Small Craft Advisory 891, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
546 AM AKST Fri Nov 20 2020

.ANALYSIS AND UPPER LEVELS...

A large area of low pressure in the eastern Bering Sea is slowly
moving northwestward. An occluded front that wraps around the low
center is moving northward. A triple point low is developing along
a negatively-tilted trough south of Cold Bay and is moving
northeastward while strengthening. An occluded front northeast of
this developing triple point low continues to move northeastward
ahead of the low center. A large upper level ridge of high
pressure extends from the Arctic Ocean to the Alaska Range. The
jetstream currently remains south of 50N west of 155W. The jet
then splits with the southern portion extending eastward to over
British Columbia and the northern portion extending northwestward
to over eastern Siberia and the Chukchi Sea.

Closer to the surface, radar imagery shows areas of light snow
along the coastal areas of the Kuskokwim Delta as well as light
snow showers moving northwestward across the northern Gulf of
Alaska. Observations show a southeasterly Turnagain wind around 20
mph with downinlet-flow, thus these winds are not entering the
Anchorage Bowl. A major temperature contrast exists along the
Anchorage Bowl with 10sF and light winds across lower elevations
while temperatures are in the upper 20sF to low 30sF with
southeasterly winds around 10 mph.

&&

.MODEL DISCUSSION...

Forecast confidence remains below average as large model
inconsistencies exist with the placement and intensity of the
developing triple point low. This is primarily due to the uncommon
aforementioned jetstream pattern as models are having trouble
resolving it.

&&

.AVIATION...
PANC...Ceilings and visibility will remain VFR. Light and variable
winds will persist at the surface, however there will be stronger
southeasterly winds aloft (130 degrees at 35 kts) starting at 21z.
Due to those stronger winds aloft, low-level windshear is
expected. Winds aloft will then weaken after 21st/10z and thus
chances for low-level windshear will significantly decrease at
that time.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Sunday)...

A front extending from a mature area of low pressure in the
eastern Bering will spread wind and rain northward across the
western Gulf and Kodiak Island, with a wintry mix reaching western
Prince William Sound by this afternoon.

Tonight into Saturday morning, models indicate a new triple point
low forming along this front near or just east of Kodiak Island,
with the GFS being the most aggressive with its development. As
this low deepens, storm force winds are expected to develop from
the Barren Islands into portions of the Western Gulf, with periods
of gusty winds and plentiful rainfall expected for Kodiak Island.
Its front will push towards the northern Gulf Coast with a storm
force barrier jet expected ahead of it. Late Saturday the low
weakens as the front moves ashore causing winds to rapidly
decrease below gale force across the Northern Gulf.

Additionally, accumulating snow is expected for portions of the
eastern Kenai Peninsula and Western Prince William Sound,
including Turnagain Pass especially tonight into early Saturday.
The accumulating snowfall shifts towards eastern Prince William
Sound Saturday afternoon.

For the Anchorage area, strong easterly/downslope flow should
prevent much in the way of precipitation today through early
Saturday, but a few snow showers will be possible later on
Saturday. An increase in winds for the Turnagain Arm and Matanuska
Valley is also expected tonight into Saturday morning before
diminishing Saturday night as the low weakens.

This dissipating surface low will meander northward up the Cook
Inlet on Sunday bringing some slightly better snow shower chances
to Anchorage. Also on Sunday, another storm system looks to lift
out of the North Pacific bringing another round of rain/wind to
the AKPEN and Kodiak.

&&

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

A strong front can be seen on satellite and radar this morning
extending from Hooper Bay to Pilot Point. Mostly snow is expected
along the front as it moves east into the interior through
Saturday. The exception to this should be from Pilot Point
northeast to Iliamna where rain is likely Friday, transitioning
to a rain/snow mix Friday night and then snow on Saturday. Coastal
areas from Dillingham to Nunivak Island may also see a rain/snow
mix through Saturday due to warmer air aloft and warmer surface
temperatures near the water.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

One last day of strong winds is expected Friday over the eastern
Bering as an occluded low continues to make its way north. The
strongest winds should be to the south of the low center and near
the coastline along the occluded front. Over the central and
western Bering, cold air advection may allow for snow showers
through Saturday. By late Saturday night the next system is
expected to approach the central Aleutians. There is some model
discrepancy in the exact track of the low which will dictate the
type and intensity of precipitation. Stay tuned for more details.

&&

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

Gulf of Alaska:

A lot of uncertainty remains during the 3-5 day timeframe. There
is moderate confidence a front associated with a strong low enters
the western Gulf Sunday afternoon. There is potential for storm
force winds over the Gulf waters beginning Sunday night and
lasting through Tuesday as the low tracks eastward into the Gulf.
There is still considerable uncertainty with the track and
strength of this low. Widespread gales are likely during this
time, however will need to continue to assess regarding the timing
and location of storm force winds.

Bering Sea:

Gale force winds are likely over the northern Bering waters
Sunday. There is potential for a storm force low to enter the
western Bering Sunday night. This low will spread gale force winds
across the central and eastern Bering through Tuesday. Model
solutions continue to struggle with strength and timing, however
there is high confidence in an active weather pattern over the
Bering through Tuesday.

&&

.LONG TERM FORECAST (Days 4 through 7...Monday through
Thursday)...

Beginning Monday, an upper level ridge over the central Bering is
flanked by two upper level lows - one over the Kamchatka
Peninsula and the other centered over the Gulf. As the Kamchatka
low moves eastward Monday into Tuesday, a more amplified pattern
will develop. Strong northwesterly flow is still possible over the
Alaska Peninsula late Monday into Tuesday as the pressure gradient
strengthens considerably. Meanwhile, ample cold air filling in on
the backside of the Kamchatka low will enhance winds across the
western Bering Tuesday. Model solutions (GFS/EC) suggest that a
second low develops over the central Aleutians late Tuesday into
Wednesday and lifts northward over the Bristol Bay area. As we
typically see with this type of pattern, there is potential for a
triple point low to form somewhere in the vicinity of Kodiak
Island late Wednesday. Depending on the track of this triple
point low, the middle of next week may present a more active
weather pattern across Southcentral.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 119 120 130 131 150 160 180 351.
Gale Warning 125 132 136 137 138 139 140 165 170 171 173 179
181 182 185 352 412 413 414.
FIRE WEATHER...NONE.

&&

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