Small Craft Advisory 456, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
420 PM AKDT Sun May 31 2020

.ANALYSIS AND UPPER LEVELS...
The weather pattern is currently dominated by a persistent upper
level ridge stretching across the west coast of the North
America into the north half of the state and a long wave trough
draped from western Alaska across the Bering Sea. Embedded within
this trough are two distinct upper level lows, one over the
eastern Gulf and a second just south of the eastern Aleutian
Chain. The first system is vertically stacked and and will further
weaken into an open wave as it drifts eastward toward the Alaska
Panhandle. Easterly flow aloft between the upper level and the
ridge to the north is ushering another easterly shortwave across
Southcentral this afternoon. Satellite imagery is showing a mass
of low and mid level clouds cover moving west from the Yukon and
Copper River Basin over the Susitna Valley and Cook Inlet.
Rainfall is also spreading west from the Copper River Basin to
Price William Sound. Cumulus clouds are popping up over areas of
the Alaska Range and Kuskokwim Valley were stronger isolation is
increasing atmospheric instability.

The second upper level low is associated with a surface low just
south of Dutch Harbor. This feature currently in the mature phase
and will begin occluding as the upper level low catches up to the
surface low and becomes vertically stacked. A occluded front
attached to the surface low is stretched over the western extent
of the AKPEN, producing rain and gusty southeasterly winds.

Extensive cloud cover and gusty northwesterly winds continue over
the western Bering and Aleutians as a shortwave trough slides
south over the region. Meanwhile, subsidence underneath a ridge
over the eastern Bering is keeping clouds at bay from Saint Paul
to Cape Newenham.

&&

.MODEL DISCUSSION...
Models continue to come into better agreement through the short
term and are clustered well with regard to the position and
strength of a surface low south of Dutch Harbor. Guidance is also
in good agreement with the vertically stacked low in the eastern
Gulf and it transitions to an open wave and moves to the coast
overnight. Model spread increases late Mon into Tuesday with the
progression of the main surface low as it tracks south of the
AKPEN. The EC continues to be the most progressive, positioning it
south of Sitkinak Island by Tuesday morning while the NAM, GFS,
and Canadian keep the center of circulation closer to the Shumagin
Islands. Likewise, there is also some model spread with the
development of a triple point low along the occluded front as it
moves into the Gulf. Again, the EC is more progressive lifting
this developing feature farther north toward the eastern Kenai
Peninsula coast more quickly than other guidance. Hence, a blend
of NAM and GFS was favored for the evolution of this system as it
tracks east.

&&

.AVIATION...
PANC...VFR conditions will persist. Gusty southerly winds are
expected to continue through late evening, then diminish. These
winds will then re-develop Monday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A broad upper level trough in place over the northern Gulf will
allow for the continuation of an easterly wave pattern across
Southcentral and thus showery conditions across the area for the
beginning of the week. A few thunderstorms have developed this
afternoon in the northwestern Copper River Basin near the Denali
Highway. These will likely continue through the evening hours
before dissipating. Gusty southeasterly winds will increase this
evening, thanks to a strengthening coastal ridge along the Gulf
and a thermal trough inland. Southeasterly winds will be
noticeably stronger this evening through Turnagain Arm, the Knik
River Valley, and the Copper River Basin. There will likely be a
brief lull before a second round of gusty southeasterly flow
arrives Monday afternoon as a front lifts inland along the Gulf
coast. This front will also bring overcast skies and rain on
Tuesday, especially for areas along the northern Gulf coast.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Widely scattered thunderstorm activity is ongoing across the
Kuskokwim Delta this afternoon. This activity formed in response
to the combination of an upper level wave, low level moisture, and
just enough sunshine earlier today to allow the instability to be
realized. Easterly upper level steering flow will keep the
showers and thunderstorms that form moving towards the coast. The
thunderstorm activity will diminish with the loss of daytime
heating tonight. Lingering moisture, especially in the areas hit
with the thunderstorm rains will promote fog potential again
tonight around Bethel.

A more stable air mass following behind the ongoing shower and
thunderstorm activity will move in across all of Southwest Alaska
on Monday. This will greatly diminish the thunderstorm threat,
though an isolated evening storm or two may develop along the
Nushagak Hills.

On Tuesday, another upper level wave will approach the area from
the southeast. The latest thinking is the strongest forcing from
the wave would move through Tuesday night, and impacts at the
ground will be minimal. Nevertheless, the combination of
additional large-scale lifting ahead of the wave and the
persistent thermal trough over interior Alaska may be enough to
set off another round of thunderstorms, in many of the same areas
seeing the storms this evening.

Outside of any lightning starts, fire weather concerns remain
minimal with plentiful low level moisture and seasonable
temperatures persisting. Gap winds may be an issue around Lake
Iliamna and the other usual gaps in the Aleutian Mountains from
this evening into Monday, but their ability to dry the atmosphere
will be limited.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
A slow-moving vertically-stacked low centered on the Pacific side
of Unalaska Island will only very slowly drift eastward, only
reaching the marine area south of Kodiak Island Tuesday night.
Continued unsettled weather associated with the low will therefore
persist across the Eastern Aleutians and the Alaska Peninsula
through Tuesday, with gradually improving conditions following
behind it from west to east. Elsewhere, a large area of high
pressure over the North Pacific south of the Western Aleutians
will keep conditions quiet across the rest of the Bering straight
through Wednesday. Any unsettled weather would be along the coast
of Southwest Alaska from any storms that move over the area.

&&

.MARINE (Days 3-5: Wednesday through Friday)...
Gulf Of Alaska: A rapidly dissipating low is expected to move
into the southwest Gulf on Wednesday. Guidance differs on the
exact location of the low, but confidence is high that winds
should remain below small craft advisory criteria. High pressure
will take over as the low moves into the north Pacific on Friday.

Bering: High pressure to the south of the Aleutian Chain will
bring weak, easterly winds to much of the Bering through Friday. A
frontal system to the east of the Kamchatka should bring
precipitation and increasing cloud cover to the western Bering
on Thursday, reaching the eastern Bering on Friday.

&&

.LONG TERM FORECAST (Days 3-7: Wednesday through Sunday)...
A broad longwave trough will persist over much of the state with
a closed upper level ridge slowly making its way east across the
northern slope. Two upper level shortwaves embedded within the
trough will be the primary weather makers throughout the week.
The first of which is the vertically stacked shortwave just to the
south of the Akpen on Wednesday. This system should move into the
southwest Gulf on Thursday and continue on its southeasterly
trajectory into North Pacific by Friday. Models remain in
disagreement in regards to the propagation direction of the low.
The ECMWF moves the system east into the central Gulf before it
makes its turn to the southeast, and the GFS/Canadian solution
moving the low on a southeast trajectory straight into north
Pacific. Differences between the two solutions should be monitored
as the more southerly wind component on the ECMWF solution will
bring more moisture and precipitation to the southern mainland,
while the easterly component of the GFS solution will confine
precipitation to the coastline. Meanwhile, a series of weak
easterly waves and diurnal heating over the southern mainland
should provide moderate chances for scattered afternoon showers
through the forecast period.

In the wake of the first system a period of brief ridging is
expected over the Bering and southwest Alaska. This should help
to keep the area on the clear side before the next upper level
shortwave arrives in the northern Bering from the Kamchatka
Peninsula Thursday morning. Although this system is well resolved
in the upper levels, it appears that it fails to break down high
pressure in the lower levels, resulting in only light winds and
precipitation over the Bering and southwest Alaska. The system
appears to then stall out off of the Y-K Delta on Friday, which
should help to bring consistent cloud cover and showery activity
to the area through Sunday.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
MARINE/LONG TERM...CJ