Small Craft Advisory 1011, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
851 AM AKST Sun Dec 27 2020

.ANALYSIS AND UPPER LEVELS...

A broad, deep upper level low is vertically stacked with a strong
942 mb surface low near Adak. Its associated front is currently
tracking over the Southwest Alaska, bringing blizzard conditions
to the Kuskokwim Delta coast and snow/blowing snow to the Bristol
Bay region. Gusty winds and rain/snow showers are likewise seen
across much of the Bering. Conditions are much more quiet across
Southcentral. A weak upper level trough is leading to widespread
snow showers across Prince William Sound, extending across the
Kenai Peninsula and into the Susitna Valley. The aforementioned
front will gradually weaken as it moves inland.

&&

.MODEL DISCUSSION...

Model agreement remains good as is typical with large synoptic
features. Confidence remains moderate to high that much of the
precipitation from an approaching occluded front will remain over
higher elevations. However, all synoptic models indicate an upper
level shortwave moving across Anchorage early Monday morning,
which may result in some light accumulations.

&&

.AVIATION...

PANC...VFR ceilings and light/variable surface winds will persist.
Wind shear will remain an issue through Monday as strong
southeasterly winds will develop over Turnagain Arm under a front.
These will peak around 65 kts at 4000-5000' by tonight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

The strong front associated with the low in the central Aleutians
is moving into southcentral Alaska today. The front is approaching
Kodiak Island early this morning and bringing strong winds and
heavy precipitation. The precipitation is starting off as snow,
but quickly turning to rain.

The major issue with this front will be the winds where they will
bring widespread Gales to marine areas and winds up to 65 mph
through Turnagain Arm. There had been a High Wind Watch out for
Turnagain Arm, but it was cancelled due to this front weakening as
it moves northward.

Snow and periods of blowing snow will also occur in Turnagain
Pass tonight. At sea level there should be enough warming to turn
the precipitation to rain through Portage Valley and eastern
Turnagain Arm. Otherwise a blizzard for Portage Valley would have
been considered.

A big part of the reason the front is weakening is that the jet
stream is remaining well to the south and therefore not giving
this system any significant upper level support. While this will
mitigate some of the warming, it will still be enough to bring
another weekly warm up to Southcentral. Coastal locations,
including upper Cook Inlet, will reach above the freezing point
again on Monday.

&&

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

Snow and blowing snow has developed along the Southwest coast in
advance of a front draped over the eastern Bering. Southeasterly
winds of 30 to 45 mph with gusts as high as 60 mph are expected
through the morning hours. The combination of snow and blowing
snow is expected to reduce visibility at times from Dillingham to
Goodnews Bay to as low as one half mile at times. Farther north
along the Kuskokwim Coast, colder air will allow for a more
powdery snow to fall initially before temperatures begin to climb.
Here, visibilities could drop to as low as one quarter mile
through the morning hours. As such, a Blizzard Warning remains in
effect through noon for the Kuskokwim Coast. For the AKPEN, the
colder air did hold on longer than initially thought. However,
temperatures have climbed in the upper 30s and lower 40s behind
the front, allowing for a change in precipitation type to rain
showers.

The southerly winds from Cold Bay to Sand Point have also
decreased significantly overnight, but are expected to increase
again around mid-morning, albeit at a lesser intensity than
observed yesterday. The snow over Bristol Bay is also expected to
transition to rain showers by Sunday afternoon as the front moves
to the interior. For the Kuskokwim Delta coast, the snow may
changeover to a mix of rain and snow before diminishing in
intensity as the front moves east.

The mix of rain/snow showers late Sunday transitions back to all
snow showers by Monday. The forecast then gets a bit tricky late
Monday into Tuesday as models disagree on the track of a potential
second area of low pressure moving north to the AKPEN. The GFS
keeps this feature south of the area, while the EC and NAM show
this feature moving northwest along the Southwest coast. If this
latter solution holds, there could be another round of snow
transitioning to rain for Southwest Alaska, especially for
locations around Bristol Bay.

&&

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

Widespread gales continue over almost the entirety of the Bering
this morning as a 943 mb low crosses the Chain just west of Atka.
Winds, however, are stronger along the Southwest coast, where
southeasterly, storm-force winds are occurring in association
with a front. Farther west, northwesterly gales and occasional
storm- force gusts persist for Shemya and Attu, enhanced by a
shortwave trough dropping south around the back side of the low.

The mix of rain and snow over the Pribilofs early this morning
have tapered off behind the front. However, rain showers will
develop again by mid-morning as a shortwave lifts north ahead of
the main low. Gusty southerly winds and rain showers will also
remain likely through Monday as the low remains nearly stationary
over the western Bering. By late Monday night, enough colder air
will begin to work in underneath the weakening low to allow for a
transition back to a mix of rain and snow showers or all snow
showers. Areas of gales will also persist across the Bering
through Wednesday, with winds and shower activity enhanced
periodically through the next 48 hours as upper-level shortwaves
rotate around the main low.

&&

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

Gulf of Alaska:
There is good model agreement on a barrier jet moving along the
northern gulf coast with gale force winds on Tuesday. Good
forecast confidence on Wednesday/Thursday for no significant winds
or waves.

Aleutians/Bering:
Model guidance are struggling with the positioning and movement of
a sub 945 mb low moving over the central Bering and western
Aleutians. Depending on the exact track/location, expect the
potential upside risk of gale to storm force winds Wednesday night
through Thursday for the western and central Bering/Aleutians.

&&

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

A weakening Pacific low will move interior near the AKPen on
Wednesday bringing a light wintry mix to portions of the
Southcentral and Southwest Alaska coast, but with little
impact for locations further interior thanks to high pressure
parked over east central Alaska.

The real forecast excitement will be found further west with
models coalescing around the idea of another powerful (sub 940 mb)
low forming near the western Aleutians Wednesday night into
Thursday. The details of the exact low track still are yet to be
ironed out, but snow/blowing snow may once again be a significant
concern across the western Aleutians. In addition to this,
widespread gales and storm force winds appear possible across the
central Bering/Aleutians.

On Friday this storm looks to lift into the central Bering while
gradually weakening. Expect a continued risk for widespread gales
across the Bering, and a wintry mix for much of the Aleutians
through the AKPen. Model agreement begins to fall apart on
Saturday, but the general idea will be continued unsettled weather
across the Bering/Aleutians and relatively quiet weather across
Southcentral as high pressure dominates.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning: 155.
         Winter Weather Advisory: 161
MARINE...Storm Warning: 160 176 185.
         Gale Warning: 119 120 125 129-132 136-138 150 155 165
                       170-175 177-181
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AP
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...BG/MF

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
150 PM AKST Sun Dec 27 2020

AKZ125-281500-
Western Prince William Sound-
Including the cities of Whittier, Seward, Girdwood,
and Moose Pass
150 PM AKST Sun Dec 27 2020

...GUSTY WINDS, SNOW AND AREAS OF BLOWING SNOW EXPECTED FOR
PORTAGE VALLEY THROUGH TURNAGAIN PASS TONIGHT...

Southeast winds will increase across the area later today into
tonight as a front approaches the Kenai Peninsula. Expect
sustained winds to increase to 25-35 mph with gusts up to 70 mph
in the Eastern Turnagain Arm and Portage Valley tonight.

Accumulating snow is also expected, with around 1 foot of
accumulation anticipated at Turnagain Pass by early Monday. Late
tonight, snow levels will increase changing precipitation over to
rain below 1000 feet elevation.

Areas of blowing snow and reduced visibilities can be expected
tonight, especially from Portage Valley through Turnagain Pass.
This will lead to difficult travel conditions along portions of
the Seward Highway that could continue into Monday morning.