Gale Warning 514, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 AM AKDT Tue Sep 13 2022

.ANALYSIS AND UPPER LEVELS...

On satellite imagery there is a decaying low over the Y-K Delta
with the remnants of an occlusion wrapped into it and spread
across the state. The longwave pattern is highly amplified and
features a trough axis over the Alaska Peninsula. There is a jet
core of 130 knots south of Dutch Harbor. On radar there is light
precipitation over portions of SW Alaska and Southcentral Alaska.
Also on satellite imagery although distant this morning, there are
multiple tropical systems in the West Pacific Basin. The one of
keen interest is Tropical Storm Merbok. Merbok will be the next
tropical to impact the Aleutians when it does become extra
tropical.

&&

.MODEL DISCUSSION...
Models are in decent agreement through the next 24 hrs. Then model
divergence emerges in the northern Bering and in the Northwest
Pacific. The global models all have a major low pushing across the
Bering followed by Merbok becoming extratropical later this week
and moving into the Bering. But the track and the intensity for
both lows is not totally dialed in yet. As the model data updates
expect the forecast to be adjusted.

&&

.AVIATION...
PANC...Light rain has fallen over night and early this morning. On
the Kenai Radar (PAHG) continues to ping onto light precipitation
over northern Cook Inlet this morning. Cigs will bounce around
and light showers are possible at times. Low level wind shear has
been added to the TAF again and is actively being monitored.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through
2: Today through Thu morning)...

Southerly flow continues today across the area with a couple of
shortwaves moving through the area producing areas of showers.
The first shortwave is near the coast this morning and will move
inland early today. A stronger negatively tilted shortwave will
quickly move northward enhancing precipitation across Prince
William Sound today and towards the Seward area. Another one to
two inches of rain is expected across Prince William Sound
through Wednesday. By this afternoon this shortwave will push
inland and begin to weaken but widespread showers are still
expected across much of Southcentral through this evening. Winds
will be relatively light, however some gusty southerly winds are still
expected across the Copper River Valley through this afternoon.
As we head into Wednesday the flow weakens as the upper level
trough moves across the area. Showers are still expected across
the region but coverage and intensity will decrease as we head
through the day on Wednesday. Wednesday night into Thursday clouds
may begin to clear which may set up some areas of patchy fog.
However, confidence is low on where this may occur so kept it out
of the forecast for now. Thursday looks to be the nicest day of
the week with a mostly dry day and some sunshine possible.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Friday morning)...

A vertically stacked low centered over Kipnuk this morning will
continue to occlude and drift towards Bristol Bay over the next
day. Bands of showers will continue to rotate around the low
center, with most areas likely to receive multiple showers
throughout the day today. Shower coverage will decrease on
Wednesday with the exception of southern Bristol Bay and the
Alaska Peninsula, where onshore flow and coastal convergence
should keep the rain going through the day.

A storm forming today south of the Gulf of Anadyr will send it's
front east and into Southwest Alaska Wednesday evening. Southerly
to southwesterly winds will increase in conjunction with the rain,
which could be moderate to heavy at times. The front will move
slowly into the Interior, and finally out of the region by Friday
morning. Behind the front, onshore winds will persist with
occasional showers, but also occasional clearing.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Today through Friday morning...

An occluding low over the Kuskokwim Delta will keep winds slightly
elevated at Small Craft through the next 24 hours over the eastern
Bering. Focus then shifts to a rapidly strengthening low emerging
south of the Gulf of Anadyr this afternoon and into the Western
Bering on Wednesday. A corridor of storm force winds are expected
in the offshore zones around 58N latitude. The storm will also
increase southwesterly winds to 30 kts for the Pribilofs, eastern
Aleutians and Alaska Peninsula through Thursday morning.
Widespread rain will accompany the front.

A major storm is expected to impact the western Bering/Aleutians
starting Thursday afternoon. This storm is the remnants of
Tropical Storm Merbok and is expected to become a potentially
hurricane force extra-tropical low as it taps into colder air over
the Western Bering. Model consensus on the low track has improved
significantly with the cross-over point between the Pacific and
Bering somewhere between Shemya and Amchitka. Temperature
differences across the low center are around 13-17 degrees C over
a 50 mile stretch, leading to an exceptionally baroclinic system.
High end storm force to hurricane force winds are likely, there
are just minor differences to work out on where the corridor of
the strongest winds will be. After the low crosses into the Bering
the overall track becomes less certain on Friday. Some solutions
have the low tracking into the Norton Sound, while others track it
into the Kuskokwim Delta, either way the Pribilof Islands and
Southwest Coastline will have gale to storm force winds on Friday.

&&
.MARINE (Days 3 through 5: Thursday through Saturday)...

Bering/Aleutians...Gales are likely around a low that moves
through the Northern Bering Sea on Thursday, with seas up to 20
ft. Thursday afternoon through Saturday, a strong North Pacific
low moves across the Aleutian Islands and Bering Sea. Broad areas
of gales are very likely across most of the Bering Sea and
Aleutian Islands, and the potential remains for sustained storm
force winds and hurricane force gusts. Seas around 20 to 30 ft.

Gulf of Alaska...Confidence is moderate that winds will be less
than 35 kt and seas less than 20 ft. There may be localized areas
of gales in the Northern Gulf on Saturday, depending on whether a
developing low is strong enough to produce a barrier jet and
enhance winds out of Kamishak Gap.

&&

.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

Expect generally unsettled weather through the extended period,
with brief breaks possible as transient ridges move across
Southcentral Alaska early Friday and across the Bering Sea and
Aleutian Islands on Monday.

The biggest story continues to be the interaction of a Northern
Bering low with the remnants of what is currently Tropical Storm
15W or "Merbok." Models have come into better agreement since
yesterday, increasing our confidence in the strength and track of
these storms. The consensus is that the Northern Bering low will
weaken by Friday, then become subsumed into the remnants of Merbok
in the Southern Bering Sea. The resulting low looks to bring
strong winds and heavy rain over much of the Central and Eastern
Bering Sea as well as the Pribilof Islands and the Central and
Eastern Aleutians. As the low approaches Southwest Alaska Friday
morning, high surf and coastal flooding become concerns. The
influx of southerly to southwesterly gales into Southwest Alaska
is a favorable pattern for coastal flooding, though the speed of
this low and the relatively short duration of strong winds could
reduce concerns. Details remain uncertain for now, especially
given model differences in the track of the low. For Southcentral
Alaska, the fronts from each of these two systems will contribute
to cloudy and rainy conditions. With the low tracking northward
along the Southwest coast, a tightening pressure gradient could
also lead to enhanced easterly winds through Turnagain Arm and
other gaps.

Winds should gradually subside through Saturday as the low
continues north and moves out of the region. Rain showers will
likely continue through Saturday as a longwave trough and several
shortwaves move over the region. By Monday, the weather looks to
become more active as two separate systems enter the region. For
the Southern Mainland, a surface low looks to develop off the base
of the upper level longwave trough and move into the Gulf of
Alaska. Further west, a low in Russia looks to push its gale-force
front into the Western Bering and Aleutians.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 185 412. Storm 411.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...RLC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
MARINE/LONG TERM...KC