Small Craft Advisory 133, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
439 AM AKST Wed Feb 13 2019

.ANALYSIS AND UPPER LEVELS...

The polar jet is highly amplified over the North Pacific and
Bering with the jet core barreling across the AKPEN. The longwave
pattern features a deep and broad trough from Eastern Russia to
Japan and into the Bering with a series of vort lobes embedded in
the upper level flow. Meanwhile, there is an upper level ridge
from the Central Pacific that bisects SE Alaska and British
Columbia. There is a baroclinic zone south of the Aleutian chain
that spans westward towards Japan connecting with the cyclone SE
of Kamchatka. On the ASCAT/advanced scatterometer both gale and
storm force winds were detected with this system. Looking at
Southcentral, fog has again been detected at multiple locations
early this morning including Palmer and Ted Stevens International
Airport.

&&

.MODEL DISCUSSION...
The run to run continuity with the models has been really
impressive. Using Himawari "AHI Airmass" imagery for
initialization purposes, both the GFS and ECMWF did well with the
placement of the low in southwest of Attu Island. The level of
proficiency in the initialization process was seconded by
comparing ASCAT data for the surface high south of Unalaska and
comparing it to the GFS and ECMWF. The global models are in good
agreement through 48 hrs with the placement of the synoptic
features. At the 48 hr mark, the GFS is quicker pushing the
surface front across the Aleutian chain and into the Bering near
the Pribilof Islands. But it more falls into the category of a
persnickety detail, because sensible weather wise, the outcome
won't be much different for the Pribilof Islands. This pattern
where the longwave trough has dominated the Western/Central Bering
and NW Pacific has been going on for weeks now. The persistence
of this pattern most likely has been influencing the performance
of the global models. Through Day 8, the GFS keeps the positive
height anomalies over the Gulf Of Alaska and Central Pacific and
negative height anomalies over Eastern Russia and portions of the
Bering. This pattern is simply not breaking down or transitioning
anytime soon. The one area of concern is the potential development
of fog over Southcentral especially over the Cook Inlet, Knik Arm
and Anchorage Bowl. Typically, what the models struggle with is
the placement of fog in the region.

&&

.AVIATION...
PANC...Yesterday, the temperatures in the afternoon were above
freezing for several hours which resulting in widespread snowmelt
and a more accessible moisture source for the boundary layer. With
the ridge in place, there has been no shortage of stable air. Fog
developed at the NWS Office and at PANC. At PANC, visibility got
as low as 1/4 SM with freezing fog. The current thinking is that
the fog will lift this morning. Amendments will be made as
required.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Thursday night)...
A rapidly amplifying pattern in the upper levels will lead to
strong winds over the next couple days across the Gulf of Alaska,
Kodiak Island, and along the northern Gulf and Prince William
Sound coastline. A building ridge over the southern to western
Bering Sea today will lead to a strengthening northwesterly jet
streak which will usher arctic air southward across the region.
The amplification will cause a low over the southern Gulf this
morning to intensify as it exits to the northeast Pacific this
afternoon. With surface high pressure building in over mainland
Alaska this will really tighten up pressure gradients over
Southcentral and the Gulf. The combination of tight pressure
gradients, cold air advection, and sinking air aloft will all
contribute to strengthening winds through the day for the
aforementioned areas.

All forcing mechanisms look a little stronger than previous
forecasts, so have trended stronger with the winds. While do not
expect to reach high wind threshold (75 mph) anywhere, Kodiak
Island will come very close. The upper level jet streak will cross
right over Kodiak island this afternoon. Model cross sections
all indicate a lowering tropopause on the polar side of the jet,
indicating strong downward motion through the atmosphere. This
will bring very dry statically stable air down toward the lower
atmosphere, which can lead to mountain wave activity and mixing
down of stronger winds. Thus, have increased wind gusts to as
high as 60 kts, including for Kodiak City.

Winds will then diminish across Kodiak Island tonight as the upper
jet streak exits off to the east and the ridge approaches from
the Bering Sea. However, conditions will become more favorable for
the north Gulf coast and Prince William Sound. Expect winds to
peak in this area tonight through Thursday morning, then begin to
diminish after that. Wind across interior Southcentral will be
much more localized, focused near the exit of mountain passes.
Temperatures will be on a steady decline over the next couple
days, likely bottoming out Thursday night/Friday morning.

&&

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

Bands of snow showers are moving through the Kusko Delta and into
the interior mountains in broad westerly flow. Those snow bands
are associated with instability driven "cold air" lows over the
Bering Sea, with the next in line moving into the Kusko Delta
coast this morning. This feature may bring some light snow
accumulations to these areas and a burst of westerly winds,
otherwise, weather is quiet through tonight as the flow weakens
and high pressure builds in.

A fast moving occluded front and associated upper shortwave trough
will move into Southwest from the Bering Sea Thursday, bringing
accumulating snow to all of Southwest Alaska. The front will fall
apart and weaken as it tracks toward the AK Range Thursday night.
The highest accumulations will be across the Kusko Delta with 2-4"
possible.

&&

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

Satellite imagery depicts a large and sprawling NPAC bomb cyclone
now situated SE of the Kamchatka. ASCAT satellite imagery is
already depicting a storm force front moving into the Western
Aleutians/Bering Sea, and some rain/snow has already developed at
Shemya. The front will spread east into the Bering, bringing
precipitation and gale force southerly winds as far as the Pribs
before it moves inland tonight. The low center will then move into
the Western part of the Bering Sea, bringing another shot of storm
force southwest winds as colder air wraps around the low to the
western Bering. Another trailing, fast moving basal low will move
into the Bering over the Central Aleutians Thu afternoon and move
northward Thu night. The gale force front will reach the coast by
Thu night.

&&

.MARINE (Days 3 through 5)...
Beginning Friday, there is overall good model agreement as a
strong low in the Bering lifts north out of the area leading to
generally small craft to low end gale force southwesterly winds
over the western half of the Bering. There is better confidence
that an area of southerly gales will push across the eastern and
northern Bering through Friday associated with a frontal system
pushing through the area. Overall high confidence with diminishing
westerly/southwesterly winds across the Bering Saturday into
Sunday. By the end of Sunday, another strong low approaches the
western Bering from the N Pacific. Models begin to diverge some at
this point with the track and strength as it moves into the
Bering, varying at this point between solid storm force and low
end gale strength.

With the major storm track remaining out west, the Gulf should
see rather benign weather with sub gale force winds through the
weekend.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A low centered south of Siberia will bring generally persistent
westerly flow to the Aleutians Friday through Sunday morning.
Models are in good agreement that winds will gradually diminish
from marginal gales Friday to light by Sunday as the low weakens
and departs to the north. Precipitation will track west to east
across the Bering and into the southwest mainland Friday ahead of
the associated cold front, with scattered showers remaining
possible across the region through Sunday morning. A stronger low
will move into the western Aleutians Sunday afternoon. The GFS
deepens the low much more (giving a large area of storm force
winds south and east of the center of circulation) than the EC and
Canadian models and as it has weakened slightly from the previous
run, we are currently leaning towards the latter models in the
forecast with gales and moderate rain tracking west east Sunday
afternoon through Monday. Models have another low tracking in the
same general location on Tuesday, however as of now, both the GFS
and EC show it as much weaker than the Sunday/Monday low.

Generally fair skies are expected Friday across the Gulf and
southern Alaska under a ridge. A warm front and resultant
southwesterly flow will gradually track into the region Saturday
and bring the potential for accumulating snow through the weekend.
All 3 major long range model solutions bring the snow in around midday
Saturday. The GFS brings in a second wave of snow over the Anchorage
area on Sunday associated with a weak low that it develops over
the Gulf. Neither the Canadian or EC models develop this low and
given the unreliability of mesoscale features shown in large-scale
models, we are currently not favoring this solution. The Canadian
model also has a closed low centered over southwest Alaska Sunday
afternoon with associated precipitation, however neither this is
not shown on the EC or GFS. Thus we are favoring the EC overall
for this event. There is more significant disagreement Monday into
Tuesday regarding the position of a low over the Bering Sea,
though widespread scattered snow showers appear likely for much of
the region.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Heavy Freezing Spray 140. Storm 411 413 175 177 178 352
130 131 132. Gale 412 414 150 155 170 173 174 176 179 180 181 185
351 119 120 121 125 127 128 129 136 137 138.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
MARINE/LONG TERM...JW