Small Craft Advisory 1002, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
506 PM AKST Wed Dec 23 2020

.ANALYSIS AND UPPER LEVELS...

A relatively active and somewhat chaotic pattern continues. A
broad upper trough extends from Russia to over the Northeast
Pacific. Numerous short waves embedded within the longwave are
supporting surface lows stretching from the Western Aleutians to
the North Pacific and Gulf. A short wave lifting north across the
Gulf/Southcentral is producing snow and rain showers across the
the aforementioned areas. A 970s mb low near Kuskokwim Bay is
moving north toward the Delta. Gale force northerly winds
associated with this system were evident on its backside over the
eastern Bering/Pribilof islands and southerly gales to the east
over Bristol Bay and Kuskokwim Bay. Cold advection snow showers
were evident across the eastern Bering Sea/Pribilof Islands. A
pair of surface lows located over the extreme Western Aleutians
generated widespread showers and gusty winds in the region.

&&

.MODEL DISCUSSION...

Models have similar synoptic pattern across Southcentral/Gulf/AK
Pen through Friday morning. After that, the NAM model becomes a
significant outlier in the east as it tracks a storm force low
west across the Gulf Friday/Friday night. Models struggle with
the next major low to approach the Bering Sea Saturday, but all
except the NAM show at least a 940s low near the Central Aleutians
Saturday. There will certainly be storm force winds with this
system and possibly hurricane force.

&&

.AVIATION...

PANC...Gusty southeast winds should diminish this evening and
remain relatively light through Thursday morning. VFR conditions
should prevail. However, a passing snow shower tonight could
drop ceiling/visibilities briefly to MVFR.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 3)...

A dynamic though relatively benign weather pattern is expected
over the next few days. A weakening area of low pressure will move
into the western Gulf Thursday afternoon while a broad ridge of
high pressure extends northwestward along the west coast of North
America into eastern interior Alaska. This synoptic set up will
favor offshore flow across Southcentral Alaska. Pressure gradients
are favorable for enhanced winds in the Copper River Delta,
Thompson Pass, Valdez Arm/Narrows and in the Matanuska Valley due
to the northeast-southwest pressure gradient. The peak of these
winds will be Thursday night into Friday morning as this is when
pressure gradients will be the steepest. These winds will begin to
ease midday on Friday as the low continues to weaken while moving
farther to the northwest, thus the pressure gradients will also
weaken. This low will bring precipitation to Kodiak Island as well
as the southern Kenai Peninsula and Prince William Sound on
Thursday. Precipitation type will mainly be rain along the coast
as temperatures will be too warm for snow. However, slightly more
inland temperatures will be colder and the dominant precipitation
type will be snow. Precipitation will taper off from east to west
Thursday afternoon through late Friday morning. There is a second
low expected to move into the northern Gulf on Friday evening,
though confidence remains low at this point in time as there are
significant differences in solutions with respect to the placement
and intensity of this low.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Christmas
Eve and Christmas Day)...

Two weather systems will affect the area to end the week. First,
a compact, but rather intense low will move onshore in Kuskokwim
Bay tonight. Some snow showers are already tracking inland with
this system and they will become much more widespread as we go
through the night. On the backside of this system, early Thu
morning, expect a shot of very cold air (-15C at 850mb or 5000')
with gusty NW winds. If there is any lingering snow falling,
expect some localized pockets of blowing snow, mainly along the
northwest coast from Toksook Bay to Hooper Bay.

After this, attention shifts to a developing system coming up to
the area from Kodiak Island. There should be enough antecedent
cold air to keep precipitation nearly all snow with this event.
The low will push plenty of moisture up and over the
Aleutian/Alaska Ranges before it runs into the cold air. Along
this boundary, expect a fairly efficient deformation band to
establish itself. This band will likely bring 2-5" of snow to
parts of interior Bristol Bay as it swings through into the YK-
Delta. The good news is that the band does look to be transient
enough (moving from east to west) that it doesn't look like any
one location will see excessive amounts of snow. Over the YK-
Delta, there will be enough cold air and strong north winds
that we introduced a widespread area of blowing snow with this
forecast package. Models are still locking onto exactly how
cold the air will be and exactly where the band will set up.
However,this will need to be closely monitored over the next
couple forecast updates for the potential for a Winter Weather
Advisory. For further details, please see the Special Weather
Statement issued this afternoon.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Christmas
Eve and Christmas Day)...

The Bering will be a busy place for weather the next couple of
days. A strong low and associated front will exit the Eastern
Bering tonight into SW AK. The front is draped across the Eastern
Aleutians and the Alaska Peninsula where it will continue to make
for snowy and blustery conditions through the evening. As this
system shifts east, it will leave plenty of cold air, gusty winds,
and snow showers in its wake.

Meanwhile over the Western Aleutians, a broad upper level low
complex will create a breeding ground for many compact polar lows
to form. One of these lows is blasting through Shemya this
evening, but it should continue to track quickly off to the east.
Due to the compact nature of these lows, it is very challenging to
predict where the worst conditions will occur. Therefore, expect
gale-force (with some isolated storm-force) winds and snow
squalls across the Central and Western Bering for the next few
days. A much broader and warmer front will move into the area by
Fri afternoon. While this will allow the pattern to change over to
more rain, it will also bring widespread storm-force winds back to
the marine zones.

&&

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

Gulf of Alaska: A front will move in from the Bering Sea on
Sunday, bringing at least small craft winds, but potentially
gales. In any case, it will weaken as it crosses the Gulf Sunday
through Monday.

Bering Sea: On Saturday a powerful area of low pressure sets up
across the western Bering with a gale force front extending
through the central Bering into the eastern Aleutians. Models
continue to struggle with the exact placement of this front and
the overall intensification rate of this low, but future forecasts
may need the introduction of storm force winds along a good
portion of this front (up from the current widespread Gales). On
Sunday and Monday this low remains mostly stationary across the
western Bering, but models indicate that a few smaller areas of
low pressure may rotate around this larger low, which could result
in some compact areas of Gales or even storm force winds across
portions of the Bering/Aleutians. Pinning down the location/timing
of these lows remains difficult at this time range.

&&

.LONG TERM FORECAST (Days 3 through 7: Sunday through Wednesday)...

The biggest feature in the long term will be the broad and persistent
low across the western Bering. This low will have a broad occluded
front extending through Southwest Alaska and the AKPen on Sunday,
with this front lifting towards the Northern Gulf Coast Sunday
night into Monday. This front will bring a wintry mix to southwest
AK, and another round of higher elevation snows to the Eastern
Kenai and Prince William Sound, but with minimal impacts for areas
further inland. For Tuesday and Wednesday a few compact lows look
to continue to pivot around a broader low across the Bering continuing
the unsettled pattern there. Relatively quiet conditions will be
found across the Gulf of Alaska and Southcentral - a welcome
break from the extremely active pattern we have been in as of
late.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 173
 Gale Warning 130 131 132 136 137 138 172 174 175 176 177 179 180.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...ED
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
MARINE/LONG TERM...BG