Gale Warning 732, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
533 AM AKST Sun Nov 26 2023

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

The low that had been sitting out near the eastern Aleutians and
southern Alaska Peninsula the last few days has opened up into a
trough and is lifting north and east toward Southcentral. There
are other out of phase short-waves out ahead of this lifting
northward across the Gulf and into Southcentral. The larger trough
is taking on a negative tilt and pulling a surface low back
toward the Kenai Peninsula this morning. Widespread precipitation
is falling across the region, mainly in the form of rain. The one
exception is the Copper River Basin where precipitation is
primarily in the form of snow. Meanwhile, a barrier jet is ramping
up along the north Gulf coast and into Prince William Sound this
morning as an occluded front extending east from the low center
tracks toward the north Gulf coast.

The low and front will push onshore across Prince William Sound
this morning and weaken. Marginally cooler and unstable air will
wrap around the storm system. The combination of cooling due to
melting snow aloft combined with the little bit of cold air
advection should be just enough to cause a transition from rain to
snow for the western Kenai Peninsula to Anchorage and the Mat-Su.
Timing of the transition will vary based on precipitation
intensity, but the general window will be from mid morning through
early afternoon. Precipitation will linger for much of the day
until the aforementioned upper trough coming in from the west
reaches Southcentral and pushes precipitation inland. The trough
will weaken as it crosses Southcentral tonight, so precipitation
intensity will diminish with time.

Right on the heals of the departing trough, a new long-wave trough
and surface occluded front will move in from the Bering Sea
tonight through Monday, bringing another round of precipitation.
While the overall airmass will start out cooler than the one in
place this morning, there will still be precipitation-type
challenges. Strong southeast flow ahead of the front will warm
the low levels overnight tonight from the Kenai Peninsula to
Anchorage and the southern Mat-Su. In addition to the typical
mountain-top gusty winds, also expect a strong Turnagain Arm jet
to bring some wind to south to west Anchorage overnight through
early Monday morning. The front will track rapidly inland Monday
morning, with winds shifting around to the south to southwest and
colder air moving in. At the same time a potent short-wave aloft
will approach. The end result will be a very quick transition from
warm southeasterly flow (with downsloping in the lee of the Kenai
and Chugach Mountains) to moderate snowfall. There is come
uncertainty in the exact start of snow for any given location,
but it will occur first across the western Kenai in the pre-dawn
hours, then likely reach Anchorage and the southern Mat-Su during
the morning commute. Would expect low visibilities due to the snow
intensity alone, but gusty southerly winds coming up Cook Inlet
could make things much worse. The upper level short-wave will
progress fairly quickly across Southcentral, so expect generally
looks like a 3 hour window for the heavier snowfall. Snow will
head inland Monday afternoon, lingering longest along the south to
west sides of mountain ranges due to the low level south to
southwest flow. Cold air advection will cross all of Southcentral
during the day Monday, bringing snow levels down to sea level for
nearly the entire region. The threat of snow showers will persist
through Monday night thanks to a series of weaker short-waves
aloft and an unstable airmass. It will be hard to narrow down
which areas have the best chance of seeing showers.

Forecast confidence takes a precipitous drop Tuesday through
Tuesday night. Model guidance is all keying on a strong compact
low tracking northward across Southwest Alaska. However, the
spread in track is large. This pattern has potential to bring a
lot of wind into Southcentral if the low takes a more eastward
track across Southwest Alaska. After this, yet another storm moves
up from the Pacific into the Gulf. The one thing that can be said
with confidence as we continue through the week is that
temperatures will be cooler (though perhaps still above normal)
and any precipitation will fall mainly in the form of snow -
especially inland.

&&

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

Key Messages:

 - Some model disagreement remains resolving where a low will form
   along an occluding front tracking across the Bering.
 - There is a low chance (20% to 30%) that ice could pile up along
   the shore across the Kuskokwim Delta (specifically Kipnuk to
   Kongiganak) toward Sunday night.
 - There is moderate chance (30% to 45%) that coastal flooding
   could occur across the Kuskokwim Delta shoreline Sunday night
   to Monday.
 - Temperatures trend cooler through the weekend.

Discussion:

The front of a Kamchatka low will continue to sweep across the
Bering with southerly, gale-force winds reaching the western
coast of Alaska by Sunday afternoon. Confidence is high that this
front will bring gale-force wind, with localized areas of embedded
storm-force wind. Strong cold air advection will bring gusty
westerly winds behind the front this weekend and into next week.
One area where model variation is greatest, is identifying where a
low will develop along the occluding front. Any developing low
would lead to increased winds in their immediate vicinity, adding
to the complexity of forecasting where to expect the strongest
winds. This also affects which areas could potentially see ice
pile up along the shore or experience coastal flooding.

Heavy to moderate precipitation will fall across the Aleutian
Chain which weakens to light precipitation for the Southwest, with
the exception of the Bristol Bay region receiving moderate
precipitation. Temperatures across the entire region will trend
downward through this weekend as colder air builds in behind the
front. Most locations will cool enough for rain to transition into
snow by late Sunday night.

-ER

&&

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

A well developed North Central Bering low moves towards the
Western Alaska coast, extending the longwave trough across the
Southern Mainland Alaska. This trough will dissipate Thursday over
Western Interior Alaska. The trough brings widespread cooler
temperatures across the region through the forecast period, with
areas of heavier precipitation. Higher surf may occur over the
Yukon- Kuskokwim coasts. Further west, a second strong low near
Southern Kamchatka brings a major shortwave trough across the
Aleutians and Bering towards Southwest Alaska, with weaker
shortwave troughs to follow along the Aleutians and Alaska
Peninsula. Large areas of gale force winds are expected to spread
along the Aleutians and Alaska Peninsula through the end of the
period. Forecast confidence for tracks and strengths becomes mixed
through Saturday with the transient lows.

A strong Eastern North Pacific low moves into the Gulf of Alaska
through Saturday. A gale force barrier jet occurs along the North
Gulf Coast, diminishing late Thursday. Gusty winds up to gale
force will spread mostly over the Eastern Gulf, diminishing
through Saturday. Widespread areas of mixed precipitation are
expected along the North Gulf Coast, and spread inland through
the period.

Kutz

&&

.AVIATION...

PANC...Chances for snow into this afternoon. Visibilities are
expected to drop to MVFR with cloud ceilings dropping to IFR
through this morning. Conditions improve late this afternoon as
snow chances drop and cloud ceilings will rise above 3000'. Light
northerly winds this morning flipping to southerly early this
afternoon.

&&