Small Craft Advisory 14, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
748 AM AKST Fri Jan 3 2020

.ANALYSIS AND UPPER LEVELS...

A broad, cold upper level low situated over the AKPEN last night
continues to slowly drift southeast toward the western Gulf this
morning. A vorticity lobe advecting from the northwest and
undercutting the low, along with a some additional upper-level
support in the form of a 60 kt jet streak, has helped spin up a
surface low south of Kodiak Island. This feature has deepened to
992 mb overnight with a cold front extending south into the north
Pacific and a warm front lifting along the eastern shores of
Kodiak Island. The frontal system is bringing snow and blowing
snow to Kodiak City this morning. The Middleton Island radar is
also showing that some of the moisture along the front is lifting
north toward the eastern Kenai Peninsula coast. Elsewhere across
Southcentral, a weakening trough over the Southcentral coast
continues to produce areas of mid-level clouds from the Susitna
Valley to the southern half of the Copper River Basin. Snow
showers are also evident on radar from Prince William Sound to the
Copper River Delta.

For Southwest Alaska, it remains clear and quite cold with only a
few isolated packets of mid-level clouds rotating east to west
around the upper-level low. The coldest overnight temperatures are
in the Kuskokwim Valley where readings have dropped to -20F to
-40F once again. Temperature have warmed to 0F to -15F across the
 AKPEN as clouds, snow, and winds have all been on the increase in
 response to the low south of the region. An interesting note: the
 low temperature of -41F last night in King Salmon was not only
 the coldest spot in the state, but the coldest reading there
 since February 2nd, 2006.

Farther west, northerly winds and snow showers persist across the
eastern half of the Bering, the Pribilofs, and the eastern
Aleutians as a shortwave continues to dive south along the
backside of the upper-level low. The warmest spot in the state
outside of the Alaska Panhandle tonight is Shemya (32F), where a
stalled and weakening frontal system has ushered in some warmer
air on southeast winds.

&&

.MODEL DISCUSSION...

Forecast guidance remains in good agreement with the larger,
synoptic scale features through the weekend. The biggest
challenges remain the handling of the complex low currently south
of Kodiak and potential development of smaller, mesoscale lows in
the western Gulf over the next 24 to 48 hours. The position of
these features will have an impact on the location of mesoscale
snow bands and wind direction. Differences in these elements will
have implications for the forecast in places like Kodiak Islands
through Saturday and across the Kenai Peninsula and Prince
William Sound by Sunday.

There is also disagreement concerning the evolution of the surface
low, and its possible interaction with a trough stretching across
the Bering, as it tracks east across the Aleutian Chain late
Friday through Saturday. This system, however, looks to remain
rather weak with little upper-level support. Therefore, any
significant wind and/or precipitation impacts are expected to be
minimal at this time.

&&

.AVIATION...
PANC...VFR conditions and light northerly winds are expected over
the airport complex throughout the day. There is still a slight
chance for some low stratus and/or fog to move over the aerodrome
this morning as nighttime satellite imagery and overnight
observations from PABV (Birchwood) indicates a scattered cloud
deck of 400 ft along Knik Arm. Confidence in this, however, is
low as northerly winds are expected to increase and help scour out
any lingering low-level moisture and inhibit further development
of low cloud cover.

&&

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

A low moving into Prince William Sound brought scattered snow
showers but these should taper off this morning with skies
clearing through the day. With generally higher pressure to the
north and lower pressure to the south, northerly synoptic winds
will bring cold, Arctic air into Southcentral for the next several
days with highs in the single digits and lows well below zero for
much of the region.

A Winter Weather Advisory is in effect through 9 AM Friday
morning due to a low southwest of Kodiak Island. Total snow
accumulations could vary significantly across the island
depending on where the heaviest squalls setup. After a brief lull
this afternoon, the same pattern is expected to redevelop
overnight Friday into Saturday morning with the same
aforementioned impacts expected. There is uncertainty with the
track of this low Saturday into Sunday with models suggesting it
should stay over the northern Gulf. If this low does propagate far
enough north chances for precipitation will be limited to coastal
regions before making landfall east of Cordova sometime Sunday
morning.

&&

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

The upper level low over the Alaska Peninsula continues to slowly
move southeast. This will continue the trend of clearing
conditions and keep temperatures cold. The associated surface low
is also moving off to the southeast resulting in relaxing the
pressure gradient across the southwest mainland that will diminish
the northerly winds. The wrap around cold advection across the
eastern Bering Sea is diminishing, as are the winds, thus the
blowing snow conditions in the Pribilof Islands to Unalaska have
abated. There will continue to be snow showers in the cold
advection, coupled with north winds, that will reduce visibilities
in blowing snow. The wind and snow showers look to diminish this
afternoon as the gradient around the back side of the low
diminishes.

&&

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

A front from a low off the Kamchatka Peninsula to a weaker low
south of shemya will elongate to the east southeast through the
day as the low weakens. A weak north Pacific low will move to just
south of Adak overnight. This will bump up the winds again across
the western and central Aleutians and will also bring in some snow
or a mix of rain and snow Friday night and Saturday. We have
bumped up the POPS for the central and western Aleutians to
account for this low. There is a possibility of near blizzard
conditions across the Aleutians east of Seguam Island in advance
of the north Pacific low coming in Friday Night and Saturday
morning.

&&

.MARINE (Days 3 through 5)...
On Saturday, a surface low across the southern Gulf keeps small craft
to gale force winds around Kodiak Island, then tapers down by
Sunday morning. The next strong storm organizes over the north
Pacific and pushes a front to the western Aleutians Sunday night.
This storm system moves slowly northeast spreading gales across
the southwest Bering through Monday night. There is still some
uncertainty on the speed and track of this system so adjustments
are likely to be made.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Southcentral:
Dry and cold conditions will persist for south central Alaska
Saturday through next Wednesday night. The exception to this is
Kodiak Island and the Gulf coastal locations where low pressure
over the Gulf should bring enough lift to support snow this
weekend. Snow is expected to taper off on Sunday from west to
east. As this low makes its track across the Gulf, offshore flow
is expected to increase. Thompson Pass and the Copper River Delta
will experience strong northerly gusts Saturday night through
Sunday. Another weak low tracking east from the Bering could bring
additional snow to Kodiak between Sunday and Tuesday, however
there is still a bit of uncertainty on the track and timing of
this next system.

Southwest:
Dry and cold conditions are expected for Saturday with snow
chances increasing on Sunday as a weak low from the Bering pushes
inland through Monday. Drier conditions are expected on Tuesday
and Wednesday, however another Bering front approaches Tuesday
night. There is high uncertainty with the progression of this
front, but the general model trends keep the dominant ridge
influence across the Southwest.

Bering:

Across the Bering, active weather persists through the extended
forecast Saturday through next Wednesday night. A weak surface
low is expected to develop on Saturday, south of the Pribilof
Islands. Models are still not in great agreement with this
development, however the main impact from this will be snow for
the eastern waters including the eastern Aleutians to Alaska
Peninsula. The next strong storm develops across the north Pacific
and lifts north to the Western Aleutians late Sunday. This is a
warmer gale force system that moves to the southern Bering on
Monday with re-enforcing energy sending this front farther north
and east into Tuesday. Precipitation around the core of this
system will mainly be rain for the central Aleutians with mixed
precipitation for the western and eastern Aleutians. The front
moves across the Alaska Peninsula on Wednesday. Precipitation
along the front will likely start as snow and then mix with/or
change over to rain during the afternoon.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 171.
         Flood Advisory 145.
MARINE...Gale Warning 130 138 139 150.
         Heavy Freezing Spray Warning 127 129 130 138 139
         140 160 180 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...MO/CK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD/SA
MARINE/LONG TERM...KH