strengthens along the Gulf coast. Pressure gradients
between coast and inland look a little stronger this afternoon and
evening, so expect winds bending into the terminal will also be a
bit stronger than yesterday. This pattern will continue again,
with winds dissipating early Wednesday and returning Wednesday
afternoon.

&&

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

The persistent upper-level low remains nearly stationary over the
southern Gulf through Wednesday. At the surface, weak low
pressure centers continue over the Gulf with accompanying troughs
rotating north and west. Widespread showers will continue along
the coast through Thursday as the surface low weakens and the
upper-level low drifts to the north. An advancing long wave trough
will replace the retreating low in the Gulf as a new surface low
develops south of Kodiak Island Wednesday night. This surface low
will take a similar track as the previous, drifting northward into
the Gulf before slowly dissipating.

For interior locations, the coastal ridge will remain in place
and combined with the interior thermal trough will continue gusty
afternoon and evening winds along the Copper River, out of the
Knik Valley, and over south and west Anchorage through Thursday. A
sheared lobe of vorticity advecting southwestward across the
Copper River basin and over the Talkeetna Range will provide
enough instability to produce isolated showers during the
afternoon and evening hours Tuesday and Wednesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A thermal trough extending across the Kuskokwim Valley into
Bristol Bay supports isolated to scattered showers and
thunderstorms through peak heating during the evening. Southerly
flow returns across the region overnight as the next Bering low
and front pushes to the central waters overnight. The exception to
this will be across inland areas of Bristol Bay where a weak
boundary keeps winds on the lighter side through day break
Wednesday. Patchy fog is expected to develop tonight along this
region with high surface moisture in place along with a
temperature inversion strengthening after midnight.

The Bering front approaches the southwest coast Wednesday night,
however an easterly moving wave blocks any further progression.
Precipitation chances do continue through Friday with the
combination of the slow easterly wave propagating toward the coast
as a weak surface trough remains across the southwest.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Scattered to numerous showers will be ongoing as low pressure
only makes slow southeast progress through Thursday, crossing over
the eastern Aleutians to the North Pacific by Friday morning. A
ridge moves across the western half of the Bering in the Wake of
the front bringing quiet weather for the end of the week.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning Friday, an upper level low will continue to dominate
the weather over the southern mainland bringing continued cloudy,
showery, and somewhat cooler weather to the region, especially
along coastal areas. A shortwave trough will move north into the
Gulf on Friday bringing another round of rain to the coast though
precipitation should stay mostly on the east side of the mountain
ranges. A more well defined low pressure system will move into the
Northern Pacific/Southern Gulf region by Saturday keeping cloudy
and showery conditions over coastal areas, though due to the more
southern and eastern location of the low precipitation should be
focused more over southeast Alaska, with only isolated to
scattered showers over Southcentral. With plenty of breaks in
between cloud cover, especially for more inland locations, temps
should be able to warm up into the 60's over the weekend. By
Memorial Day, models are all showing the low over the Gulf moving
east out of the area allowing a ridge of high pressure to build in
by the afternoon. This should lead to a pretty decent day for
outdoor activities over most of the southern mainland.

Further west, a frontal system will rapidly move east across the
Aleutians and Bering Sea Sunday through Tuesday, following closely behind
the transiting ridge. Models are not in agreement regarding timing
of this system, with the GFS bringing a front across the west
coast by Monday afternoon and the EC delaying it into Tuesday.
Regardless, there is consensus that there will only be a brief
period of quasi summer-like weather for the region as we head into
next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 176-178.

&&

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