Gale Warning 100, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
500 AM AKST Thu Jan 31 2019

.ANALYSIS AND UPPER LEVELS...

An upper-level trough extending from Kodiak Island across the
northern Gulf continues produce some widely scattered snow
showers across areas of the immediate southcentral coast. These
showers, however, are tapering off quickly as the trough and its
associated surface feature slides south and east. Farther north,
an upper-level ridge centered over northwest Alaska is elongating
and drifting toward Southcentral. This, combined with weak low-
level forcing and lingering surface moisture, is producing
widespread stratus and areas of dense fog from the Cook Inlet
north across portions of the MAT-SU Valleys. Areas of low stratus
are also present over the Copper River Basin where colder air is
advecting into the region and helping drive gusty offshore winds
through Thompson Pass and the Copper River Delta.

For Southwest Alaska, low stratus remains from the YK-Delta south
to Bristol Bay as a weakening trough lifts to the northwest and a
surface ridge slowly builds over the eastern Bering. West of
Shemya, a 952 mb low is producing widespread gales and areas of
storm-force winds west of Adak, as seen on the latest ASCAT
scatterometer pass. An occluded front attached to the low
stretches across the Bering and over the central Aleutians
producing rain west of Dutch Harbor and the Pribilofs.

&&

.MODEL DISCUSSION...

Guidance remains in good agreement through the duration of the
short-term forecast package. The models have continued the trend
in keeping the bulk of the energy from the upper-level trough
digging south toward the Copper River Basin east over the Yukon
and northern Canadian Rockies moving into the end of the week.
There continues to be some minor timing differences concerning the
front moving into the Gulf from the west Friday afternoon.
Forecast models all indicate that the front will fall apart as it
approaches the northern Gulf; however, all but the GFS hint at
the development of a weak surface low south of Prince William
Sound late Friday or early Saturday that would keep the threat for
showers along the southcentral coast for the weekend. There is
also some differences in the timing and position of the next front
to move east into the Gulf from Southwest Alaska late Saturday
into Sunday as the parent low moves from the eastern Bering into
interior Southwest Alaska. Given the good agreement between the
models, the NAM was favored for its higher resolution. Forecast
confidence remains higher than normal.

&&

.AVIATION...

PANC...IFR and LIFR conditions are expected over the airport
complex through midday today due to persistent areas of dense fog.
Conditions should improve to VFR after midday as a light easterly
wind advects drier air over the region. Vicinity fog is possible
again beginning this evening as winds slacken and patches of low
stratus develop over Cook Inlet and Knik Arm.

&&

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

A cold upper trough dropping south along the ALCAN boarder has
signaled a shift toward drier and colder conditions across the
Southcentral mainland. Cold air will continue to pour into the
Cooper River Basin today increasing pressure and thermal
gradients between the maritime coastal environment and continental
polar interior. These strong gradients in combination with
increasing northerly upper level flow and dry air intrusion will
enhance outflow and gap winds across much of the North Gulf Coast
and Kenai Peninsula. Expect the strongest winds to be out of the
Cooper River Delta and in the Thompson Pass to Valdez area. These
winds will peak late this evening then diminish slowly overnight
as upper support wanes.

Foggy conditions across the MAT-SU Valleys, Anchorage Bowl,
Turnagain Arm, and the Western Kenai should break up by noon today
as increased flow brings enough mixing. There is the possibility
if winds remain light tonight that the fog could reform across the
Anchorage Bowl and Knik Arm.

A weather front heading toward Kodiak on Friday will bring
increased chances of rain and snow to the western Gulf. This front
will also spread clouds across the northern Gulf, Kenai
Peninsula, and Cook Inlet to MAT-SU Valleys corridor Friday
night. Snow potential inland does not look very promising and
should not amount to much accumulation, if any.

&&

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

Generally quiet weather starts the day in southwest Alaska.
Satellite indicates low level stratus is stubbornly hanging on
over much of the area and that flow has transitioned to the
northeast. Satellite imagery indicates a plume like feature of
stratus moving down the north side of the Alaska Range and
expanding once it reaches southwest Alaska, really highlighting
the wind direction as cold air filters down the north side of the
range. Meanwhile, surface observations show this flow has pushed
fog out of most areas, with just some fog along the coast.

Weather will change Friday, with the arrival of a frontal system
extending from a low in the western Bering. This low does not have
a lot to support it, so it will fall quickly apart as it reaches
the coast. Saturday another very similar frontal system will do
the same, again falling apart as it pushes inland. Expect mostly
snow with the first system but with both fronts warming
temperatures, there will be more of a rain/snow mix mainly along
the coast with the second system Saturday.

&&

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

The storm force low we have been advertising for the past couple
of days has arrived in the western Bering. The latest
scatterometer pass indicates storm force winds moving through the
Rat Islands, while a ship observation at 0400Z south of Adak
indicated winds as high as 70kts. We have not observed storm force
winds west of these areas as forecast, but with the observation
out at Shemya again this evening, there is no way to verify if
storm force winds passed through there. It is likely given storm
force winds elsewhere.

This low is the first of two lows that will move through the
Bering Sea in the next couple of days. The next low moving through
Friday into Saturday will again have storm force winds impacting
the western Bering and Aleutians. Both will also have fronts that
will detach and move east across the Aleutians and eventually into
southwest Alaska. So a rinse and repeat pattern with brief breaks
in the weather before the next low moves in.

&&

.MARINE (Days 3 through 5)...

The global models continue show a series of lows developing in
quick succession for the Bering and Aleutians. The pattern
actually holds up for 10 days. Suffice to say, very confident in
the notion of cyclones developing south of the Aleutians, jumping
the chain and then unleashing strong winds/rough seas into the
region. Very confident in how quickly the storms follow in the
wake of their predecessors. At the beginning of this forecast
period 00z Saturday, there is a surface low near Kamchatka with an
estimated pressure of 954 mb and a second surface low west of
Adak ranging from 960 mb - 966 mb which is enough to make a
difference between gale of storm force winds materializing. By 12z
Saturday the low west of Adak moves due north and then is west of
St.Paul. The GFS and the ECMWF both prog that track but the GFS
definitely intensifies the low more than the ECMWF. Meanwhile the
Kamchatka low slowly decays wobbles around the western Bering.
But there is another slug of vorticity propagating about the base
of the longwave trough that support a new low moving west of Adak
18z Sunday. Also at 18z Sunday, looking further down range there
is a new low in the Sea Of Japan that is going to undergo rapid
cyclogenesis once it moves over the Kuroshio Current and it is
headed directly to Attu Island.

.LONG TERM FORECAST (Days 3 through 7)...

The longwave pattern features a ridge from the Central Pacific
into Southwestern Alaska with the longwave trough over Kamchatka
and the Northwest Pacific. As time elapses, the troughing extends
eastward encompassing more of the Bering while the ridge axis
shifts over Southcentral AK. This is going to result in a
prolonged period of southwest flow and moisture advection over
the AKPEN and SW AK. There are numerous shortwaves embedded in the
longwave pattern. Expect this forecast period to be highly active
over the Aleutians/Bering with 3 distinct baroclinic fronts
moving across the chain and into Southwest Alaska resulting in
above normal level of precipitation. There is growing confidence
that this mean pattern actually holds through Day 10. And guidance
from the Climate Prediction Center actually suggests that it will
hold through Day 14.

With that ridge in place over the Central Pacific, what is less
clear is how much remnant moisture will work its way into
Southcentral Alaska. At this time, Sunday looks like the best
chance for precipitation in the Anchorage Metro Area and the
forecast thermal profiles are suggesting it will be snow! Places
like Kodiak, Homer and Seward could see more of a rain/snow mix.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...

PUBLIC...Blizzard Warning 131.
MARINE...Storm Warning 176 177 411.
Gale Warning 127 170 172 173 174 175 178 179 185 412 413 414.
Heavy Freezing Spray Warning 180.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
MARINE/LONG TERM...PS