Small Craft Advisory 27, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
419 AM AKST Mon Jan 8 2018

.ANALYSIS AND UPPER LEVELS...

The upper level pattern remains in a weak, discombobulated mess
with a number of features affecting portions of the area. For
mainland Alaska, the main feature of note is a building upper
level high moving south out of the Arctic. The ridging has caused
the upper flow over Southcentral to turn to a rare clockwise
east to northeasterly flow. The primary effect of this in
conjunction with the approach of upper level troughing over the
Yukon is to advect some bitterly cold air southwestward into the
Copper River Basin. The cold will settle in over the next couple
days, more on that below. Elsewhere, the subsidence associated
with the clockwise flow is keeping the weather quiet as regards
precipitation. However, this same pattern favors localized
development of fog and low stratus. This is especially true in
Southwest Alaska, where a number of sites from Bethel to Naknek
are reporting dense fog. Weak flow will keep this moisture largely
in place.

Over the Bering, all eyes are on a powerful nearly vertically
stacked low located south of Adak this morning. The low is
producing widespread gales and even some localized storms through
the Aleutians between Dutch Harbor and Adak. The front and the
strongest winds are moving east into the eastern Aleutians.

&&

.MODEL DISCUSSION...

The models remain in excellent agreement through the next couple
days. All the models agree on very cold air moving into the Copper
River Basin, which will then drain through the Matanuska Valley,
Thompson Pass, and the Copper River Delta in the form of strong,
gusty winds. Elsewhere, the winds will be relatively light and
offshore, keeping conditions dry. The models are also tracking the
very strong low over the Aleutians very well. The NAM was used
once again for updates with this forecast package and confidence
is high once again. Short-term challenges remain forecasting the
fog and low stratus currently affecting portions of Southwest
Alaska, whether fog develops from Anchorage south through Kenai,
how strong the gap winds will be, and how cold it gets through the
Copper River Basin.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through much of
the day. Fog will be in the vicinity of the airport and may
briefly impact ceilings and visibilities through the morning. This
evening, down-Inlet winds will gradually increase in intensity
with the approach of a front from the south and cold air builds
in the Copper River Basin, accelerating the Matanuska Valley winds
across west Anchorage.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
As high pressure builds in over Southcentral through Wednesday,
an upper level trough will dig down from NW Canada across the
ALCAN border bringing strong northerly flow. The trough will bring
polar air from interior NW Canada into Alaska, which will bleed
into the Copper River Basin and interior sections of Southcentral.
The coldest air will be located over the Copper River Basin where
temperatures could drop to -40 F Wednesday morning. The large
thermal gradient between cold dry air in the interior and
relatively mild moist air near the coast will drive strong
northerly to northeasterly gap winds across the region, including
through the Matanuska Valley, Valdez, and out of the Copper River
Delta. Increasing cyclonic flow over the northern Gulf combined
with cold air over warm water will increase instability, leading
to some showers over the Gulf. Some of these showers will reach
Kodiak Island and the southern tip of the Kenai Peninsula later
today, and northward through the eastern Kenai Tuesday and Tuesday
night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Today through Wednesday evening)

Early this morning, Southwest Alaska remains sandwiched between a
very strong area of surface high pressure across the Arctic, and
a strong area of low pressure located just south of the central
Aleutians. The pressure gradient between these two intense systems
is allowing for two things two happen. The first being an
offshore flow pattern has developed, which in turn is allowing
much colder air to drain into both the lower and upper portions of
the Kuskokwim Delta. The other benefit of this wind direction is
fog formation will be much more localized, if it develops at all,
due to frictional heating via the down-sloping effects which help
to dry out the air. This pattern looks to hold for the next few
days.

Otherwise the main item of interest will be chances of
precipitation for the Tuesday through Wednesday morning time
frame, mainly for coastal locations. The numerical models have
trended further south with the track of the Aleutian low compared
to yesterday, thus its associated surface front struggles to swing
northward while undergoing frontolysis (the dissipation of a
front), which in turn reduces any surface convergence to act as a
source of lift. However, some upper-level spokes of vorticity
rotating around the low may be just enough to allow for some
showery type precipitation to develop.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Today through Wednesday evening)

A strong area of low pressure continues to spin just south of
Adak, with strong winds currently affecting much of the chain and
surrounding waters. As expected, rainfall intensity has been of
moderate intensity, with some visibility reductions noted.
Overall, not much has changed with the previous model runs, as
they mainly keep the low wobbling in place through tonight,
before weakening Tuesday as it heads out of the region to the
southeast. However, another developing storm will move into the
area from the west Tuesday night into Wednesday, with the original
low getting absorbed into the system in an inverted trough-like
fashion. This will maintain at least showery type precipitation
across all of the Bering Sea and Aleutian Chain for the next few
days.

As for the strength of the winds, look for a gradual decrease in
wind speeds to commence through the next 24 hours for both the
land and marine zones, with the exception being the Pribilof
Islands, as they'll see a rapid increase over the next several
hours due to the approaching front mentioned in the Southwest
Alaska portion of the discussion. Heading into Tuesday through
Wednesday, a continued decrease in wind speeds are expected as the
stronger winds will be located well south of the forecast area,
closer to the position of the jet stream.

&&

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

The long term forecast period begins Tuesday night with a high
latitude blocking high over the interior and a broad upper trough
extending across the Pacific Ocean to the eastern Bering Sea. The
main jet energy will remain well to the south of the AOR with the
blocking high acting as the main dominant weather feature for
Alaska. Expect a return to colder than average mainland
temperatures, especially in prone low lying areas and basins. Any
precipitation will be relegated to coastal regions like Kodiak and
the North Gulf Coast through Wednesday night.

Thereafter, the North Pacific jet will begin to shift the overall
trough position farther eastward. This will eventually result in
increasing cyclogenesis and more southerly flow Thursday through
the weekend. There will be a noticeable uptick in frontal
progression northward with increasing chances of coastal
precipitation (probably turning to predominantly rain as it warms
up by the weekend). Inland areas will gradually warm up as well as
clouds increase and mid level temperatures warm dramatically.
Precipitation looks to remain low, however, as cross-barrier flow
will likely play a strong role with this pattern. This means
stormy weather is likely for the immediate coast Thursday and
beyond.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 127 155 165 170-180 185 411-414
         Heavy Freezing Spray Warning 180 181

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...JA