Small Craft Advisory 957, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
405 PM AKST Fri Dec 11 2020

.ANALYSIS AND UPPER LEVELS...
A high amplitude trough extends from the Bering Sea southward
into the North Pacific, with a corresponding high amplitude
ridge downstream from the eastern Gulf northwestward across
Southcentral Alaska and on up to the west coast of Alaska. A weak
surface low continues to spin nearly stationary near the Aleutian
chain. The most significant feature today is a storm force
vertically stacked low approaching the southern Alaska Peninsula.
This is producing widespread gale force winds from the western
Gulf of Alaska to the far eastern Bering Sea, with a smaller area
of storm force winds embedded in the easterly flow ahead of the
low center. Also of note, a strong short-wave east of the low
center is tracking northward into the western Gulf, overhead of
an occluded front at the surface. This strong short-wave is
pushing up against the ridge to the north, causing it to weaken
with time. This is a more aggressive weakening of the ridge thane
expected yesterday, which will affect the track of the next low
headed toward the Gulf on Saturday. In any case, rain (locally
heavy) extends from Kodiak Island westward along the Alaska
Peninsula to the eastern Aleutians. Dry and seasonably cold
conditions prevail across the southern mainland of Alaska under
the influence of the strong ridge overhead.

&&

.MODEL DISCUSSION...
Models are now fully locked on to the track and depth of the
storm force low as it crosses the Alaska Peninsula and then
weakens as it continues northwestward across the Bering Sea.
There has been a shift in solutions over the past 24 hours with
handling of the next storm tracking from the northeast Pacific to
the Gulf of Alaska weekend. The track has shifted westward,
resulting in impacts across the entire Gulf instead of just the
eastern Gulf. This is largely due to the weakening and shifting
of the upper level ridge as noted in the Analysis section above.
While all models have followed the same trend, differences in the
details of low shape, depth, and track remain. The GFS is a bit of
an outlier with its depiction of a single concentric low. The NAM,
ECMWF, and Canadian solutions all indicate one or more elongated
low centers. Ultimately, they all track the low to near Kodiak
and lift a front northward across the Gulf and toward Southcentral
Sunday night through Monday - but again - there is a spread in the
details of this. Thus, confidence in timing of winds/precipitation
is low, but the overall trends in the forecast should be good.

Meanwhile, a strong subtropical jet will enter the northwest
Pacific Sunday through Monday, setting up a pattern of fast moving
deep lows over the North Pacific and Aleutians as we head into
early next week. Model spread with handling of individual storm
systems is large, so forecast confidence is low.

&&

.AVIATION...
PANC...VFR conditions will persist. Surface pressure gradients
will strengthen in response to the storm crossing the Alaska
Peninsula. As a result, expect northerly winds to strengthen.
While they will generally be less than 12 kts, would not be
surprised to briefly see winds above that threshold at the
strongest point - which looks to be late tonight through
Saturday morning.

&&

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

An active pattern returns to the Gulf, after a few days of high
pressure. An area of low pressure in the northeastern Pacific will
move across the Alaska Peninsula this evening. This will bring
gale force winds and higher seas to the western Gulf through
tonight. Additionally, rainfall will move across Kodiak Island,
and the southern and eastern Kenai Peninsula. The offshore flow
and local gap winds from the last few days, especially across the
north Gulf coast, eastern Kenai Peninsula and western Prince
William Sound, will increase tonight as the pressure gradient
tightens between high pressure inland and low pressure moving into
the Gulf.

Models have come into better agreement with a second low pressure
system that was originally projected to stay in the eastern Gulf.
The system loses its circular shape as it is forced to the
northwest by the high pressure over the interior. This elongates
the low, diagonally, across the Gulf. Gale force winds, with the
potential for storms, will enter the southcentral Gulf Saturday
morning and move up towards the north Gulf coast through Sunday.
Precipitation looks to remain all rain for Kodiak Island, as
easterly flow dominates and warm 850 mb temperatures continue to
inundate the Island through Sunday. Precipitation should largely
remain as rain over coastal locations with the exception of a
wintry mix of precipitation anticipated for western Prince William
Sound. This is due to the warm air being advected into the area
from the low in the Gulf. Temperatures are expected to rise across
Southcentral due to this, with inland locations nearing the 32
degree mark. Expect a continuation of strong offshore flow as the
pressure gradient is met with this temperature gradient to finish
off the forecast period.

&&

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

An oblong low moving over the AKPen Friday night will bring the
first round of unsettled weather to Southwest on Saturday. This
will be primarily in the form of strong winds due to the pressure
gradient between the low over the eastern Bering and the ridge
over the interior. Snow is likely over the Kilbuck Mountains
during the day Sunday due to upslope flow associated with the low
as it moves northwest towards Nunivak Island.

Saturday evening the next surface low is expected to move into
the western Gulf, creating an elongated, negatively tilted trough
from Kodiak Island to the Kamchatka Peninsula. A parade of upper
level energy and southeast moisture advection aloft should help to
expand snow throughout Bristol Bay and into the Kuskokwim Delta
by early Sunday morning. Snow should be the heaviest over and to
the northwest of Dillingham. Current snowfall totals are estimated
to be around 6 inches on Sunday with amounts increasing over the
higher terrain. Winds appear to be below advisory criteria at
this time; however, depending on the location of the low over the
western Gulf there could be potential for blowing snow over and to
the north of Dillingham.

On Monday there is considerable uncertainty in the forecast in
regards to the surface low over the western Gulf. Broad consensus
is that snow will continue over Bristol Bay with northerly wrap
around flow bringing snow to the AKPen. Snow totals for these
regions are expected to be around 5 inches.

&&

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

A storm force low crossing over the AKPen Friday night will move
into Bristol Bay on Saturday. This low is expected to slowly
weaken as it moves north over St. Matthew Island while phasing
with a broad region of low pressure over the western Bering. Over
the Aleutian chain persistent cool westerly winds will provide a
chance for snow showers to the western Aleutians while warmer air
over the central and eastern Aleutians will bring mixed
precipitation or rain. Forecast confidence decreases on Monday
over the western Bering as a broad, negatively tilted trough
approaches from the Kamchatka Peninsula.

&&

.MARINE (Days 3 through 5/Monday through Wednesday)...
There will be a low somewhere over the western Gulf/Kodiak Island
region Monday, with a front pushing onshore along the north Gulf
coast. While there could be some gales with this early on, the low
and front will be in the weakening phase as we head through the
day.

Attention will then shift to the north Pacific, as a series
of deep lows form, with one or more lows likely crossing the
Aleutians into the Bering Sea Tuesday through Wednesday. Would
expect to see another low track into the Gulf in this time period
as well. However, the complex nature of the pattern is such that
we cannot yet determine the track and strength of individual
storms in this time period. Bottom line, expect multiple fairly
fast moving storms across the forecast area, likely with gale or
storm force winds.

&&

.LONG TERM FORECAST (Days 4 through 7/Tuesday through Friday)...
The weather pattern next week looks quite dynamic, with a strong
subtropical jet stream over the North Pacific and a deep Arctic
low/trough somewhere over the Yukon and Mainland Alaska. There
will certainly be interaction between the northern stream and
southern stream energy. This pattern favors active weather across
the entire forecast area, with fast moving deep lows likely across
southern areas (the Aleutians eastward to the Gulf of Alaska). As
far as details on individual storms, it is impossible to say due
to the shear number and complexity of features. Stay tuned.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 120 131-139 150 155 178 181 185.
		 Storm 130 160 165 180.
		 Heavy Freezing Spray 139 140 160 180 181.
FIRE WEATHER...NONE.

&&

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