Gale Warning 531, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
415 AM AKDT Sun Sep 17 2023

.ANALYSIS AND UPPER LEVELS...

A nearly stationary upper level closed low over northwest Alaska
anchors a long-wave over the Bering Sea and Alaska region. A wavy
subtropical jet centered between around 45N and 55N latitude is
leading to a series of fast moving deep lows. A large vertically
stacked low with surface pressure around 970mb is tracking
eastward across the eastern Gulf of Alaska and toward Southeast
Alaska. Rain is beginning to diminish across Southcentral as the
low pulls away and flow shifts around to the north. A trailing
short-wave trough extends from the northwest Alaska low southeast
to Bristol Bay. This trough is lifting northeastward across
Southwest Alaska, producing steady rain over interior Bristol Bay
and the western Alaska Range.

Strong pressure gradients between the Gulf low and a ridge over
the eastern Bering Sea combined with a strong jet streak and
subsidence along the front side of the upper level ridge is
leading to strong and gusty winds across the western Gulf and
Kodiak Island waters, with gusts of 50 kt (57 mph) or higher
through bays and mountain passes. Gap winds are also rapidly
strengthening along the eastern Kenai Peninsula and western Prince
William Sound, especially through Resurrection Bay and Passage
Canal. Winds are diminishing across Southwest Alaska as the ridge
approaches from the Bering Sea.

Out west, a short-wave trough is tracking eastward across the
western Aleutians, with a trailing upper low over the Kamchatka
Peninsula. A broad and fairly weak low center is located in
between the two features over the far western Bering Sea. A more
significant feature is an intensifying low just ahead of the
Aleutian short-wave. This compact low is passing south of Adak
and Atka, tracking east-northeast toward the eastern
Aleutians/Unalaska.

&&

.MODEL DISCUSSION...

Guidance remains in good agreement with the current system as it
begins to weaken and enter the eastern Gulf of Alaska. The wind
and precipitation fields are pretty consistent between models as
both this system slides east and the trailing elongated upper-
level trough moves across Southcentral Alaska through thy day
Sunday. There are still some differences with the next shortwave
trough from the northern Pacific, which looks to be similar to the
system today; however, guidance is now clustering the center of
this system well, putting it on a track that would take it from
just south of Unalaska to southern and eastern Gulf by Tuesday.
Models are also in decent agreement with the next systems that
moves into the southern Bering by late Tuesday and to Bristol Bay
by Wednesday.

&&

.AVIATION...

PANC...Expect a good chance of MVFR ceilings (or possibly even
IFR) this morning as low level flow shifts around to the
northwest and brings rain back to the terminal. However, this
should be short-lived, with rain exiting quickly eastward to the
mountains later this morning where it will linger into the
afternoon hours. Therefore, ceilings should rebound to low end
VFR by 18Z. A short-wave trough moving up the Inlet may hold
ceilings at that level until this evening. In any case, generally
expect ceilings to continue to lift through the day.

&&

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

GOES satellite imagery currently shows a large low pressure system
moving across the northern Gulf of Alaska, analyzed at ~970mb at the
surface. This low pressure, along with and in quick-succession
with the preceding low, was responsible for much of the rain
observed across south-central Alaska over the last day.
Precipitation amounts have ranged from just a few hundredths
across the Anchorage bowl and the Mat-Su Valley to several inches
along portions of the Prince William Sound.

Rain will begin to taper off today as the low pressure system
weakens as it shifts towards SE AK, though precipitation will likely
linger on the Chugach, Talkeetnas, and portions of the Wrangells
through Sunday night into part of Monday. Snow-levels will fall
from about 5000 ft to near 3000 ft by the time precipitation
ceases altogether.

A rather strong wind field currently exists around the low,
particularly on the backside (west side) given upstream ridging
across western Alaska. Widespread gale force winds are forecast
for much of today, with storm force winds expected across the
Barren Islands corridor. NW to SE gap winds will also be out in
full force, with strong winds gusting in excess of 30 kts out of
Resurrection Bay and Passage Canal. Winds will then diminish
substantially overnight as the low shifts away.

Monday and Tuesday will be fairly quiet weather days for
southcentral Alaska. Yet another low will move into the Gulf on
Monday, but will remain far enough south that most of the region
will stay dry outside of perhaps Kodiak Island and some of the
coastal mountains. Ridging will amplify across the area Monday
night, leading to clearing skies. Combined with cool/seasonable air
in place, temperatures will likely fall to the coolest observed
since late May. Locations in the Mat-Su Valley and even Anchorage
may dip to near or below freezing by dawn Tuesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3: Today through Wednesday morning)...

An upper-level low continues to remain nearly stationary near the
Bering Strait this morning. However, a shortwave upper-level ridge
builds in for most of today, giving most of mainland Southwest a
break from active weather. The break will not persist, especially
for the coast, as the next round of rainfall comes later this
evening.

Behind the quick moving ridge, two systems will be the next
weather makers for the region. The first of which is another
North Pacific low, not too different from the North Pacific low
last night/yesterday, which rides eastward along the central and
eastern Aleutian Chain today and makes it to the southern Alaska
Peninsula (AKPEN) tonight. While similar to yesterday's system,
this system is expected to be a little weaker at gale-force, with
pockets of storm-force winds, as opposed to widespread storm-
force; and the ridge behind this low is not as amplified either.
Nonetheless, moderate to heavy rain and gusty winds will accompany
this system before it exits to the Gulf of Alaska Monday morning.
The eastern Aleutians, AKPEN, and southwest dry out for Monday
afternoon as the ridge behind the system moves eastward and over
the area.

Meanwhile, the second system, slowly dropping out of Kamchatka,
ejects a shortwave southeastward through Bering this morning and
through the day and provides another round light rain for the
western Aleutians, the Pribilof Islands, and eventually, coastal
Southwest Alaska tonight. the main upper-level low and surface low
enter the western Bering Monday. This is expected to be a large
system, that sends numerous fronts through the Bering. The surface
feature is expected to be gale-force as well. The upper feature
becomes positively tilted and elongates on Tuesday. This upper
flow regime puts the eastern Bering and southwest Alaska within
deep moist southwesterly flow. The western Aleutians will enjoy
clearing on the backside of this trough with ridging building in
for Tuesday into Wednesday before the next front arrives during
the day Wednesday. The Pribilofs, central Aleutians, eastern
Aleutians, AKPEN, and Southwest Alaska will be unsettled and back
in the showery regime for Tuesday and Wednesday as numerous
shortwaves pass over those areas.

&&

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

Few changes since yesterday's long-term discussion. Active weather
remains in the forecast due to a progressive and amplified
pattern, though there will be brief breaks between systems. The
hazards we'll be keeping an eye on for this period are: (1) an
atmospheric river moving into the Gulf on Wednesday and (2) the
potential for higher water along Southwest Alaska coastlines for
Friday and Saturday.

The long-term forecast begins on Wednesday with an occluding low
near the Alaska Peninsula and an atmospheric river moving into
the Gulf of Alaska. The influx of moisture and vorticity advection
with these two systems will likely bring widespread rain to all
but the Central and Western Aleutians/Bering Sea. The highest rain
rates and amounts are expected around Prince William Sound through
the day on Wednesday. Today's model runs seem to have trended
towards higher rain amounts than yesterday's. While it's too early
to accurately forecast rain amounts, guidance is currently
suggesting a 15-30% chance that communities in Prince William
Sound will see 2 inches of rain in 24 hours. We'll get a clearer
picture of rain amounts as we get closer to the event. Either way,
a rainy day is in store for much of Southern Alaska.

The Western and Central Aleutians/Bering Sea will get their own
rain day(s) from Wednesday evening into Thursday as a potent
gale-force front moves through. The GFS and ECMWF operational
models are in good alignment with the strength of the low,
suggesting around 970-980 mb at its deepest. These two models
also agree with the general track of the low and its front, but
overall model agreement deteriorates past Thursday night, leading
to uncertainty for the latter half of the extended period. While
the low's track and strength bring the potential for persistent,
stronger onshore winds and higher surf, confidence is too low at
this time to say whether higher surf and elevated coastal water
levels will occur.

&&

SYNOPSIS AND MODEL DISCUSSION...SB/TM
SOUTHCENTRAL ALASKA...MJB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DN
LONG TERM...KC