Small Craft Advisory 610, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
420 AM AKDT Thu Aug 13 2020

.ANALYSIS AND UPPER LEVELS...

An upper low coupled with a ridge building from the North Bering
to the western Brooks Range is leading to northeast mid/upper flow
over much of the interior. Convection aided by the upper low and
cool mid-levels over the Bristol Bay inland and Susitna Valley
earlier Wednesday evening has long since dissipated due to
boundary layer decoupling. In fact, surface temps have mostly
cooled to below 50F across both regions with a few areas of fog
materializing once again near the Kuskokwim Delta and along the
Western Kenai Peninsula. At least isolated areas of fog will
likely continue to develop where skies remain clear from the
Susitna Valley to portions of the Cook Inlet coastline as
temperatures cool to the respective dew points prior to sunrise.
Clear, cool and quiet conditions exist elsewhere across the
Southwest Alaska with patches of low/mid water clouds over the
Chugach Mountains and Prince William Sound in the Southeast.

Elsewhere, a stubborn closed and occluded low near Adak continues
to spin North of a jet streak located in the North Central Pacific
with a couple new, progressive shortwaves developing in this
strong southwest flow. Unsurprisingly, low ceilings are persisting
over the Southeast Bering Sea and Eastern Aleutians, as will
continue to be the case until the low fills or is picked up by the
jet to the south. Upstream, a broad ridge continues to slowly
move West from the Northeast Pacific to AK Gulf.

&&

.MODEL DISCUSSION...

The models are in fairly good agreement with the synoptic features
within the first 24 to 36 hours, then later struggles in
maintaining a solid traffic flow of the large scale patterns
over the weekend. Two interesting features to keep an eye out for
are the Aleutian low and the emerging low south of the Aleutians.
At 500 mb, the low south of the Aleutians is not quite picked up
yet but solutions agree to take the upper low from the Aleutians
southeast then trekking toward the eastern Gulf by the weekend.
AKNAM, GFS and ECMWF-HiRes follows this upper low very closely
until Friday afternoon. NAM and ECM takes off by lifting the low
through eastern Gulf through at least Saturday morning.

Over the Gulf, a diffused upper level low is expected to meander
over the central Gulf through at least Friday night. GFS and
AKNAM12 holds the position of this low through at least Friday
night before NAM bails out while GFS keeps the upper low
stationary before moving it over the North Pacific through
Sunday night. ECMWF-HiRes follows through then differs in position
by staying more northeast of GFS and AKNAM12 through Friday
afternoon.

An upper level high over the Bering Sea will continue to strengthen
as it interact with a ridge from North Pacific, and as soon as
the Aleutian low pushes eastward this pattern will trail behind it
and will move across the region over the weekend. Behind this
high follows another low from south of Kamchatka which will move
closer to western Aleutians before the end of the week. Overall,
models are doing a great job with the short term period but
stumbles to hold the developing surface and upper low systems in
days ahead.

&&

.AVIATION...

PANC...Fog/stratus over Cook Inlet will gradually dissipate this
morning. Hence, relatively quiet weather and light to gentle winds
will prevail around Ted Stevens International Airport through
Friday, and VFR conditions will persist.

&&

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

A broad upper low moves slowly south over the Gulf of Alaska today
which sets up generally east-northeast flow aloft over Southcentral.
Weak waves are expected to propagate westward in this flow
continuing chances of showers across much of Southcentral Alaska.
Cool 500 mb temps combined with warming 850 mb temps with an
intensifying thermal trough will continue the threat of
thunderstorms over the Copper River Basin, MAT-SU Valley, Chugach
Mountains boarding Anchorage, and inland areas of the western
Kenai Peninsula. Drier and warmer conditions will prevail on
Friday as an upper ridge builds in from the west and the thermal
trough continues to intensify. A few afternoon and evening showers
will persist mainly near the mountains with only a slight chance
of thunderstorms along the Talkeetna Mountains. The bigger story
will be the increased offshore flow and gap winds developing along
the North Gulf Coast, Prince William Sound, and Eastern Kenai
Peninsula Friday and Friday night. The strengthening offshore
gradient will hold afternoon seabreezes at bay in the
aforementioned areas and allow afternoon temperatures to get
fairly warm.

&&

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

Expecting a stagnant pattern to set up over Southwest that will
bring hot, dry and increasingly windy conditions to the region
through Saturday. A positively tilted upper level ridge is
expected to remain nearly stationary over the region. This will
lead to northeasterly flow and subsidence, which means mostly
clear skies and warming temperatures each day. Expecting
temperatures to be in the 70s throughout the region. The exception
to this should be the coastal areas of the AKPEN to the south of
King Salmon, and coastal areas of the Y-K Delta. The location of
these areas should allow for the marine environment to keep high
temperatures in the 60s.

Winds will progressively increase out of the north/northeast each
day as the gradient between the surface high over the Bering and a
developing low in the Gulf increases. Northerly sustained winds
between 15 and 20 kts are possible Fri and Sat afternoon/evening
across Southwest. Guidance indicates that minimum relative
humidities should stay in 30s and 40s for interior locations;
however, these will be monitored closely as high temperatures and
wind are at or near Red Flag criteria. Winds overnight may prevent
relative humidity recoveries from reaching their full potential,
but clear skies may also aid radiative cooling. Regardless, at
this time fuels are not supportive of hot, dry, and windy red flag
warning criteria.

&&

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

Thursday is expected to be the last day of relatively calm weather
across the Bering. The last remnants of this week's primary storm
system should exit to the southeast by Friday. High pressure is
expected to occupy the Northern Bering through Saturday. Models
continue to struggle with the interaction of the system currently
over Dutch Harbor and low trying to develop within a baroclinic
zone of the jet over the North Pacific. Satellite analysis shows
the North Pacific system moving much faster to the northeast than
past model solutions have indicated, which has resulted in two
independent lows moving east rather than one low consolidating to
the south of the AKPEN. This means that the current low over Dutch
Harbor will linger a little longer than previously projected, but
impacts from this occluding low should remain below small craft
wind criteria along with some light to moderate rainfall.

The pattern will then begin to change over the Western Bering
overnight on Thu as the next frontal system moves in. Winds should
remain at small craft criteria near Shemya through Fri morning, by
which time the gradient between the high over the Northern Bering
and low east of Kamchatka intensifies. This should lead to Gales
over the Western Aleutians Fri afternoon. This front is expected
to gradually push east through Sat with the area of Gales moving
into the Central Bering by Sat afternoon.

&&

.MARINE (Days 3 through 5: Saturday through Monday)...

Aleutians/Bering: A North Pacific low and front move across the
Aleutians and Bering through Mon. While the track and development
of the low are mixed, confidence is good for Southeasterly gale
force winds and waves to move with the front over the Aleutians
and Bering. As the front enters the Eastern Bering Mon, the winds
diminish to small craft over the Bering with local gales near St
Matthew Island while the wind pattern wraps around the low through
Mon. Westerly small craft winds and waves continue over the
Aleutians through Mon.

Gulf of AK: A front moves across the Alaska Peninsula into the
Western Gulf through Sun. Confidence is good for Northwesterly
small craft winds and waves over the Western Gulf diminishing Sun.
A weak low pressure complex develops along the Southern Gulf
front through Mon. As the low centers rotate through the Gulf,
Confidence is good for local East or Southeast small craft winds
along the North Gulf Coast as the wind pattern raps the low
through Mon.

&&

.LONG TERM FORECAST (Days 4 through 7: Sun through Wed)...
Model agreement has improved significantly with the strength of
the Aleutian Low that will dominate the long term forecast as it
moves through the Bering early next week. This improvement is
largely a result of todays runs of the GFS coming in nearly 20 mb
weaker with the peak intensity of this system, so it no longer
appears that this will become a major system. The track of this
system is more in question, but the general trends are to track
this system towards the AKPen instead of much further north into
the Bering Strait. As a result of this general consensus, the long
term forecast will largely use a consensus blend into the middle
of next week. Confidence is also beginning to improve that this
system will primarily impact the Bering/Aleutians, SW Mainland,
and the Gulf. Southcentral on the other hand should see a glancing
blow, allowing for generally benign weather from late this weekend
into next week.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...AS/CB
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
MARINE/LONG TERM...MK/DK