Small Craft Advisory 672, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
424 PM AKDT Wed Aug 24 2022

.ANALYSIS AND UPPER LEVELS...

The anchored upper level low which has been sitting south of
Kodiak Island is now opening up into a trough as it lifts
northward across the western Gulf and toward Southcentral Alaska.
Areas of rain which had been rotating over the top of the low
from southeast to northwest across Southcentral are now beginning
to track off to the north-northeast. The heaviest rainfall today
has been at the tip of a jet streak and atmospheric river, across
eastern Prince William Sound and the Copper River Basin. However,
this is all steadily shifting eastward to the eastern Gulf.

Out west, a deep vertically stacked low is crossing the Kamchatka
Peninsula and into the Bering Sea. A leading frontal system
extends well out ahead of the low into the central Bering Sea,
producing rain and small craft advisory to gale force winds.
CIMMS Layer Precipitable Water satellite imagery shows a
robust atmospheric river originating over the western Pacific
near Typhoon Tokage and extending northward across the western
Aleutians and Bering and into the low center. This airmass
associated with this is quite warm and juicy, with precipitable
waters in excess of 2" and 850mb temperatures as high as
17.5 degrees Celsius (as seen on the 12Z sounding from Shemya).
Some of the rainfall is likely pretty heavy, but is falling
mostly over the open waters of the Bering Sea.

In between the Gulf trough and the Bering storm, conditions
are quiet over Southwest Alaska and the eastern Bering Sea.
In fact, much of Southwest Alaska is experiencing clear skies,
a rarity in recent weeks.

&&

.MODEL DISCUSSION...

Models are in good agreement with the overall pattern the next
few days. There are some differences with amplitude of the upper
level trough as it settles over mainland Alaska Saturday. The
NAM seems to be an outlier with its less amplified solution and
resultant much lower rainfall amounts for Prince William Sound and
the Copper River Basin. Otherwise, there are the expected
differences with depictions of the plethora of short-waves moving
through the long-wave trough. However, this doesn't affect the
fundamental forecast for southern Alaska, with a continuation of
cloudy and wet conditions.

&&

.AVIATION...
PANC...Periods of rain will continue as the upper trough lifts
northward across Southcentral. To this point, southeast flow has
largely maintained VFR ceilings at the terminal (with the
exception of some fog in the vicinity). This will change tonight
through Thursday morning as the trough exits northeastward, with
flow gradually weakening and veering to the S to SW. As a result,
expect ceilings to slowly lower, likely reaching MVFR levels by
Thursday morning. Confidence in timing of this is low. With a
continued very moist airmass, fog will likely reform in the
vicinity. With deep southwest flow in place Thursday ahead of
an approaching front, ceilings will be slow to lift.

As far as surface winds, the terminal will sit along the very
northern edge of the Turnagain Arm jet this afternoon/evening,
so gusty southeast winds may come in and out, but should generally
be on the weaker side. A stronger push of Turnagain winds will
come in Thursday afternoon as pressure gradients build ahead of a
strong front pushing onshore of Southwest Alaska.

&&

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

An upper level trough and attendant surface low between Kodiak
Island and the Eastern Kenai Peninsula will move east over the
next 12 to 24 hours, bringing a slow decrease in rain and shower
activity through early Thursday afternoon. Behind this trough is a
large front and westerly to southwesterly flow aloft, which will
increase precipitation coverage once again from west to east
Thursday afternoon through Friday. Precipitation is expected to
be widespread and heaviest overnight Thursday and into much of the
day on Friday. With the increased southerly flow and coastal
convergence, southerly gap winds will also increase through
Turnagain Arm and the Copper River basin. These winds will likely
maintain themselves through Saturday morning when a surface low is
expected to develop in the lee of the Kenai Peninsula. Overall
forecast uncertainty for precipitation increases on Saturday as a
stronger shortwave rounding the southern periphery of the Bering
trough moves over the western Gulf. The north/south trajectory of
this shortwave will determine precipitation amounts and timings,
particularly for the Copper River Basin. All in all, expect more
of the same...

&&

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

The calm conditions seen currently in Southwest Alaska are to be
short-lived as a rapidly approaching front appears it will move
inland by late this evening. This front will be associated with
strong precipitation across Southwest Alaska, and will persist
into the weekend. The front will bring elevated sustained winds as
well as southerly gusts of 40-50 mph, reaching from Kipnuk to
Tuluksak. A High Surf Advisory has been issued for the coast in
this area. Steady, onshore flow will continue across the Southwest
coastline following the passage of the cold front, however gusty
conditions are expected to ease into Thursday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

A prominent cold front currently over the Pribilofs will move
east out of the area, into Southwest Alaska by Thursday. Steady
precipitation along the boundary of the cold front will continue
as the front tracks eastward. Elevated wind speeds and westerly
40-50 mph gusts are expected following the front due to strong
mid- level support and a 150 kt upper level jet. Lingering showers
will remain behind the front for much of Thursday. A new system
of precipitation will then enter the eastern Aleutians Thursday
afternoon, as the core of precipitation will follow the chain
reaching the Central Aleutians by Friday morning. The core of this
wave of precipitation is expected to then drop south of the
Eastern Aleutians and out of the area. Weak ridging to develop in
the Western Bering Saturday afternoon.

&&

.MARINE (Days 3 through 5: Saturday through Monday)...

Gulf of Alaska: Forecast confidence is high for Saturday and
Sunday with respect to winds remaining below gale force with no
high seas expected. Confidence is then much lower on Monday with
respect to the intensity of the low in the Gulf and this
translates to low confidence in the wind and wave forecast.

Aleutians/Bering: Forecast confidence is moderate overall during
the 3-5 day period due to some minor to moderate inconsistencies
between guidance. Winds are likely to remain below gale force
across much of the area. However, there is a chance of gale force
northwesterly gap winds along the eastern Aleutians and along the
western Alaska Peninsula on Sunday.

&&

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

There is high forecast confidence with respect to the overall
synoptic pattern during the extended period due to a high degree
of ensemble agreement. Odds tilt in favor of cooler than normal
temperatures across Southcentral and Southwest Alaska as upper
level troughing is likely to extend across the area. Confidence
remains lower with precipitation timing and amounts as low
pressure will remain nearly stationary in the northern Gulf of
Alaska Sunday through Tuesday. A second low is expected to move
eastward from the Bering Sea to over Southwest Alaska on Tuesday
and Wednesday. Models show significant differences in the
placement and intensity of this low which makes for low forecast
confidence in precipitation timing and amounts at this time.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory: 121.
MARINE...Gales: 180, 181, 185, 411, 414.
FIRE WEATHER...NONE.

&&

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