Gale Warning 248, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 PM AKDT Thu Mar 16 2023

.ANALYSIS AND UPPER LEVELS...

The high amplitude ridge/trough pattern persists across the state
this afternoon. Northerly flow between the two continues across
the Bering with gusty winds and snow showers from the eastern
Aleutians to the Alaska Peninsula. For Southwest Alaska,
conditions are generally clear and dry with scattered mid to high
level clouds.

To the east, the upper-level trough axis extends from Kaktovik to
the north south and west over the north Pacific. Multiple waves
continue to rotate around the upper-level low anchoring the
trough. The first of these waves is moving inland over
Southcentral with an increase in mid and high level cloud cover.
The second of these waves is associated with an occluded surface
east of Kodiak City. A surface trough extending from this feature
is moving from the northern Gulf to the Southcentral coast, with
yet another band of snow showers from Homer and Seward across
Prince William Sound to Cordova.

Satellite imagery is also showing the development of a baroclinic
leaf, the precursor to the next surface low, as yet another
shortwave rounds the base of the trough and begins to move north
toward the Gulf of Alaska. It is this system that will produce
widespread precipitation and gusty winds across the Gulf and
southern half of Southcentral Alaska beginning Friday.

&&

.MODEL DISCUSSION...

Models remain in overall agreement thorough Saturday morning,
having now homed in on the evolution and track of the compact
storm-force low as moves from the eastern Gulf to the Barren
Islands. Beyond that time, however, model divergence grows,
especially with the potential development of a new storm-force
surface low. There was quite a bit of uncertainty regarding this
feature within the 00z and 06z guidance. Most recent runs (12z and
18z), however, are starting to key in on a solution of surface low
development over the eastern Gulf for late Saturday morning,
moving east-northeast toward the Barren Islands for Sunday
morning. The 12z GFS is the strongest with this feature, taking a
slightly more southerly track. The 12z and 18z NAM have a weaker
feature that tracks through the northern Gulf and along the
Southcentral coast. The 12z GEM and EC depict solutions much more
similar to the GFS.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through early
Friday morning. Light snow is expected to develop by early morning
Friday, potentially lingering through late Friday morning. MVFR
conditions are expected during this time with the potential for
IFR conditions if a period of more moderate snow develops.
Northerly winds will also increase beginning early morning Friday
with gusts approaching 25 kt by mid-morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Friday and Saturday)...

A near-stationary low that had been persisting off the east coast
of Kodiak Island is finally beginning to weaken this evening.
Through the rest of tonight low pressure will continue to weaken
and ultimately dissipate over interior Bristol Bay. The big
forecast challenge over the next 24 hours entails the next North
Pacific low, which will cross over Kodiak Island by midday
tomorrow. A winter weather advisory is in effect for Kodiak Island
with snow and blowing snow expected through Friday morning.

Strong southerly flow east of the N Pacific low will bring an
influx of moisture along with snow into coastal regions of
Southcentral. Warming temperatures through Friday afternoon will
see some areas transition from snow to a rain/snow mix, or even
all rain. Coastal areas of Prince William Sound, including
Cordova, Whittier, and Portage will be the most prone to the
changeover. Likewise, Seward is also expected to see a warm-up
through midday on Friday. Further inland, moderate to heavy snow
is forecast from Valdez up into Thompson Pass. Gusty winds are
expected to reduce visibility due to blowing snow, and a winter
weather advisory in effect for the region. Moderate to heavy snow
is also forecast across the Eastern Kenai Peninsula, especially
from Moose Pass up into Turnagain Pass.

Areas of light snow remain possible across the Anchorage Bowl up
into the Mat-Valley on Friday. Lower elevation areas may see an
inch or less of accumulation. Higher elevations could see around 2
inches of snow. With the southern position of the Gulf low over
Kodiak, the pressure gradient will reintroduce enhanced Mat-
Valley winds on Friday. Wind gusts ranging from 25 to 35 mph are
expected during the late morning and early afternoon hours. Snow
and blowing snow may reduce visibility at times.

A second Gulf low is forecast to push across Kodiak Island on
Saturday. This second low is looking stronger with a broader
region of storm force winds possible across the Gulf, and the
potential for hurricane force gusts. A slightly more southward
track may spare the Kenai Peninsula and Prince William Sound from
the strongest winds. Another round of precipitation, including
rain and snow, appears likely over the second half of the day.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Sunday)...

A trough situated over Mainland Alaska will continue to bring
begin conditions across Southwest. As the aforementioned trough
continues to sit in place, warmer air wrapping around will
eventually move into the region later this weekend and bring back
chances of precipitation to the Bristol Bay area by at least Friday
evening through Saturday morning. Otherwise, expect clear
conditions tonight and then again by Saturday afternoon, along
with a gradual warming trend through the forecast period.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 3: Tonight
through Sunday)...

A trough anchored over Mainland Alaska continues to bring colder
air down from the north across the Bering. This is creating
convective showers with gusty winds that have been impacting the
Alaska Peninsula, eastern Aleutians, and the central Bering/eastern
Bering the past couple days. By Saturday, chances for precipitation
begin to increase for the western Bering as shortwaves drop from
the north and the ridge currently in place shifts to the west.
Meanwhile, the eastern Bering will continue to experience benign
conditions with northerly flow. The next big weather maker will
track into the western Bering by Sunday and will bring back
precipitation, gusty winds, and southerly flow to the Bering as it
slowly tracks east through next week. Be sure to check back with
the forecast as this system develops!

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

Looking into the extended forecast, both deterministic guidance
and their respective ensembles concur on the continuation of an
active weather pattern through Thursday for both domains. A pair
of negatively tilted shortwave troughs lift northward from the
Gulf of Alaska and into Southcentral Alaska through Wednesday
morning. The best chance of precipitation associated with these
shortwaves will be for coastal Southcentral due to having the best
upslope enhancement. A more significant trough digs southward from
Siberia, through the Bering Sea, across the Alaska Peninsula, and
into the Gulf Wednesday as it becomes negatively tilted in nature.
However, both deterministic guidance and ensemble guidance are not
in agreement with the ultimate evolution and behavior of this
trough. The 12z Canadian deterministic guidance is the furthest
north with the closed upper-level low in the northern Gulf, while
the GFS is way further south with the upper low in the North
Pacific and south of Kodiak Island. The 12z ECMWF is in between
the GFS and Canadian. Where this upper trough decides to
negatively tilt and where the upper low closes off will ultimately
determine the track and strength of the associated surface low
for Wednesday into Thursday and its effects for Southcentral.

Out west, Southwest Alaska will initially be unsettled as a deep
Bering Sea low sends its front into the coast and inland Monday
afternoon bringing a chance of moderate to perhaps heavy
precipitation, especially for coastal sections of Southwest. An
additional front on Wednesday and Thursday may impact Southwest,
but as alluded to previously, there is a lot of uncertainty with
this system at both the lower levels and upper levels. The Bering
Sea and Aleutian Chain both look to remain active as well with
continued lower heights and overall trough conditions persisting
through Wednesday. The front that affects Southwest on Monday
afternoon, first sweeps the eastern Aleutian Chain Monday morning
with moderate precipitation and gusty winds. After that,
conditions remain unsettled with additional shortwaves digging
southeastward from Kamchatka, through the Bering, and across the
Chain through Wednesday. Zonal flow, from west to east, tries to
establish itself through the Bering on Thursday, but there is a lot
of uncertainty there as guidance hints at a North Pacific low
trying to lift northward around the western Aleutian Chain around
that time as well.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 131 171.
MARINE...Storm Warning: Northern Gulf.
Gale Warning: Barren Islands, Kamishak Bay, Shelikof Strait,
Prince William Sound, Lower Cook Inlet.
Heavy Freezing Spray Warning:Alaska Peninsula, Bristol Bay,
Pribilof Islands, Kuskokwim Delta.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...NS
LONG TERM...DN