Small Craft Advisory 423, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
800 AM AKDT Wed May 20 2020

.ANALYSIS AND UPPER LEVELS...

The two main weather features today are an occluding low moving
over the central Aleutians and a building ridge over the southern
mainland. Upper level support for the low will diminish as the jet
stream becomes more zonal and decouples from the surface low. The
main weather impacts from this system will continue to be rain
and gale-force winds along the occluded front as it pushes its
way east towards the AKPEN today.

To the east, a brief period of ridging is taking place over the
southern mainland, which will help to clear skies today. Low
clouds and some patchy fog are evident in the satellite imagery
this morning. Cumulus and light showers are possible over areas
of high terrain. This period of calm weather will be short lived
as the occluded front pushes its way east towards the mainland
late tonight.

&&

.MODEL DISCUSSION...

Synoptic models remain in good agreement with good run-to-run
consistency. GFS indicates higher gap winds between the Aleutian
Range and Kilbuck Mountains. This tends to verify in southeasterly
surface flow and although the winds aloft are not completely
aligned with the surface, the stronger solution remains
preferred. Forecast confidence remains above average.

&&

.AVIATION...

PANC...VFR conditions will persist. Gusty southerly winds of up to
20 kt will likely persist through Thursday morning.

&&

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

A building ridge of high pressure will lead to partly cloudy skies
for most of Southcentral today. Diurnal heating combined with
sunshine will generate some weak instability over the higher
terrain. This will once again provide a chance for isolated showers
as we saw on Tuesday afternoon. This weather pattern will change
early Thursday morning as a front moves in from Southwest Alaska.
Rain and windy conditions are expected to move up the Gulf Coast
from midday Thursday through Friday night. The strongest gusts will
be located in the typical SE to NW oriented gaps such as the
Turnagain Arm and the Knik River Valley. A building ridge over the
Yukon Territory and Alaska interior should help to confine
precipitation to coastal areas along the western and central Gulf.
Southeasterly flow along the front should keep leeward areas along
the Kenai Peninsula (Kenai, Homer, etc) and Chugach Mountains mostly
dry (Anchorage, Palmer, etc).

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

Upper level ridging over Southwest Alaska will continue to shift
eastward today through Friday as an occluded low in the Bering
approaches from the west. Southwesterly flow aloft will help advect
in more clouds and moisture into the area ahead of this low. Rain
will move into the area late this morning into early afternoon.
These will taper to showers and linger through Friday as the low
remains nearly stationary in the eastern Bering. Strong
southeasterly winds will develop Thursday through Kamachak Gap as
the southeasterly winds associated with the low funnel through the
Aleutian Range.

&&

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

A large area of low pressure in the western Bering/Aleutians will
continue gradually moving eastward into the eastern Bering/Aleutians
by Friday. This low has become occluded, thus it will weaken as it
stalls over the western Bering. GOES-17 water vapor imagery shows
dry air intrusion into the low center. Moreover, the cyclonic
vorticity maxima (center of counterclockwise rotation) has become
detached farther away from the core of the subtropical jetstream,
thus synoptic scale forcing will diminish, allowing this low to
weaken. Scatterometry (ASCAT) data shows low end gale force winds
associated with this low across the western and central Bering and
it will likewise continue to bring widespread rain near its center.
Fog will remain a concern in the Pribilof Islands over the next few
days, Guidance shows little to no dewpoint depression in combination
with easterly winds.

&&

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

Gulf of Alaska: High confidence remains that a frontal system will
move through the Gulf of Alaska Friday with a barrier jet
developing along the Gulf Coast with winds remaining below gale
force in the small craft range with waves building as this wind
remains persistent over the same area. Another low pressure system
will drive northward along the front from the North Pacific to
the southwest of Ketchikan and reinforce the aforementioned front
along the Gulf Coast. This will reinforce the barrier jet across
the northern Gulf and Gulf Coast of Alaska with a possible area of
gale force winds in this area Saturday. As the front pushes
inland and the low moves over Kodiak, winds will diminish on
Sunday.

Bering Sea: Model guidance remains in great agreement from
yesterday in showing a low pressure system slowly moving over the
Bering Sea south of the Pribilofs leading to a high confidence
forecast. With the low in a weakening phase during this time, only
patches of small craft winds are expected around the low during
the forecast period Friday to Sunday. This will keep the Bering
Sea and Aleutian Chain unsettled as well with rounds of rain.

&&

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

The long term forecast period continues to look quite unsettled
for the Bering Sea/Aleutian Chain, Southwest Alaska, and over to
Kodiak Island. An area of low pressure depicted very well by the
various different pieces of model guidance will slowly move over
the Bering Sea south of the Pribilof Islands before retrograding
(when a system moves from east to west) westward beginning Sunday
afternoon. The system will continue to weaken as it does so
through the end of the forecast period. Model guidance is also now
picking up on another low moving northward from the North Pacific
south of Kodiak Island which models were hinting at yesterday.
This will help to keep the unsettled pattern in place.

Further to the east, an upper level ridge is anticipated to be in
place across northwest Canada to the North Slope of Alaska and
the Arctic Ocean. However, the strength and orientation of this
ridge is still somewhat unclear which will have an influence on
temperatures and winds during the extended. Upper level waves of
energy are expected to periodically move northward from the Gulf
into Southcentral Alaska but the details on each individual wave
is uncertain leading to an average confidence forecast. The Copper
River Basin remains the region that is expected to have the best
influence from the upper level ridge leading to above normal
temperatures there with further west towards the Anchorage Bowl
and the Kenai Peninsula favoring near to below average
temperatures. With this pattern in place and upper level waves
moving northward, there may be a heightened convective
thunderstorm risk over the Wrangell Mountains, Chugach and eastern
Alaska Range with this pattern.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 170 172-174 178 185 411 413.
FIRE WEATHER...NONE.

&&

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