Small Craft Advisory 732, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
521 AM AKDT Sun Sep 27 2020

.ANALYSIS AND UPPER LEVELS...

Satellite imagery shows a strong, occluded cyclone rapidly
approaching the Gulf of Alaska this morning. ASCAT data shows
storm force winds in the cold sector to the south of the storm
center. Bouy data is currently showing gusts up to 45 kts
throughout the gulf, with wave heights up to 17 ft at buoys 001
and 085. The occluded front can be seen on the Middleton Island
radar bringing stratiform rain over coastal areas with showers
over the Kenai Peninsula and Anchorage Bowl visible on the Kenai
radar. Over Southwest, radar indicates showers along the front
extending into the AKPEN and Bristol Bay region. Cold air aloft
that lead to snow over Bethel late Saturday night has been
replaced by warm air from the system in the Gulf. As a result,
precipitation over Southwest has transitioned to rain, although
snow is still possible over the higher elevations.

&&

.MODEL DISCUSSION...

Models remain in excellent agreement with the cyclone moving into
the Gulf this morning. There are slight disagreements on the
possible development of a triple point low to the east of Kodiak
along the occluded front; however, this should have little impact
to the forecast as the overall pressure and wind field remains
fairly uniform.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. Low clouds
this morning are not expected to materialize into a ceiling.
Shower activity should taper off mid-morning as northerly surface
winds begin to pick up.

&&

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

The well-advertised large and powerful low is on track this
morning. The low is near or has reached peak intensity as measured
by the central pressure reaching its lowest point. However, the
forecast hurricane force winds north of the low are not yet
expected until later on this morning. These winds will form as the
easterly flow ahead of the occluded front gets squeezed up against
the coastal mountains, creating a narrowing corridor off the coast
that the strengthening winds will need to pass through. The
primary low will continue to track nearly due north, moving into
Prince William Sound tonight, while its forward speed slows. A
secondary low may develop west of the main low center, off the
coast of Kodiak Island, which will allow the aforementioned
corridor of strongest winds to extend westward to near the Barren
Islands. The winds will rapidly weaken as the weakening low
approaches this afternoon, so the hurricane force winds are only
expected to impact the waters off the North Gulf Coast for the
morning hours today.

Periods of moderate to heavy rain have begun on Kodiak Island
while rainfall rates are increasing across the northern Gulf and
eastern Kenai Peninsula as the mountains help lift the copious
moisture and squeeze out heavier rain. Once the low moves inland
Monday morning, the southerly return flow behind the low will
keep periods of heavy rain ongoing, particularly across the
northern and eastern Prince William Sound on Monday. That
southerly flow will allow for rain and high elevation snow to
develop along the mountains into the Copper River Basin on Monday
as well.

On Monday night, the remnant surface low and associated upper
level low will be absorbed into the upper level trough over
Southwest Alaska. As this happens, and an upper level shortwave
moves into the area as well, off-and-on rain is expected to
impact the Cook Inlet region. This timing is a bit later from
previous forecasts, which had most of the rain Monday afternoon.
Nevertheless, most areas around Cook Inlet should hold at one
tenth of an inch or less of rain.

Then on Tuesday, there will be a brief break in the action ahead
of the next storm, with weak ridging and a cooler air mass moving
into Southcentral. Thus, we currently expect much of the daylight
hours to feature a mix of clouds and sun for all inland locations,
while easterly flow keeps coastal locations under the clouds.
Meanwhile, the next storm in this "parade of storms" that will
impact Southcentral will already be racing across the Gulf. The
parent low will track northwestward south and west of Kodiak
Island. However, its leading front will impact much of the coast,
reaching Kodiak Island Tuesday evening, and spreading across
Southcentral Tuesday night. The front will also be strong
regarding the winds, and storm force winds are in the forecast for
much of the western Gulf late Tuesday night into Wednesday
morning. Thus, the break in the action will be brief.

&&

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

An upper level shortwave continues to push westward across the
Yukon-Kuskokwim Delta this morning. As a result, a few rain
showers were occurring this morning with any threat diminishing
by the afternoon. Some sleet has been mixing in at times with this
shower activity in places like Kipnuk and Toksook Bay with
temperatures in the mid 30s. Bethel is on the edge of a stratus
deck that may occasionally result in lower ceilings/fog at times
this morning. The flow is expected to become more northeasterly
off the land late today and into tonight. This is a drying flow
which will result in gradual clearing for the region overnight.
850mb temperatures won't be all that cold for Monday morning
(generally 2-5 degrees Celsius) keeping generally mid 30s for low
temperatures across the Middle Kuskokwim Valley and Yukon-
Kuskokwim Delta.

For Bristol Bay today, a piece of upper level energy and a surface
trough are expected to make it over the Aleutian/southern Alaska
Range and bring a quick shot of rain to Dillingham and King Salmon
later this morning into the early afternoon. This surface trough
will drift southward and pivot over the AKPEN keeping areas from
Pilot Point to Point Heiden in the threat for rain for much of the
day and into the overnight hours ending from north to south.

For Tuesday, much of Southwest will remain quiet as the area will
be in weak synoptic flow aloft. This will lead to mainly dry
conditions for many areas.

Our attention then turns to the next storm system that will be
approaching the southern AKPEN Tuesday evening. Basically, expect
a return to unsettled conditions with rain and increasing winds
beginning Tuesday night and continuing through Wednesday. There is
expected to be a downsloping component to this system as well,
 but expect leeward side of the mountains to be drier with the
windward side to be wetter due to upslope.

&&

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

Much of the Bering Sea and the Aleutian Chain remains under the
influence of northerly flow. This will continue to aid in cold air
from the Arctic and Siberia to flow over the relatively warm
Bering Sea waters continuing the cumulus cloud development with
occasional showers underneath. The eastern Bering and Pribilofs
will actually experience warming 850mb temperatures tonight
leading to more widespread stratiform rain instead of the
convective variety before another shot of colder air moves back in
Monday morning.

A frontal system will graze the western Aleutians and Pacific side
marine zones with small craft winds and chance of rain Monday into
Tuesday.

Our attention then turns to the next storm system that will be
approaching the southern AKPEN/eastern Aleutians Tuesday
afternoon through Wednesday with storm force winds, increased wave
heights and periods of rain.

&&

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

A strong North Pacific low moves through the Western Gulf into
the Eastern Bering for Thu. The track and development of the low
vary somewhat as it moves over the Alaska Peninsula, but the
system spreads widespread gale force and local storm force winds
over the Western Gulf. It will be gustier around Kodiak Island and
Shelikof Strait on Wed, and will continue through Thu.

For the Bering and Aleutians, the remains of a front over the
Western Aleutians spreads small craft with local gale force winds
through early Wed when it moves into the North Pacific. A Gulf of
Alaska low moves into Bristol Bay Wed and continues into the
eastern Bering for Thu. The track and development of the low vary,
but widespread gale force winds move with the front into the
eastern Bering slowly diminishing Thu.

&&

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

Cold air pooling over the North Pacific will generally maintain a
broad upper level trough across the Central North Pacific. This
will be the spawning ground for strong surface lows as strong
upper short waves round the base of the trough and head north
toward the Alaska Peninsula. The first of these strong surface
cyclones pushes north toward the AK Peninsula Tuesday night into
Wednesday and slowly weakens over the eastern Bering Sea early
Thursday. This system along with the front will bring very strong
winds and moderate to heavy rains, especially from the Alaska
Peninsula to the northern Gulf Coast. The models struggle with the
timing and intensity of the next system developing under
favorable jet support Thursday/Friday. That said, it does appear
that this low track will be further east. If so, then the brunt of
the wind and rain will favor the Gulf of Alaska and Southcentral,
and Bristol Bay Thursday night through early Saturday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Hurricane Force Wind Warning...119, 120.
Storm Warning...351, 352, 139, 130, 131, 132.
Gale Warning...138, 137, 136, 150, 125.
FIRE WEATHER...NONE.

&&

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

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE ANCHORAGE AK
1244 PM AKDT SUN SEP 27 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0839 AM     NON-TSTM WND GST 73 SE CHENEGA           59.44N 146.33W
09/27/2020  M81 MPH          VALDEZ-CORDOVA     AK   ASOS

            REPORTED AT MIDDLETON ISLAND.

0513 AM     NON-TSTM WND GST KODIAK                  57.80N 152.39W
09/27/2020  M59 MPH          KODIAK ISLAND      AK   ASOS

            REPORTED AT KODIAK.

&&

GJERDINGEN