Small Craft Advisory 69, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
524 PM AKST Sun Jan 19 2020

.ANALYSIS AND UPPER LEVELS...

A broad upper level trough extends from eastern Russia to across
eastern Bristol Bay to south of the Alaska Peninsula. This feature
is helping to support a weak baroclinic zone which extends north
from the eastern Alaska Peninsula to over interior Bristol Bay.
Observations and radar imagery shows light snow falling along this
band. A moderately strong surface low over the east-central Gulf
of Alaska is moving northwest. Its leading edge of its associated
front band is spreading light snow west across the North Gulf
Coast and Prince William Sound. Generally fair skies and cold
temperatures were observed across the remainder of the mainland.
Partly to mostly cloudy conditions with open and closed cell
cumulus snow showers were observed with cold advection across the
Bering Sea.

&&

.MODEL DISCUSSION...
Synoptically the models are all in fairly good agreement in the
short term. The biggest challenge will be in the details of
snowfall across Southcentral tonight and Monday. We will be
leaning closer to the NAM/Canadian Regional.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through this
evening. Lower ceilings and snow will work it's way in from the
east overnight. There is some uncertainty in timing, but expect a
period of IFR conditions in snow sometime Monday morning, most
likely between 12Z and 18Z. Lighter snow and MVFR conditions will
then likely hold through Monday afternoon.

&&

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

A surface low over the central Gulf will move northwest tonight
to just southwest of Prince William Sound. Precipitation will
continue to spread to the west, making its way towards the
Chugach Mountains. Snow could be heavy at times on the eastern
Kenai through Monday morning, especially over the higher terrain
of eastern Turnagain Arm (Girdwood to Turnagain Pass).

For areas around Cook Inlet, there will be chances of snow
through Monday night as southerly flow aloft brings moisture into
the region. This, combined with waves of energy rotating around a
lifting upper level trough, and cold enough temperatures
remaining at the surface, look to provide the conditions necessary
for snowfall across much of Southcentral. For areas around Cook
Inlet and the Mat-Su Valleys, a general 2 to 4 inches is
possible. Despite these cold surface temperatures, the air mass
associated with this system is considerably warmer than what has
been present across the region for the last few days, so
temperatures look to increase into the upper teens to lower 20s
for the Anchorage Bowl by late Monday. There is the potential
for a rain/snow mix across Prince William Sound and along the
immediate coast of the Kenai Peninsula as temperatures warm
even more.

Snow looks to remain across the Matanuska-Susitna Valleys into
Tuesday morning, albeit much lighter than the accumulations
expected earlier in the day. By Tuesday night, we should dry out
with scattered snow showers remaining in the Copper River Basin
and a snow/rain mix across the eastern Prince William Sound.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (through Wednesday)...
An arctic air mass will remain in place, however, a weak front
and trough from the Bering will move ashore tonight over the
Kuskokwim Delta. There will be increasing clouds in this area as
well as a chance for snow. Given the weak nature of the system,
accumulation is not expected. The Kuskokwim Valley will likely
remain frigid with low temperatures approaching -30F again despite
the mid-level overcast currently overhead. Behind this front,
into the middle of the week, dry conditions will return and a
colder airmass will settle back into place along with offshore
flow.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (through Wednesday)...
Shishaldin Volcano continues to erupt. So far ash seems to be
limited with the plume, which is carrying to the southeast over
marine areas. Ashfall amounts beyond a trace are not expected at
this time. A weak trough over the eastern Bering Sea will keep
small chances for snow showers as it moves onshore over the next
couple days. Behind the trough a colder air mass will spread
numerous snow showers over the western and central Bering Sea. The
pattern shifts by mid-week with a gale-force front affecting the
western Chain.

&&

.MARINE (Days 3 through 5: Wednesday through Friday)...

Gulf of Alaska:

After quiet weather across the Gulf on Wednesday, a low moving
into the eastern Gulf will bring the threat of north to
northwesterly gales through the Barren Islands and south of Kodiak
Island. There is some disagreement in the track of the low, but a
westerly track and stronger low could result in storm force winds
in those same areas, primarily Thursday night into Friday morning.
Gales will continue through the day Friday.

Bering Sea and Aleutians:

A strong low meandering south of the western Aleutians is expected
to produce easterly gale force winds, with intermittent storm
force winds possible, for all three days across all of the western
and central Aleutians and adjacent waters. Where the strongest
winds set up will be determined by the exact track of the low and
associated fronts. Strengthening high pressure over the northern
Bering will also produce gale force outflow winds through the
eastern Aleutians, Alaska Peninsula, and near Cape Newenham. The
strongest winds in these areas are expected Friday.

&&

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

A stagnant, slow-to-change weather pattern is expected to set up
across Alaska and the Bering Sea throughout the entire long-term
forecast period. A stalled out low is expected to set up south of
the western Aleutians, it's northern periphery will cause strong
winds and rain across the central and western Aleutians from
Wednesday through Sunday. Any fronts associated with the low may
briefly increase both the winds and the precipitation intensity.
Moving east from there, an area of high pressure over the
Pribilofs Wednesday morning will move over the west coast of
Alaska and intensify with time through the weekend. This will have
several effects: first, it will allow an Arctic air mass to
reestablish itself across much of mainland Alaska from Friday
through the weekend, resulting in another round of well-below
normal-temperatures across all of Southern mainland Alaska.
Second, it will cause gusty winds to develop, especially through
the mountain gaps such as Thompson Pass, the Seward area, the
Barren Islands, and south of Kodiak Island. The combination of
falling temperatures and gusty winds will almost certainly cause a
renewed round of wind chill concerns, both in Southwest and
Southcentral. The cold and gusty outflow winds currently appear to
be strong enough around the Anchorage area that freezing fog
concerns appear unlikely.

The last and certainly most uncertain element to the long term
forecast will be the tracks of a series of lows that are expected
to move through the Gulf through the long-term period. All of the
models agree that lows will move through the Gulf, so it's highly
likely that unsettled weather will be the norm across coastal
areas. The lows will each have the potential to produce snow for
coastal communities, spread some cloud cover inland at times,
which will make the temperature forecast more difficult, and
increase the winds through the gaps even further, adding both to
the wind chill concerns and possibly localized blizzards where the
strong winds and heavier snow could combine. Thus, despite an
otherwise stagnant weather pattern, the forecast will remain
challenging and active, especially for Southcentral.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Heavy Freezing Spray Warning 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...CK/TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ML
MARINE/LONG TERM...JW