Gale Warning 153, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 PM AKST Tue Feb 12 2019

.ANALYSIS AND UPPER LEVELS...
Aloft, the synoptic pattern is composed of a ridge building over
the Gulf of Alaska, and a shortwave trough positioned over the
central Aleutians and central Bering. The jet stream is in a zonal
pattern and the areas of the highest winds are over the western
Gulf/Lower Kuskokwim Valley and the western Bering. In both of
these locations, winds are reaching 120 kts at 30,000 ft. Weak
southerly flow continues over the southwest Alaska and offshore
flow dominates Southcentral and the Gulf. Satellite imagery shows
clear skies over Southcentral and the Gulf as the ridge builds
over the area. The southern Bering is covered stratus which mimics
the location of the jet stream.

At the surface, an intensifying surface low travels eastward
along the southern coast of the central Aleutians. A second weak
low spins in the northeastern Gulf. This surface low has little
upper level support but aiding the westerly flow through the Gulf.
The offshore flow present at the surface is increasing winds over
the Barren Islands and the central Gulf. Temperatures throughout
Southcentral are beginning to drop as cold air from the north is
slowly advecting into the area, especially in the Copper River
Basin. Southern mainland Alaska is fairly dry while the central
Aleutians is experiencing precipitation this evening with the
surface low progression.

&&

.MODEL DISCUSSION...
Models continue to have very good run-to-run consistency. Some of
the high resolution models are being used for the outflow winds in
the eastern part of the forecast area. The gusty winds along the
Gulf coast are being kept in for longer than models indicate as
they typically last longer than models expect. The next real model
issue is showing up this weekend with a wave/low moving toward the
western Gulf of Alaska. The track and structure of this feature is
handled a bit different by the models and will impact forecasts in
both Southcentral and Southwest Alaska.

&&

.AVIATION...
PANC...The stratus that is holding just to the west of PANC early
this afternoon is likely to make its way back over the airport
this evening. The big question then becomes whether the increasing
northerly winds late tonight will be enough to mix out the
atmosphere and break up the stratus. It is looking more like a
stratus situation than a fog event overnight, but there is always
the possibility of periods of fog with the stratus. Due to the
depth of the mixing and the strength of the tightening pressure
gradient, it looks like the stratus should break up late tonight
as winds pick up and then the terminal should remain VFR after
that.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through
3/Tuesday night through Friday)...
Clearing skies, cooling temperatures and strong offshore winds
along the coast are in order for the next few days. Northwest flow
will develop tonight into tomorrow over all of Southcentral AK.
This will cause cold air to dive south out of the Arctic, as a
front over moving east out of northern AK couples with a strong
low developing in the Northern Gulf of AK. The exiting low (which
models have been increasing in strength) in northern AK will bring
in the northwest flow, while the low in the gulf will anchor this
flow, creating a broad trough of cold air.

This cold air will cause temperatures to gradually decrease over
land, while the force of cold air will push through mountain
passes and valleys along the coast, creating some strong winds.
Winds will be mainly a result of surface based colder air and
pressure gradient and will not have strong winds in the upper
levels for support, so expecting strong but not warning level
winds. Expect these conditions to last through Friday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Wednesday
and Thursday)...
Trailing precipitation associated with a weak area of low
pressure south of the AKPEN continues to bring scattered rain and
snow showers over Southwest and the northern AKPEN. As this low
continues to move eastward, gusty northerly winds will become
channeled through the usual gaps and passes along the Pacific side
of the AKPEN this evening. By Wed, a weak ridge over the eastern
Bering will begin to briefly build over Southwest before another
front approaches Wed evening. Ahead of the front, subsidence
associated with the ridge may allow for fog to develop late Wed
night into early Thu morning. As the front moves in Thu morning,
expect warm air and gusty south to southeast winds to return
across most of the area. Snow combined with gusty offshore flow
along the YK Delta could create areas of blowing snow, reducing
visibility at times especially along the coast. Some good news is
that this front appears weaker than Monday's system, so there are
minimal concerns regarding coastal flooding at this time.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and
2...Wednesday and Thursday)...
Westerly flow continues ahead of an incoming front over the
Bering. As another storm force low approaches from the west, its
associated front lifts over the western Aleutians this evening and
reaches the central Bering by Wed afternoon. Initially, this
front will bring strong southerly winds, as an area of storm force
winds embedded within a larger swath of gales moves in over the
western Aleutians Wed morning. As cold air wraps in behind the
slowly advancing low, another round of south to west storm force
winds will move over the western Aleutians Wed evening into Thu
morning before diminishing to gales by Thu afternoon.

Of note, a sharp baroclinic zone currently exists along the
Aleutians, providing fuel for successive shortwaves around the
parent low to develop later in the week. By Thu afternoon, a weak
disturbance forms just west of Adak, producing a pocket of southerly
gale force winds through Thu evening.

&&

.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...Storm Warning 130 132 175 177 178.
 Gale Warning 120 121 127>129 131 136>138 150 155 170 172>174 176
179 181 185.
 Heavy Frz Spray 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RA/EZ
SOUTHCENTRAL ALASKA...BB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...AP/KVP