Small Craft Advisory 1006, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
446 AM AKST Sun Dec 18 2016

.ANALYSIS AND UPPER LEVELS...
A fairly stable upper level pattern is setting up across Southern
Alaska this morning. A deep vertically stacked low is centered
over the southern Bering just north of the central Aleutians. A
negatively tilted trough extends eastward from the low out over
the eastern Aleutians. The upper level ridge over Southcentral is
rapidly shunting off to the east with the jet, which will take the
warmer air associated with yesterday's above freezing temperatures
along with it.

Temperatures are rapidly falling this morning across Southcentral
under mainly clear skies. Teens and 20s are the norm from Kenai
north through Anchorage and the MatSu Valleys. The next front is
moving northward across Kodiak, with winds across the island
gusting up to 40 mph. With cooler air moving in aloft behind the
departing jet and ahead of the negatively tilted trough and front
over Kodiak, precipitation on the Middleton Island radar has
become much more cellular, which is a sign of increasing
instability. The same front extends westward across Bristol Bay
this morning. This has kept the winds at the surface blowing,
preventing temperatures from falling nearly as much as they are
further east. The front is also quite dry so far with very little
in the way of reflectivities showing up on King Salmon or Bethel
radars from the front.

The aforementioned deep low over the central Bering is dominating
the weather in the region. With the front extending north of the
low and nearly all upper-level support has been cut off, the
system continues to slowly weaken. The storm's claim to fame is
very much it's wind-producing ability. A large swath of winds
greater than 55 mph is evident on the 0826 UTC ASCAT pass. Adak
has been reporting winds sustained above 50 mph since yesterday
afternoon, with gusts there frequently in the 80-90 mph range.

&&

.MODEL DISCUSSION...
The numerical models are in fairly good agreement. There are some
differences in the handling of a surface low in the Gulf Monday
morning into Monday night, with the EC appearing to be the
outlier with a slightly faster and more easterly solution. Out
west, the models seem to be handling the Bering low well through
Monday morning. This is where the EC became the outlier, by
slowing the progress of the surface low and bringing it into the
Central Aleutians Monday night. Whereas the other models are
keeping the system tracking eastward before tracking into the
Alaska Peninsula Tuesday Morning. The GFS/NAM were the preferred
models for this forecast package, and forecast confidence remains
near normal.

&&

.AVIATION...
PANC...VFR conditions and light winds are expected through this
morning. North winds are expected to increase this afternoon as a
strong low moves into the Gulf, then diminish by midnight.

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A frontal boundary and triple point low pressure system will spin
up into the Gulf today bringing snow (mixing with rain) to the
northern Gulf. The gale to storm-force front will first affect
Kodiak Island early this morning with gusty easterly winds and
rain. As the front pivots into the northern Gulf this afternoon it
stalls and weakens as the surface low begins to move eastward.
Precipitation should remain all snow for the coast until the
afternoon when some mixing with rain can be expected, especially
for the Cordova area. There is some possibility of moisture making
it over the mountains with weak isentropic lift, once downslope
flow ends, but dynamics are weak.

As the system slides eastward across the Gulf tonight,
southwesterly flow aloft over the Cook Inlet region resumes. The
result will be a weak deformation band that moves up the Inlet,
bringing light snow to the Kenai Peninsula. The jury is still out
on whether or not this feature holds together through Anchorage
and the Matanuska Valley Monday morning. Monday night the storm
track stays from the Gulf toward the Yukon where most of our area
stays in weak flow with embedded shortwaves.

&&

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

A strong occluded frontal zone is moving into Southwest Alaska
this morning. King Salmon RADAR is beginning to pick up some low
reflectivity returns entering Bristol Bay. Easterly winds will
increase this morning. Along with some light snow from Dillingham
north and west, some blowing snow and reduced visibility can be
expected through the afternoon. Thereafter, the front will weaken
and winds will diminish with the remnants of the front moving
north into the interior parts of Bristol Bay and the Lower
Kuskokwim Valley. Temperatures will continue to fall region wide
as colder interior air moves into the region.

&&

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

ASCAT passes and wind observations continue to support low end
hurricane force winds just south of the Central Aleutian chain.
Some channeling is helping to keep hurricane force gusts at Adak
this morning, but winds will diminish by this afternoon as the low
weakens and slowly shifts south. Otherwise, with the occluded
front now lifting north into the Eastern Bering Sea, most
locations should remain warm enough for rain with the exception of
some mixed rain and snow showers in the cold sector of the
cyclone along the Aleutian chain. The low will remain storm force
through Monday morning before weakening and drifting southeast on
Tuesday.

&&

.LONG TERM FORECAST (Days 3 through 7...Mon night through Sat...

A complex area of low pressure over the extreme eastern Bering, Akpen
and western Gulf of AK Mon night will consolidate over the
northern Gulf Tue into Wed. While a ridge of high pressure builds
over the Bering, the Gulf low will drift and weaken before
dissipating Sat. The Bering high moves over mainland Ak and beyond
during this period, allowing a resumption of eastward moving
systems across the Bering. A second low will develop Tue night
east of Kamchatka, and move near Cold Bay by early Sat before
crossing into the northern Gulf. A third low will develop near
Kamchatka on Sat before making its way across the Bering.

The initial Gulf low will bring accumulating snow to coastal
mountains and the Aleutian Range, diminishing through Sat as the
low weakens. The second low will bring wind and mainly rain to
the eastern half of the Aleutian chain, and the Akpen through the
week. The final low will bring wind to the western Bering and
Aleutians with some rain and snow on Sat.

The run to run consistency demonstrated by the models remains an
issue. Confidence is low to moderate with the timing of the lows,
as well as the location of precipitation. The forecast utilized
the WPC ensemble solution.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High wind warning 187.
MARINE...Hurricane 176.
         Storm 119 131 132 170 172 173 174 175 177.
         Gale 120 127 128 130 136 137 138 139 141.
              150 155 160 165 171 178 179 180.

&&

SYNOPSIS AND MODEL DISCUSSION...TP
SOUTHCENTRAL ALASKA...MTL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...DS