Small Craft Advisory 959, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
447 AM AKST Fri Dec 2 2022

.ANALYSIS AND UPPER LEVELS...

Upper level ridging stretches over the eastern Bering Sea and
Southwest Alaska, shifting eastwards to encompass Southcentral as
well, over the next couple of days. Warmer air aloft exists under
the ridge, showing between ~2-4 degrees Celsius (~35-39 degF)
across the models, that could bring about increased temperatures
across Southern Alaska. An elongated trough moves against the
Southwest coast Sunday morning, and inland Sunday afternoon. The
upper levels will begin cooling off, but still show hovering
around the freezing mark. Thus, precipitation type may vary on
Sunday until a colder airmass moves in later that night. Ridging
scoots off to the east as the trough makes its way across the
Mainland towards Southcentral Alaska.

&&

.MODEL DISCUSSION...

Models are in good overall synoptic agreement through the short
term. There are some minor discrepancies with the North Pacific
low as it tracks northwards across the Bering Sea, but there is
good consensus for the general track and speed of the system.
There is also a good handle on the large upper level ridge as it
builds over Southern Alaska.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Friday through Sunday night)...

Gusty offshore winds will continue to wind down through the
morning hours as the upper-level ridge currently over the AKPen
moves east towards the Gulf of Alaska. A series of weak upper-
level waves will move west to east over the top of the ridge,
allowing for periods of high and mid-level clouds to move over
Southcentral through the next several days. Otherwise, conditions
are expected to remain generally clear and dry through late
Sunday. The biggest forecast challenges in the short term will be
the extent of cloud cover and pinning down overnight low
temperatures, and whether any low stratus and/or fog sets up in
favored valley and coastal locations. This ridge will advect
warmer air aloft (850mb temps rising to +2 degC), lending to a
warming trend at higher elevations through Sunday.

For Sunday, the ridge begins to slide east as the next shortwave
and associated weather front moves to the west coast.
Precipitation is expected to develop around Kodiak Island by late
Sunday afternoon with more widespread light snow developing from
Cook Inlet north to the Susitna Valley by Monday morning. One more
forecast challenge with regard to the aforementioned ridge, will
be the extent of marine stratus development. If widespread marine
stratus does develop over the western Gulf underneath the ridge,
it is possible these clouds move toward the immediate coast on
Sunday as a southwesterly wind flow develops.

&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Today and
Saturday)...

Another day of quieter weather is store today thanks to a ridge
of high pressure anchored over Southwest Alaska. Mid to high level
clouds riding on top of the ridge will stream across the
Kuskokwim Delta, while Bristol Bay remains mostly clear.
Temperatures along the coast will be near or above freezing with
inland valleys remaining in the teens. Heading into the weekend,
the ridge axis shifts slowly eastward in response to a broad upper
level trough setting up over the Bering Sea. At the surface, a
North Pacific low lifts north of Amchitka Saturday morning and
then near Saint Paul Island by Sunday morning. Increasing
southerly winds will help draw enough warmer air for rain as the
main precipitation type along the Alaska Peninsula between late
Saturday and Sunday.

Meanwhile, models seem to agree to have the moisture and
associated precipitation staying along the Kuskokwim Delta coast,
especially for Toksook Bay and Kipnuk. Further inland is looking
less likely to see precipitation given the lack of moisture and
drier airmass. For late Friday night into early Saturday morning,
a brief period of light freezing rain is possible for Bethel,
albeit trace amounts are expected. By mid- morning and early
afternoon, the area will see rain as the atmospheric column of air
warms above the surface. A brief break of drier is in store for
the rest of Saturday before the next round of precipitation
returns by Sunday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Today
and Saturday)...

A ridge of high pressure will slowly shift eastward in response
to a broad trough over eastern Russia extending southward into the
Bering Sea. Ahead of the trough, a low pressure system lifts
north of Amchitka late tonight, bringing unsettled conditions for
the weekend. Rain will increase throughout the today, with
widespread gales along the Aleutian Chain and over the Pribilof
Islands. The low will track near Saint Paul Island by Sunday
morning, with southerly gales out of the bays and passes for the
eastern Aleutians.

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

An active pattern continues as several systems transit the Bering
Sea. Wet and windy conditions are in store through Tuesday, with
a transition towards cooler temperatures. Average model agreement
early in the extended period deteriorates significantly by
Wednesday, leading to significant uncertainty in the track of an
incoming North Pacific low.

The long-term forecast begins on Monday with two fronts, a weak
one extending over Mainland Alaska and a much stronger one
extending across the Western Bering and Aleutians. The first front
will continue to weaken as it tracks east through early Tuesday,
with precipitation likely to be heavy at times along the Alaska
Peninsula, Kodiak Island, and the north Gulf coast. The front over
the Western Bering and Aleutians will also weaken as it tracks
east. Heavy rain and winds up to storm force should weaken to
light precipitation and winds up to small craft speeds as the
front impacts Southwest Alaska Tuesday morning. As this second
front follows quickly on the heels of the first, there looks to be
an extended period of light to moderate precipitation (likely
snow, given cooler temperatures aloft) for Southern Alaska from
Monday through Wednesday. The biggest areas of uncertainty with
these fronts will be peak winds and the timing of incoming
inclement weather.

Model agreement deteriorates significantly by Wednesday.
Ensembles seem to support a North Pacific low moving up into the
Bering Sea on Wednesday, then dipping south into the Western Gulf
towards the tail end of Thursday. Deterministic models are in poor
agreement with the speed and strength of this low. The
deterministic ECMWF model cleaves most closely to the ensembles
but features a strong low with central pressure below 960 mb. The
GFS is much more conservative, with minimum pressure dropping to
the high 980s. Meanwhile, the deterministic Canadian model is an
outlier in that it keeps the low in the North Pacific. Much
uncertainty remains, though it looks highly likely that the Bering
Sea region and Southwest Alaska will see active weather at some
point on Wednesday and Thursday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 145
MARINE...Gale Warning: 170 172 176-179 185 411 413 414
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KM
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MF
LONG TERM...KC

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
302 PM AKST Fri Dec 2 2022

AKZ101-111-050300-
Anchorage-Matanuska Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Palmer, Wasilla, Sutton, and Chickaloon
302 PM AKST Fri Dec 2 2022

...AREA CREEKS EXPECTED TO REMAIN HIGH THROUGH THE WEEKEND...

Recent cold temperatures have caused area rivers and streams to
increase in stage due to freeze-up. Localized ice dams have formed
on Peter's Creek near Stoltze Bridge, as well as Campbell Creek
in Far North Bicentennial Park near Basher Dr. Localized bankfull
conditions to minor flooding have been reported in these
locations. Expect localized overflow on area trails in Far North
Bicentennial Park. Water levels will continue to fluctuate.