Small Craft Advisory 415, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
809 AM AKDT Mon May 18 2020

.ANALYSIS AND UPPER LEVELS...

An upper level jet near the western Aleutians is helping to
maintain an associated 500 mb low northeast of Adak. Although a
surface low is currently weakening as it is becoming vertically
stacked with the aforementioned upper low, it is still aiding in
widespread shower activity across the central and western
Aleutians. A well-defined cold front is tracking eastward across
the Alaska Peninsula, Bristol Bay and Kodiak Island. Given the
long southerly fetch between the broad upper low and a large ridge
over the Gulf, moderate to locally heavy rain and gusty southerly
winds are observed over this area. Water vapor shows this swath
of moisture originating from around 35N. While the crest of the
upper ridge ahead of this feature is flattening over Southcentral,
conditions remain quiet with partly clear skies over interior
regions. Skies remain overcast near the Gulf coast in weak onshore
flow.

&&

.MODEL DISCUSSION...

Synoptic features remain in good agreement through the end of the
week. There remains up to a half inch spread in total rainfall
amounts over portions of Bristol Bay and Kodiak Island as a cold
front tracks across the region through Tuesday. Model spreads are
even slightly higher over the Alaska Peninsula, with NAM being the
most aggressive, showing localized amounts of around 1.5 inches
between Port Heiden and Nelson Lagoon. The current forecast
generally reflects a model average, though the higher-end models
could easily verify (over non-downsloped areas) given the strong
southerly fetch aloft as indicated by the latest water vapor
imagery. In any case, there are no concerns of impacts from this
rainfall as the front will gradually weaken as it continues to track
eastward and forecast confidence remains steadfast.

&&

.AVIATION...

PANC...VFR conditions will persist. Light and variable winds will
continue through this morning. Southeasterly Turnagain Arm winds
are expected to develop this afternoon, with gusts near 20 kt
possible over the terminal. These will likely diminish by early
Tuesday morning.

&&

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

A weather front over Kodiak Island and the western Gulf this
morning will move northward today. This will bring increasing
winds to the area, especially along Turnagain Arm as well as the
Knik and Copper River Valleys. Rain with this front will push into
the Kenai Peninsula early this afternoon, and then across the
remainder of Southcentral later this afternoon through tonight.
Most of the rain will fall along the eastern Kenai Peninsula and
coastal locations, with just chances of rain for inland areas.
The front will weaken on Tuesday, with winds diminishing. Some
showers will continue on Tuesday. Showers will diminish Tuesday
night as the front dissipates.

&&

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

Upper level high pressure continues to shift eastward today. This
will allow for more clouds and moisture to move into the area from
southwesterly flow aloft. A negatively-tilted trough axis is
moving northeastward towards the AKPEN. The orientation of this
trough axis is conducive to cyclogenesis (favorable for low
pressure systems to develop and strengthen). There is good
agreement that an area of low pressure will develop along the
AKPEN this morning and move northeastward into Bristol Bay this
afternoon. Widespread rain and showers are expected today along
the AKPEN and in Bristol Bay as the low moves through. The highest
rainfall amounts will be along the AKPEN and Bristol Bay. Sunshine
will increase on Wednesday as a second upper level ridge moves
over the area. However, conditions will remain humid as guidance
indicates relatively small dewpoint depressions.

&&

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

A large area of low pressure over the western Aleutians will
continue to weaken while moving eastward today bringing widespread
showers.

A second low is currently developing east of the Kurill Islands
while moving northeastward. The center of cyclonic vorticity
maxima (area with the strongest counterclockwise rotation) is
located near the jet-core inside of the left-exit region of the
subtropical jet stream. This will provide ample synoptic scale
forcing for this low to continue strengthening as it approaches
the western Bering/Aleutians. This low will bring gale force
southerly winds to the central Aleutians Tuesday afternoon and
evening. The low will begin to weaken Tuesday night, thus winds
will begin to gradually ease. Widespread rain and showers will
move over the Aleutians Tuesday morning and continue through
Wednesday night. The rain is expected to begin in the Pribilof
Islands Tuesday night into Wednesday morning and continue through
Wednesday night.

Fog is expected to continue across the Pribilof Islands through
Wednesday and may be dense at times. Guidance shows little to no
dewpoint depression, thus there is a high probability for
condensation to occur at the surface.

Large long period swell will build across the western
Bering/Aleutians Tuesday and Wednesday. The largest swell will
occur on the Pacific side of the western and central Aleutians.

&&

.MARINE (Days 3 through 5: Wednesday through Friday)...

A weak area of low pressure in the Gulf Tuesday looks to dissipate
by Wednesday night. Winds and waves may be a bit higher,
although, nothing significant to note. A front associated with a
low over the central Aleutians will enter the western Gulf
Wednesday afternoon, bringing small craft winds and waves across
the Barren islands through Thursday.

A strengthening low across the Aleutians will bring gale force
winds and waves across the central Aleutians and the western and
central Bering Tuesday. Expect these to propagate north and east
as the low moves towards the southwest coast before dissipating
Thursday.

&&

.LONG TERM FORECAST (Days 4 through 7: Thursday through
Monday)...

Widespread rain across the Bering, Aleutian Chain and the
southwest coast will continue on Thursday as a very large complex
low remains over the Aleutians. This moisture will move into
Southcentral, however, forecast timing on this is currently a
challenge. The challenge remains with timing of the complex low
splitting into smaller lows and those smaller lows propagating
into the Gulf.

All in all, expect cooler temperatures, chances for rain and
increased winds through the extended forecast period as this
pattern looks to continue through then.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 173-176.
FIRE WEATHER...NONE.

&&

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