Small Craft Advisory 412, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
357 AM AKDT Sun May 17 2020

.ANALYSIS AND UPPER LEVELS...

Two lows, both at 500 mb and at the surface, are currently phasing
over the far western Aleutians. This is leading to widespread
rain, generally west of Unalaska along the surface low's
associated front. A weak vorticity maximum is moving northeastward
over the Kuskokwim Delta bringing light scattered showers to that
region. Weak upper ridging over Bristol Bay and the Alaska
Peninsula is somewhat breaking up cloud cover, though light
onshore flow is preventing skies from completely clearing. A
trough extends significantly to the west of a deep 500 mb low
over the Canadian Arctic. However, its orientation is leading to
drier air advecting from the north per water vapor imagery and
thus skies are generally clearing in the Copper River Basin with
more overcast conditions observed to the southwest near Anchorage
where the upper flow is weaker.

&&

.MODEL DISCUSSION...

Synoptic features continue to be well-handled by all major
models. Confidence remains high.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist.

&&

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

A weak trough over Southcentral will move eastward today. High
pressure will move into the area today and remain over the
mainland through tonight. An upper level trough and associated
front will move into the southwest Gulf late this afternoon and
then into Kodiak tonight. This front will reach the north Gulf
coast around midday on Monday, then into the Southcentral mainland
during the afternoon and evening hours. This will bring
increasing chances of rain to the area.

&&

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

Upper level ridging will continue to shift eastward across
southwest Alaska today through Tuesday. Due to the 500 mb ridge-
axis being located to the east of the area today and on Monday,
southwesterly flow aloft will continue to bring cloud cover and
moisture from the Bering. Due to the ample cloud cover minimizing
the magnitude of diurnal heating, the chance for convection is
low as the atmosphere remains relatively stable. A warm front
associated with a weakening low pressure system in the eastern
Bering will move over the AKPEN and Bristol Bay late tonight
through Monday morning, then into the Kuskokwim Delta and
Kuskokwim Valley Monday afternoon through Monday night. This front
will bring widespread stratiform rain to the area. Due to the
dynamic process leading to overrunning, the chance for convection
is minimal as the atmosphere will remain stable. This rain will
gradually taper off from southwest to northeast on Tuesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

The jetstream pattern remains amplified during the forecast period as
teleconnections support a persistent trough over the western
Bering/Aleutians, ridging over the eastern Aleutians into the
AKPEN and western Gulf. A low pressure system is currently moving
northward and approaching the central Aleutians. The system has
occluded and the core of the subtropical jetstream is moving
farther east and away from the cyclonic vorticity maxima (center
of counterclockwise circulation). Due to this, the synoptic scale
forcing from the left-exit region of the jet will become weaker
and thus the low pressure system will gradually weaken.
Scatterometry (ASCAT) data shows winds currently below gale
force. Since the system is currently weakening, gale force winds
are not expected at this time as the system enters the central
Bering. Widespread rain associated with this low is expected
across the Aleutian chain today. There is good agreement that
rain will move over the Pribilof Islands during the late afternoon
hours today. This low will then weaken over the eastern Bering on
Monday resulting in stratiform rain tapering to showers from
south to north through the day. Fog is expected today through
Tuesday across the Pribilof Islands as this low moves across the
area. GFS/NAM/ECMWF MOS guidance is consistent with dewpoint
depressions being near zero in Saint Paul, especially during the
nighttime hours. A second low pressure system will approach the
western Aleutians from the North Pacific Monday night into Tuesday
morning. There is good agreement with this low bringing gusty winds
and rain to the western and central Aleutian chain on Tuesday.
There is a higher degree of uncertainty with wind speeds reaching
gale force, though the best chance for this would be on the
Pacific side of the western Aleutian chain.

For the sea state, large long period swell will continue along the
Pacific side of the Aleutian chain. There is good agreement
between GFS WW3 and ECMWF WAM with this swell gradually building
from today through Tuesday and remaining long in period.

&&

.MARINE (Days 3 through 5)...

Gulf of Alaska: Two rounds of weak low pressure will move through
Gulf of Alaska. The first on Tuesday will be just exiting the
area. It will produce some elevated winds and sea states, but
nothing significant. The next low will move in Thursday. It will
again elevate winds and waves, but the only area meeting small
craft advisory will be around the Barren Islands. Forecast
confidence is above average.

Bering Sea: A large but weak low pressure system is expected to
move east through the Bering. The low center should be right over
the Aleutian Islands. Some marine areas may meet small craft
advisory criteria, but this is low is not expected to have signifcant
impacts to marine traffic. Forecast confidence is above average.

&&

.LONG TERM FORECAST (Days 4 through 7)...

A change in the weather pattern will present itself on Monday,
starting over the central Aleutians, as an area of low pressure
traverses eastward across the southern Bering. This feature will
bring precipitation and increased winds across the region as it
approaches the southwest coast Monday morning. That moisture will
then advect across southwestern Alaska and into Southcentral
Monday night.

Behind this, expect the same conditions as another low propagates
into the western Aleutians Tuesday morning, following the same
track as the aforementioned low. Expect landfall sometime
Wednesday evening on the southwest coast and propagation eastward
into Southcentral Thursday and into Friday.

All in all, expect cooler temperatures, chances for rain and
increased winds through the extended forecast period.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...AP
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ED
MARINE/LONG TERM...CK/BB