Small Craft Advisory 643, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
239 PM AKDT Tue Aug 16 2022

.ANALYSIS AND UPPER LEVELS...

An upper level trough and N-S oriented jet streaks continue to
promote the advection of shortwaves into the southern Alaska
region. The E-W pressure gradient in the eastern Bering has
loosened, leading to calmer winds out of the southern gaps and
passes. Currently,the high pressure in the central Bering
continues to promote widespread stratus across much of the area.
However, this weak surface ridging will do little to hold back the
advancing warm front from the Western Bering. This front and its
associate low will reintroduce rainy conditions across the
Aleutians and Southern Bering. A weakening low pressure complex
over mainland Alaska is positioned to be reinvigorated by incoming
Bering and Gulf lows, perpetuating unsettled weather over much of
southern Alaska. The brief lull in precipitation for southwest
Alaska will be ceased by the aforementioned Bering low. Moist,
onshore conditions will continue for Southcentral, accentuated by
the incoming Gulf low.

&&

.MODEL DISCUSSION...

Model guidance is in strong agreement through Wednesday morning
where solutions begin to diverge. Forecast confidence is high for
today and Wednesday and average for the later this week. Some
slight discrepancies include timing the persistence of gales
along the northern Gulf coast, and locating the areas of highest
precipitation associated with the Bering low Thursday afternoon.
Agreement at the mid-levels, and nailing down shortwave passages
through Southern Alaska is the source of much of the model
disagreements Thursday and beyond.

&&

.AVIATION...

PANC...Current VFR conditions are expected to persist through
Wednesday afternoon with some spotty showers possible. Gusty winds
from the southeast should turn away from the terminal by this
afternoon but may return by late Wednesday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through
2...Today through Wednesday Night)...

A surface low over the southern Gulf will slowly move towards
Kodiak Island through tonight with a frontal system moving into
the northern Gulf today. This will bring moderate to heavy rain
into Prince William Sound and the eastern Kenai Peninsula. A
couple of weak shortwaves at 500mb will move inland today which
will help to spread precipitation north of the coastal mountains
with light rain expected today from Kenai to Anchorage as well as
the MATSU valleys. Winds will also increase across the Turnagain
Arm and higher elevations this afternoon and evening with wind
gusts up to 45 mph. As the initial wave moves through this morning
there may be breaks in the rain this afternoon and evening for
inland valleys as weak downsloping may occur. However, showers
will continue to be possible across the inland valleys through the
rest of today as weak shortwave energy moves northward across the
area. The upper level low that was south of the Gulf will move
north by Tuesday night and Wednesday morning this will bring a
shortwave into Southcentral by Wednesday morning with another
round of rain possible from Anchorage northward. Meanwhile, the
surface low near Kodiak Island will slowly move north into the
Kenai Peninsula on Wednesday. This will keep moderate to heavy
rain going along coastal locations. By Wednesday afternoon and
evening the main shortwave will move northward and inland this
will continue to bring rain inland, so expect Anchorage and the
MATSU valleys to be wet most of the day on Wednesday. Instability
will also develop ahead of this shortwave with isolated
thunderstorms possible across the Talkeetnas and the Alaska Range
Wednesday afternoon and evening. As we head into Wednesday night
there is more uncertainty with the forecast as there is
disagreement with the evolution of the next low pressure system
moving towards Southcentral. Thus, the timing of precipitation is
in question.

&&

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

An upper-level low is located over Northern Alaska this morning,
and a shortwave continues to pinwheel around the southern side of
the low across the Y-K Delta. As a result, winds and rain will
persist for the next few days, and the low itself will enhance
rainfall as it drops south over Southwest on Wednesday.
Precipitation will start to clear by Thursday morning as the upper low
exits to the northeast. Expect drier conditions and generally
cloudy skies to persist for Thursday through Friday morning while
the next system approaches from the west. A large front is
expected to near the Southwest coast on Friday morning.

&&

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

A deep low near the Kamchatka Peninsula extends its front and
associated upper-level shortwaves across the western and central
Bering/Aleutians this morning. Steady rain and stronger southerly
winds with the front will continue spreading east through
Wednesday morning, at which time the front and associated low exit
into the North Pacific.

Another large, deep low will enter the domain from the west
on Wednesday. Rain is expected to begin for the western Aleutians
starting Wednesday morning with the arrival of the front. By
Thursday morning, the low center will be located near the tip of
the western Aleutians and heavy rain will encompass much of the
western and central parts of the domain. Gale force winds will
also develop along the backside of the low. By Friday morning,
the center of the low will track to the central Bering and its
front will near the Southwest coast. Expect rain and stronger
winds with this feature to persist through the end of the
forecast period.

&&

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

Gulf of Alaska: Winds may increase to gale force along a front
tracking through the western Gulf on Friday or Saturday. The
preferred area for this would be through the Barren Islands and
Kamishak Bay, though timing of the frontal passage is uncertain
and confidence in winds reaching this criteria is low. Seas are
expected to remain less than 20 ft.

Aleutians/Bering: A low tracking into the western Bering on
Thursday is likely to be accompanied by gale force winds wrapping
the center. Winds diminish gradually as the low reaches the
eastern Aleutians on Friday, with northerly gales west of the low
moving through the central Aleutians. Some northerly gap winds may
reach gale force across the eastern Aleutians through Saturday. Seas
may approach 25 ft on the Pacific side of the western Aleutians
on Thursday. Seas to 20 ft may persist in the vicinity of the low
center through Friday afternoon before subsiding. Confidence is
moderate.

&&

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

A wet pattern is on tap. The NCEP Coupled Forecast System model
Version 2...more commonly known as CFSv2 has a broad area of
negative height anomalies over much of the state this forecast
period. This is seconded by the GFS which has negative height
anomalies at 500 mb over much of the Bering, Southwest and
Southcentral Alaska through this forecast period. The ECMWF, the
Canadian and the GFS look active over the Bering and the Gulf Of
Alaska with multiple surface lows. While the global models are
consistent with the longwave pattern....the very same global
models are not in lock step with the tracks of the surface lows.
To account for discrepancies and conflicting signals late in the
period, a multi-model blend was used for this forecast package.
Anchorage and the Mat- Su Valleys will have a few brief breaks in
precipitation but overall the pattern suggests that the rainy
season has arrived and will continue.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales: 119, 120, 125, 131.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CL
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AF
MARINE/LONG TERM...PS/CQ