Small Craft Advisory 792, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
439 AM AKDT Thu Oct 19 2017

.ANALYSIS AND UPPER LEVELS...
Weak high pressure and offshore flow continues to bring generally clear
skies with gap winds to much of Southcentral and the Gulf Coast
as a low in the Eastern Gulf begins to weaken as it moves inland.
The one exception to this is a weak deformation band that brought
an area of light snow to the Southern Kenai Peninsula overnight,
which should quickly dissipate this morning as the upper trough
axis moves to the east. Further west over the Eastern
Bering/Southwest coast a broad area of cold air advection in the
northerly flow is producing rain/snow showers as it cold air moves
over the warmer waters of the Bering. These are beginning to
diminish however as a gale force low begins to spread into the
Western-Central Bering. This system will continue moving eastward
through the Aleutians this week and be the focus for active
weather in the Gulf this weekend.

&&

.MODEL DISCUSSION...
The models remain in very good synoptic agreement through Friday,
with just some minor differences in the low position over the
Eastern Gulf that should have little impact in forecast
confidence. This agreement quickly begins to break down starting late
Friday as the Aleutian low begins to deepen as it moves into the
Gulf. Beginning Wednesday evening, model runs are began shifting
this further to the east into the Panhandle, however since this
was such a large departure from the previous track the forecast
was left mostly as is tonight until better model consensus is
available.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

The high pressure in the Interior and low in the eastern Gulf are
keeping the northerly winds going today. The lack of extremely
cold air and only moderate pressure gradient for such a pattern is
keeping outflow winds on the modest side. An upper level trough
is over the Talkeetna Mountains through eastern Kenai Peninsula
early this morning and it will remain nearly stationary into the
evening and then move eastward into the Copper River Basin
overnight. This will keep some areas of clouds in the Copper River
Basin. Periods of light snow through tonight with minor
accumulations will be possible within Wrangell- St. Elias National
Park including the McCarthy area. There is also an area of clouds
that has set up along the Western Kenai Peninsula coast from
Kasilof southward. This is due to the cold advection and gusty
winds coming through the gaps in Western Cook Inlet causing some
convergence right there. Other than that, skies should be mostly
clear over the region today into tomorrow. Clouds will move into
the area in a widespread manner late Friday night or Saturday
morning. The timing of these clouds will have a direct impact on
Friday night temperatures so those are subject to be adjusted as
the timing of the clouds becomes more clear.

&&

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

A bowling ball low near Point Barrow continues to drop southward
which continues to advect colder air toward Southwest Alaska. The
atmosphere remains relatively unstable with the colder air moving
in aloft but the lack of large-scale forcing is limiting the
coverage of snow showers across the region. Other than scattered
snow showers over the next several days, a rather benign weather
pattern is in store for the region with partly cloudy skies
prevailing across the region. The advertised cool down has started
and it still looks like each of the next 3 days should see both
high and low temperatures a degree or two cooler than the previous
day.

&&

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

The area of low pressure near Shemya continues to strengthen this
morning and the satellite signature shows it's now just
approaching the mature phase. This has allowed winds along the
frontal boundary to be just a touch stronger than originally
expected with storm force winds over parts of the far western
Bering Sea. Now that the low has matured, it will steadily weaken
over the next several days as it moves along the Aleutian Island
and ultimately into the Gulf of Alaska Saturday morning. Cold air
advection showers will build across the Bering Sea today and the
vast majority of the Bering Sea will see rain/snow showers over
the next several days with colder air filtering in aloft.

&&

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

An active pattern is expected through the long range forecast
through the middle of next week. The main challenge surrounds a
storm system which slides east into the gulf this weekend bringing
the potential for snow across Southcentral into early next week.

For the start of the weekend, cold air advection keeps
temperatures below normal across the southern mainland into
Saturday morning under dry conditions as low pressure tracks south
of the Alaska Peninsula to the Gulf. Weak disturbances across the
gulf will initiate rain showers along a surface low near the northeast
coast heading into Saturday morning, with a few snow showers moving
inland over the eastern Copper River. For locations along the
Bering, including the southwest coast, precipitation chances
increase through the upcoming weekend as an upper low pressure
system in the eastern Bering slowly propagates south toward
Bristol Bay. A colder air mass advecting from the northern Bering
changes precip type mainly to snow except along the Aleutians
where a mix of rain and snow prevail under moderate temperatures.

The biggest challenge of the upcoming forecast is the storm system
which tracks into the gulf on Saturday. This system is currently
expected to lift a frontal boundary to the northern waters by
Saturday afternoon ahead of a surface low. This low moves across
the central gulf Saturday night and deepens to 970 mb. This warm
core system will spread rain across the Gulf on Saturday, with
showers along the gulf coast starting initially as snow mixing
with or changing over to rain through Saturday night. The upper
level low associated with this front begins to lift north towards
Sunday morning, which brings the potential for snow to develop
across inland areas of Southcentral. Models continue to struggle
from run to run on the timing and track of this system, therefore
only small changes were made to increase precipitation chances
for the latter part of the weekend. With high uncertainty during
the mid range forecast, kept changes minimal through the middle
of next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 130 132 170 173-178.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MC
LONG TERM...KH