Small Craft Advisory 339, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
448 AM AKDT Wed Apr 15 2020

.ANALYSIS AND UPPER LEVELS...

A weak upper level trough continues to bring light showers to the
western Bering/Aleutians, however overall conditions remain quiet.
Steady rain is occurring farther east across the Alaska Peninsula
and extending through the Kuskokwim Delta in an extremely long
2500 mile southerly fetch of moisture originating from the
Hawaiian Islands. While the flow is slightly weaker to the east,
rain is also developing along the southern Gulf coast and will
continue to increase in intensity and move northward as the flow
aloft becomes stronger through the day. As such, temperatures will
remain slightly above normal with freezing levels shown at roughly
800mb across Southcentral.

&&

.MODEL DISCUSSION...

Models remain in good agreement synoptically. Timing of the
rainfall across Southcentral has come into consensus, however
total amounts between the models still slightly vary. All models
likewise indicate an upper level ridge developing in the wake of
the southerly fetch.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist through this
morning. Rain will develop this afternoon, likely dropping both
ceilings and visibilities to MVFR conditions through Thursday
morning.

&&

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

Two different systems will impact Southcentral today. The first is
an upper level disturbance embedded in the large upper level
ridge over the eastern Gulf of Alaska. This wave is strongest over
the Prince William Sound region where it is bringing rainfall
this morning. It should move northeast and across the Alcan
border by this evening just in time for the next system to move
in.

This second system is the main one for the area with strong
southerly flow at the upper levels which is bringing in a large
amount of moisture. This front will make it into Kodiak early
this morning and proceed to move from west-to-east across
Southcentral through tonight. The rain is expected to begin this
afternoon and continue through the overnight hours with the
heaviest rain rates during the evening.

This front will also bring rain the the north Gulf coast with
amounts of an inch or more for today through Thursday expected
from Seward through Cordova.

&&

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

Widespread rainfall will continue across Southwest Alaska today as
an upper level disturbance continues to cross over the region
throughout the day. Satellite imagery this morning showed this
impressive large plume of moisture spreading across Bristol Bay,
which will keep spreading north and eastward through the late
afternoon hours. Up to an addition half inch of new rainfall is
expected today before this system exits towards Southcentral
overnight tonight into Thursday morning.

Clearing conditions will move across interior Bristol Bay early
Thursday, while an upper level trough will keep lingering showers
across the Kuskokwim Delta throughout the day Thursday. A chance
for precipitation will return Thursday evening into Friday as a
front associated with a Bering low begins to push inland, which
will once again be a warm system accompanied by widespread rain
showers.

&&

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

As the low over the eastern Bering continues to exit the region
this morning, the next system quickly moves in over the western
and central Aleutians by the afternoon hours. Gale force winds and
gusts will initially accompany this system as it moves across the
Aleutians, but will quickly weaken to small crafts as the front
begins to shear apart as it moves into the Bering. However, expect
strong southerly winds to gale force speeds out of bays and
passes along the eastern Aleutians and AKPEN as the system moves
eastward beginning Thursday morning through Friday. There is more
confidence in today's model runs that there will be widespread
small craft winds and rain showers as this front moves into the
eastern Bering and Aleutians through Friday.

&&

Marine Discussion (Days 3 - 5: Friday through Sunday)

Gulf of Alaska…

A dissipating front over the southwestern Gulf may produce near
gale force southeasterly winds through the Barren Islands and
southern Cook Inlet Saturday. Elsewhere no gales or high seas are
expected through Sunday. Confidence is low regarding gale
potential over the Barrens.

Bering Sea/ Aleutians…

A large low over the western Bering may produce low end gales
across the Eastern Aleutians on Friday, extending over the Central
Aleutians on Saturday. As another stronger North Pacific low
approaches the Aleutians on Sunday, gale force southeasterly winds
are expected over the Eastern Aleutians. Northerly gales are
expected over the Western Aleutians Sunday from the same low, with
a possibility of storm force gusts as well. Confidence increases
by Sunday on the likelihood of gales along the Aleutians.

&&

Long Term Discussion (Days 4 - 8: Saturday through Tuesday)

The upper level pattern of ridging with surface high pressure
over the Gulf of Alaska and troughing with low pressure over the
Bering will start out the long-term period on Saturday. A stronger
North Pacific low will approach the Aleutians on Sunday. The
models generally agree that this low will effectively shunt the
downstream ridging over the Gulf into North America, allowing the
Bering trough to move eastward into at least western Alaska by
Tuesday. How quickly this happens and if there is any phasing from
a deeper polar trough building into Northern Alaska at this time
remains highly uncertain. The GFS solution, on the one hand, keeps
a much stronger low nearly stationary over the central Aleutians
into Tuesday. Weak impulses moving northward will only nudge the
ridging over the Gulf eastward, which in turn would result in a
nearly steady-state forecast for much of Alaska into Tuesday. The
EC, on the other hand, has a much weaker low that is more
progressive, tracking largely south of the state and into the
Gulf on Tuesday, which would completely eliminate the effects of
the ridging currently over that area.

There is high confidence that through at least Sunday, the
weather pattern will be warm and wet over the western half of the
state, with cooler and wet conditions across the Bering and
Aleutians. Confidence diminishes for Monday and Tuesday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 130 170 177.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AP
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...JW