Small Craft Advisory 736, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
532 AM AKDT Mon Sep 28 2020

.ANALYSIS AND UPPER LEVELS...

A high amplitude trough extends from the Kamchatka Peninsula to
Southeast Alaska with the jet center reaching as far south as 38N.
A well defined low can be seen on water vapor imagery near the
base of the trough at 45N 165W. This system is expected to become
coupled with the jet later today and move north towards the AOR.
This will continue the trend of above average temperatures due to
warm air advection into Southcentral.

The low center from the storm force low that impacted the Gulf on
Sunday can be seen on radar and satellite moving onshore this
morning between Cordova and Yakutat. A triple point low has
formed on the western edge of the storms occluded front with the
low center now over Bristol Bay. This is helping to bring rain to
the Aleutian Range, Kodiak Island, and the southern tip of the
Kenai Peninsula. Over the Bering, northerly flow is bringing in
low stratus and cooler temperatures through Bering Strait as a
front approaches Shemya from the west.

&&

.MODEL DISCUSSION...

Model agreement has improved in the overnight runs in regards to
the storm force low impacting the AOR on Tuesday. There are some
slight differences in whether there is a single closed low or
multiple low centers revolving around one another. Regardless,
the global models all show a minimum central pressure between 970
mb and 975 mb. The track of the storm and arrival time of the warm
front to Kodiak Island and the AKPEN remain in good agreement.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through this
evening. A Turnagain Arm wind is expected to develop tonight and
peak in intensity around midnight. MVFR ceilings and rain are
possible as a low pressure system moves north of the terminal
overnight Monday and into Tuesday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today
through Wednesday Night)...

The last remnants of the surface low that produced hurricane force
gusts at Middleton Island yesterday morning is dissipating and
transferring its energy westward and inland this morning. An upper
level front is following behind the low. As the front and upper
level trough over mainland Alaska shift west, so too will the low
bring some Gulf moisture over the mountains and into the Cook
Inlet region. This westward shift is mostly due to a large upper
level low over the North Pacific near 45N pushing eastward into
a large, strong upper ridge over the western US and Canada. This
will amplify the ridge northwestward, which will push the front
northwestward into Southcentral. Sensibly, this means the rain
over the northern Gulf and Prince William Sound will shift
northwestward, and rain will return to the Cook Inlet region.
There has been quite a bit of variability as to when this will
occur, with the latest models suggesting that happens this
afternoon into this evening, rather than during the overnight.
Thus, the most substantive change to the forecast this morning is
to increase the chances of rain for this evening into the first
half of the overnight tonight for all of the Cook Inlet Region.
Once the front and surface low shift over to Bristol Bay tonight,
southeasterly winds will return to the mountains through Turnagain
Arm and the Knik River Valley. Thus, the winds will increase, and
in the case of the former, will move into south and west Anchorage
tonight. Thus, it will be both windy and rainy tonight.

On Tuesday, as the low over Bristol Bay shifts south and gets
absorbed into the next low, most areas of inland Southcentral will
enjoy a partly cloudy day, while some shower activity persists
along the mountains. This will be a brief break in the weather
action for the area. On Tuesday afternoon and evening, the leading
front associated with another strong low over the North Pacific
will move northwestward into the Gulf. Rainfall associated with
the front will approach Kodiak late in the day Tuesday, but the
northeasterly flow ahead of the front will bring another round of
heavy rain and strong northeasterly to easterly wind to Kodiak.
Most of the energy with the front will impact Kodiak and then
quickly shift west to the AKPEN Tuesday night.

Tuesday night into Wednesday, the leading front will persist
over the Gulf and move north into the coast through the day. This
front will have a connection to deep tropical moisture, so it too
will lead to the next round of heavy rain impacting much of the
eastern Kenai Peninsula, with Whittier likely being the proud
recipient of several inches of rain from this front. The heaviest
rain in Whittier will be midday into the afternoon. The strong
easterly flow with the front should allow for a distinct downslope
hole in the precipitation, so little if any rain is expected west
of the Chugach and Kenai Mountains. It will also mean another
ramp-up in the winds through Turnagain Arm and higher elevations.
Periodic upper level waves, particularly one expected to track
northwestward across Southcentral Wednesday night may briefly
allow the rain to overcome the downslope, especially if the flow
can turn more southerly.

Temperature-wise, the area will remain in unseasonably warm air
masses for the next few days overall, though one exception will be
during the day Tuesday, which despite the expectation of the most
sunshine being on that day, the cooler air mass should largely
offset it. Thus, the arrival of the winter snowpack that stays
until the Spring on the higher elevations of the Chugach will
continue to be delayed.

&&

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

Northeasterly flow aloft, which is a drying flow, will bring much
of Southwest Alaska some sunshine and dry conditions to start the
new work week with above normal temperatures for daytime highs.
Many areas will be between 45-50 degrees Fahrenheit with the
mountains about 5 degrees cooler (40-45F). For tonight some weak
upper level shortwaves are expected to cross the Alaska Range. As
a result, some isolated rain shower activity is expected
especially over the Alaska Range and the Kuskokwim Mountains
tonight into Tuesday evening. However, the flow aloft may be just
enough that a few of these make it off the mountains and affect
the Bristol Bay, Middle Kuskokwim Valley tonight through Tuesday
afternoon. The Yukon-Kuskokwim Delta's best chance to see a brief
shower will be late Tuesday morning into Tuesday evening.

A weak upper level ridge is expected to build in briefly across
Southwest moving from southeast to northwest beginning Tuesday
morning. The ridge will be narrow in nature, therefore any
clearing that occurs Tuesday will be brief before the next system
begins to move in with the increasing clouds. Bristol Bay and the
eastern Middle Kuskokwim Valley will have the best chance Tuesday
to see a good amount of sunshine. Areas further west will take
longer to clear out with the best clearing occurring Tuesday night
into Wednesday morning before clouds increase quickly Wednesday
morning. However, peeks of sunshine are still expected Tuesday.

Attention then turns to the next system that will affect the
region Tuesday night through Wednesday. This system is already
developing over the North Pacific leading to above average
confidence for this system. A storm force low is expected with
hurricane force wind gusts to the south of the AKPEN and north of
Chirikof Island Tuesday night into Wednesday with storm force
gusts for Bristol Bay. Wave heights will also be on the rise
south of the AKPEN with anywhere between 20-25 ft seas expected
with upwards of 30+ feet at the southern Shelikof Strait entrance.
Wave heights in Bristol Bay will be between 8-12 ft. Periods of
rain will also accompany the system with a general 0.5-1" rainfall
amounts, locally higher for the south facing areas of the AKPEN.
As the system comes northwestward the flow will be cross barrier
for much of Southwest leading to downsloping for areas downwind of
the Alaska Range (i.e. King Salmon, Igiugig, and Levelock).
Dillingham is further west and has a better chance to see heavier
rainfall from this system with up to a 0.5" possible there with
King Salmon less than 0.25". Bethel, which is downwind of the
Kuskokwim Mountains in this type of setup, is expected to see a
downslope component as well. The windward facing slopes will
experience more rain than the leeward sides. Breezy conditions
will also be expected across much of Southwest Wednesday due to
the tight pressure gradient that will be present across the area.

&&

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

Northerly flow will continue across much of the Bering again today
with occasional rain showers. Cold air aloft flowing over the
relatively warm water of the Bering Sea continues to provide an
unstable environment resulting in cumulus clouds that occasionally
grow enough to produce a shower, especially if a weak upper level
shortwave passes over the area to help enhance the lift. Breezy
conditions will continue again also due to the tight pressure
gradient present which will begin to diminish this evening.

A frontal system will graze the western Aleutians and Pacific
side marine zones with small craft winds and chance of rain this
afternoon into Tuesday afternoon. Weak high pressure moves in for
Tuesday evening into early Wednesday before another weak system
approaches Wednesday morning with another shot of rain and
increasing winds.

Tuesday into Wednesday, a potent piece of upper level energy will
move southward to the east of a building upper level ridge over
the western Bering. At this time, it appears this will be nothing
more than bringing an increase in clouds to the Pribilofs with a
slight enhancement in shower activity along with a reinforcing
shot of cool air.

Further southeast, our attention then turns to the next storm
system that will be approaching the eastern Aleutians Tuesday
night through Wednesday with storm force winds, increased wave
heights and periods of rain.

&&

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

A strong North Pacific low tracks over the Alaska Peninsula late
Wed. The Gulf will have widespread gale force, with storm force
winds across the Kodiak Island and western Gulf waters. These
winds will diminish a bit Wednesday, but remain elevated. A new
North Pacific low approaches the Alaska Peninsula on Thursday.
There is uncertainty in the exact track and strength of this
second storm, but it looks like it will affect the same areas.
The current forecast will go with widespread gales for now, but
storm force winds are possible Thursday afternoon through Thursday
night from the western Gulf to Bristol Bay, likely spreading into
the Bering Sea on Friday.

For the Bering and Aleutians, A North Pacific low crosses the
Alaska Peninsula and dissipates in the Eastern Bering Thu. This
will produce widespread small craft winds, with smaller areas of
gale force winds likely. The second storm mentioned above has
potential to bring much more widespread gales to the region by
Thursday night/Friday, but confidence is low at this point in
time.

&&

.LONG TERM FORECAST (Days 4 through 7/Thursday through Sunday)...

The pattern will become even more amplified as the upper trough
over the eastern Bering/Aleutians deepens and the upper ridge over
the west coast of North America builds. The tilt of the upper
trough will change from positive to negative which will help
support cyclogenesis along the eastern Aleutians/AKPEN on
Wednesday as a 972 mb low is expected to strengthen south of the
AKPEN on Wednesday and move northwestward into the eastern Bering.
As this low moves northwestward into the Bering it will gradually
weaken due to moving farther away from the core of the jet. The
low then will remain stalled in the eastern Bering through Sunday.
One major impact of the orientation of this jet is a good
potential for strong Turnagain Arm winds during this period as the
core of the jet is over Southcentral Alaska on Wednesday
(favorable for strong Turnagain Arm winds). Moreover, the
northwestward motion of this low into the eastern Bering will
enhance the pressure gradients across Turnagain Arm and cause
these strong winds to be prolonged. This system will also produce
heavy rain across Kodiak and along the windward (south-east
facing) side of the Kenai Mountains. The highest rainfall amounts
will be over Kodiak and along the southern Kenai Peninsula.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...CJ
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...MK/BG/SEB