Small Craft Advisory 12, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
427 PM AKST Thu Jan 4 2018

.ANALYSIS AND UPPER LEVELS...

There is a shift in the upper level pattern today as a North
Pacific jet streak is carving out another upper level trough over
the Bering Sea. The upper low which brought snow to Southcentral
yesterday has weakened and is now under the influence of general
upper level high pressure over the state of Alaska. The low is now
closed off with the center over Prince William Sound. A rather
large area of warm air advection is forming ahead of the
aforementioned upper trough over the Bering Sea, and this is
enhancing shortwave ridging which is moving into Southwest Alaska.

At the surface, the leading edge of the Pacific warm front is
moving into the southern Bering Sea with the parent 959 mb low
centered just south of Shemya. Trailing the warm front is a
strengthening 965 mb low...which is expected to move into the
central Bering Sea this evening. Across Southcentral, weak
offshore winds are developing as the pressure gradient switches
around. However, with clearing skies and plenty of low level
moisture, fog is now beginning to form across inland areas.

&&

.MODEL DISCUSSION...

The numerical models are in excellent agreement with the general
synoptic features through the weekend as the North Pacific/Bering
trough shifts southeastward. The main discrepancy in the model
guidance is with the development of a Gulf low as the front over
the Bering moves into the Gulf of Alaska Fri night and Sat. The
NAM and GFS are more aggressive in closing the low off as it moves
toward the Gulf Coast whereas the Canadian and ECMWF depict a
faster moving and slightly weaker frontal trough axis. This
forecast favors a blend of all the solutions which enhances the
front to near gale force, but does not show a strong closed low
like the GFS and NAM.

&&

.AVIATION...
PANC...Fog has moved in and out of parts of the Anchorage Bowl
today. By tonight, winds become northerly and should help dry
things out so fog is anticipated to move back out from the
terminal overnight, but there is a chance that some patchy fog
remains into Friday morning.

&&

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

The remnants of the closed upper low which brought snow to
Anchorage is now slowly digging into the Gulf of Alaska. This is
already inciting enough offshore flow for local gap winds to begin
increasing. This is also coinciding with clearing skies in terms
of higher level clouds, so inland areas are now seeing the
development of some locally dense fog. Given the weak diurnal sun
efficiency, it seems that if this fog can develop during the day,
it will probably last into the overnight period across parts of
Cook Inlet and Copper Basin where offshore winds will be weaker.
Speaking of winds, the local gap winds are expected to peak during
the overnight then diminish through the day tomorrow.

The next weather maker for Southcentral will be a secondary triple
point occluded front moving into the Gulf of Alaska Friday night
and Saturday then lasting through Sunday as it slowly weakens in
place. The orientation of the front is not very favorable for
heavy coastal precipitation, but places favorable in this pattern
include Whittier and into Portage Valley, although even these
locations will not see much. Otherwise, inland locales will likely
remain dry given the dry air to the north and the strong blocking
high over interior Alaska.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A weak warm front lifting northward across the eastern Bering Sea
and southern Alaska Peninsula this afternoon will head to the
northern Bering with nearly all of the precipitation remaining
offshore of the Southwest mainland. There may be some light snow
on Nunivak Island and the immediate coast of the Kuskokwim Delta,
but if so, it won't amount to anything.

Focus quickly shifts to a strong low moving into the Bering. A
much stronger front will march eastward tonight through Friday
and elongate right along the Southwest coast Friday night. A
surface wave moving up the front will pass west of Cold Bay, which
will enhance the southeasterly winds across the southern Alaska
Peninsula. However, with stable low levels it looks like winds
will largely stay below warning thresholds, with perhaps just a
few isolated peak wind gusts up to 75 mph, so have cancelled the
High Wind Watch. With the parent low headed to the western bering
and a triple point low developing south of Kodiak Island, a lot of
the energy with this storm system will head in opposite directions
away from Southwest Alaska. Thus, the big change to the latest
forecast is to stall the front offshore, with less precipitation
moving onshore along with cooler temperatures. The biggest impact
will be strong southeasterly winds, particularly in the Bristol
Bay area, the Western Capes, and on up to the Kuskokwim Delta. As
we head to the weekend the front will dissipate, low level winds
will weaken and the remnants of the upper trough will lift
northward across Southwest. This will bring a chance of light rain
or snow, especially to the Bristol Bay area.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
While one deep mature low weakens along the Western Aleutians,
another strengthening low will charge northward into the Bering
Sea this evening, then settle in over the Western Bering on
Friday. The biggest impact will be ahead of the associated front,
from the eastern Aleutians and Bering to the northern Bering,
where widespread storm force winds or high end gale winds will be
observed tonight through Friday. An accompanying surge of warm
air will produce lots of rain. Cooler air will warp in around the
south side of the low late Friday through Saturday, bring showers
and locally gusty winds to the Aleutians.

&&

.LONG TERM FORECAST (Days 4 through 7)...
The weather looking into early next week will be dominated by
continued storminess over the Bering Sea/Aleutians with a ridge of
high pressure over the Mainland. The most impactful system in this
period will begin late Sunday with a storm force low that will
move it the Central Bering. This system looks to bring widespread
gales to much of the Bering Sea region to the Southwest coast, but
likely will not move much further inland as high pressure over the
Mainland holds. This should allow for temperatures to remain at or
below normal for most of the Mainland with clear skies through
Tuesday with impacts largely confined to possibility of outflow
winds to the Gulf Coast. This pattern then looks to break down
slightly towards the middle of next week as the high pressure
begins to slide eastward into the Yukon. This would allow
storminess to return to the Southwest Mainland and possibly the
Western Gulf, however confidence in this pattern shift is low as
the models are typically too quick to break down Mainland ridging
in the extended forecast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale :127 131 132 138 171 174 175 176 177 178 352 411
...............413
.........Storm:150 155 160 165 170 172 173 179 180 181 185 412
...............414

&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SB
LONG TERM...DEK