Small Craft Advisory 509, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
628 AM AKDT Sun Jun 28 2020

Corrected Model Discussion

.ANALYSIS AND UPPER LEVELS...

On the planetary scale, the Rossby Wave pattern remains amplified
as a deep 500 mb trough extends across the Bering/Aleutians while
a 500 mb ridge is currently building over the Gulf and over the
eastern Alaskan mainland with the ridge-axis slightly west of the
AlCan border. Teleconnections support this pattern to continue
amplifying as the Pacific-North American Teleconnection trends
stronger into the negative phase while interacting with the
Western Pacific Oscillation that is expected to remain in the
positive phase (this interaction supports upper level troughing
over the Bering/Aleutians and into the central North Pacific). The
Eastern Pacific Oscillation is currently weakly positive and is
trending into the negative phase which will allow for this upper
ridging over the Gulf to become more pronounced and extend across
Southcentral Alaska with the ridge-axis remaining near the AlCan
border. The Rossby Wave progression over the past 5 days shows
good representation of these teleconnection trends. Moreover, the
200 mb zonal winds in the polar-jet region will remain easterly
and thus decrease the amount of Atmospheric Angular Momentum and
thus allow for these Rossby Waves to amplify further due to a
weaker jetstream.

On the synoptic scale, the subtropical-jet is oriented from
southwest to northeast and extends from the subtropical central
Pacific to the northern Gulf of Alaska. An upper-low that is
currently located over the eastern Aleutians is moving east-
northeastward. This upper-low is weakening due to the low becoming
more vertically stacked. Moreover, the center of cyclonic
vorticity (center of counterclockwise circulation) associated with
this low is increasing in distance from the core of the
subtropical-jet. This means there will be less synoptic scale
forcing and thus allowing the low to weaken. A warm front ahead of
this low is currently moving northeastward across the Gulf of
Alaska. A cold front is currently moving eastward across the North
Pacific. The triple point (where the cold, warm and occluded
front meet) is located near 55N 152W. An occluded front (cold
occlusion) extends northwest across the AKPEN, then southwest
across the eastern Aleutians around the center of the
aforementioned low.

On the mesoscale, observations show Turnagain winds developing in
the eastern portions of Turnagain Arm. However, the Turnagain wind
is not in Anchorage during the 12z observations as winds are light
and variable at PANC throughout the Anchorage Bowl.

&&

.MODEL DISCUSSION...

The synoptic pattern is in very good agreement among all the
models. As is often the case, the challenges of the forecast are
in the minutiae and the local effects. The two streams of energy
(northern and southern) will interact tonight bringing a wide
swath of precipitation and cooler air across southern AK Sun-Mon.
The American Models are now advertising a bit more SE flow late
Sun morning/afternoon than previously depicted. This cross-barrier
flow could help to chip away at some of the QPF in places like
Anchorage and the interior of the Kenai Peninsula. However, it
still looks like a high PoP scenario. The cool and wet pattern
across the Southern Mainland will hold for Mon, but it does look
like the coldest air will remain confined over SW AK and north of
the AK Range. Finally, as westerly flow moves in earnestly on Tue,
this should dry things out quite a bit more quickly than originally
thought. This forecast package lowers the PoPs late Tue.

&&

.AVIATION...
PANC...VFR conditions will continue for this TAF package. There is
a high degree of uncertainty with respect to the Turnagain wind.
It appears the best chance for the wind to bend into PANC is
between 15z and 19z this morning. After 19z, the low level down-inlet flow
will increase and lower the likelihood of Turnagain winds at
PANC.

&&

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

Precipitation is expected to expand throughout Southcentral today
as a warm front pushes into the area. Initial rainfall is
expected to be primarily on the eastern Kenai Peninsula and Prince
William Sound on Sunday morning. By Sunday afternoon an upper
level westerly wave is expected to transition the flow aloft from
southeasterly to southwesterly. This transition will bring up-
inlet flow aloft and heighten chances for rainfall in the Susitna Valley
and Anchorage Bowl this afternoon and into the evening. At the
surface, a coastal ridge will continue to build, which will help
to bring in gusty winds to the Turnagain Arm and Copper River
Basin throughout the day on Sunday.

On Monday, the upper level system is expected to be over the
eastern Gulf. This should reduce chances for rain during the day
on Monday over most areas with the exception of the high terrain
of the Talkeetna Mountains and Alaska Range. On Monday night the
next wave of westerly energy is expected to reach the western
Alaska Range, which will increase chances for precipitation once
again over the Alaska Range and Talkeetna Mountains. By Tuesday
a ridge will begin to build over western Alaska, which will help
to clear out most of Southcentral throughout the day.

&&

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

Southwest Alaska is waking up to cloudy conditions and rain
showers across the region. This is due to moisture associated with
a low over the Bering Strait merging with remnants from a North
Pacific front moving across the area this morning. Precipitation
will begin to taper off over the coastal areas starting this
afternoon, before another round of rainfall moves in over the
Kuskokwim Delta overnight into Monday morning as an upper level
disturbance moves inland. The soggy pattern will begin to weaken
across most of the region Monday afternoon and into Tuesday.
Onshore flow will persists will persists through the next few
days. Stay tuned for the next system to move in across Southwest
Alaska from the Bering Sea, bringing another round of widespread
rain for mid-week.

&&

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

A weakening low over the eastern Bering is keeping a showery
pattern and strong winds across the region this morning. Winds
will begin to weaken, as well as precipitation diminishing, this
afternoon as the system moves eastward out of the forecast area.
West to southwest small craft advisory level winds will persists
over the central and eastern Bering and Aleutians through Monday
afternoon, as the next strong system starts to move in over the
western Bering. A broad low will keep a large area of strong,
gale-force level winds in the forecast as it tracks eastward. This
system will reach the Pribilofs by Tuesday afternoon when it
starts to weaken into Wednesday. Expect widespread rainfall and
elevated seas with this system as it moves across the Bering Sea
and Aleutians.

&&

.MARINE (Days 3 through 5: Tuesday through Thursday)...

Gulf of Alaska:

Gale force westerly wind gusts are possible out of the Barren
Islands Tuesday morning. No gales are expected anywhere else in
the Gulf through Thursday.

Bering Sea:

Widespread southerly gales are expected Tuesday west of Atka.
Gusts could approach storm force at times in the gaps between the
Aleutian Islands. The gales will spread east to the eastern Bering
by Wednesday morning, then rapidly weaken by Wednesday evening. No
gales are expected on Thursday. Waves of 20 ft are expected on the
Pacific side of the Western Aleutians Tuesday, and waves of 15 to
20 feet are expected across the Bering with the gales through
Wednesday morning.
&&

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

Models in good agreement as we start the period on Wednesday, but
beginning Thursday the GFS seems to be the outlier as ECMWF,
Canadian and Ensembles in good agreement with ridge building over
the Gulf of Alaska extending north into most of mainland Alaska.
While the operational GFS produces a series of troughs with
a weaker ridge over the area. So the consensus is to follow a
blend of ECMWF and ensembles in the long term.

With that said, a strong ridge will build across the southcentral
and areas east with drier and warmer weather expected Wednesday
through Saturday, some models have the ridge retrograding a bit to
the west so parts of southwest Alaska may get in on the dry
weather for Friday and Saturday. Meanwhile, a series of low
pressure systems and fronts across the Bering Sea will move around
the ridge into the Arctic region and bring precipitation to the
YK Delta region and the Aleutian islands for much of the long
term.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 178.
FIRE WEATHER...NONE.

&&

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