Small Craft Advisory 130, NWS Anchorage, AK

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

.ANALYSIS AND UPPER LEVELS...

The polar jet is highly amplified over the North Pacific, mainland
Alaska and the Eastern Pacific. The jet core is north of the
Aleutian chain and trekking into the Kuskokwim Delta. The
longwave pattern features a deep and broad trough from Eastern
Russia to Japan and into the Bering. Meanwhile, there is an upper
level ridge from the Central Pacific that bisects mainland Alaska
and actually spans into the Yukon Territory, NW Territories and
over some of the northern islands of Nunavut like Bathurst Island
and Cornwallis Island. There is a baroclinic zone spanning from SW
AK, across the AKPEN and into the north Pacific with a new wave
developing along the frontal boundary southwest of Adak. Looking
further down range there is another low pressure system east of
Japan and a third low pressure system near Kamchatka over the Sea
of Okhotsk. There is a menagerie of shortwaves from the Alaska
Range to regions west of the International Dateline. On the
ASCAT/advanced scatterometer both gale and storm force winds were
detected over the Bering.

&&

.MODEL DISCUSSION...

The run to run continuity with the models has been excellent.
Using ASCAT data for initialization purposes, both the GFS and
ECMWF did well with the placement of the low in the Bering. The global
models are in good agreement through 72 hrs with the placement of
the synoptic features and the development of new low east of Japan
over the Kuroshio Current. 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.
The one area of concern is the potential development of fog over
Southcentral. Typically, what the models struggle with is the
placement of fog in the region.

&&

.AVIATION...
PANC...  As of 1153z or 2:53 AM local, there was a ceiling of
2,000 feet observed at PANC and at 2,400 feet at Merrill Field.
This set up is not conducive for fog development. However, the
cigs have the potential to bounce around from 2,000 feet to 3,000
feet at PANC early this morning. As the day unfolds, the cigs are
expected to lift a bit and then VFR conditions will prevail for
the remainder of the TAF period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Wednesday night)...
The big story as we head through the next couple days will be
colder and drier conditions with strong winds across the Gulf and
strong gap winds along the Gulf coast and Prince William Sound.
Starting with this morning, a weak upper level short-wave trough
moving over-top of and flattening a strong upper ridge, will pass
across interior Southcentral. This will produce some light snow,
mainly along west facing slopes of mountain ranges. Cold air
advection, which ensured behind a front that moved through the
region last evening, will continue across Southcentral and the
Gulf. This will lead to strengthening winds across the western
Gulf, Cook Inlet, and western Prince William Sound.

Meanwhile, a short-wave tracking eastward along the Aleutian
chain will lead to cyclogenesis over the North Pacific. While
this low will initially be quite weak, the short-wave will begin
to dig southeastward as an upstream ridge rapidly amplifies ahead
of a strong Northwest Pacific low. Thus, the low will deepen as
it passes to the south of Kodiak Island tonight then across the
southern Gulf on Wednesday. This will really tighten up pressure
gradients across the region. One last piece of the puzzle will be
the current storm over northern Alaska. As it exits eastward today
arctic air will pour in behind it. When the upper flow amplifies
Tuesday night and Wednesday this air will be forced southward
across Southcentral. The tight pressure gradients combined with
the cold air advection will lead to very strong gap winds along
the north Gulf coast and Prince William Sound, peaking most places
Wednesday through Wednesday night. Winds should then begin to
diminish on Thursday.

&&

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

A once powerful low is quickly moving into the North Slope, with
strong onshore winds slowly coming down as the pressure gradient
slackens this morning. The strongest winds remain over the Kusko
Delta, and they will remain so through the morning. High
temperatures will probably occur this morning across most of the
region with a slow drop in temperatures as colder air moves in
behind the low.

The flow will remain onshore for the next two days as a series of
low amplitude waves moves inland through Wednesday. None of the
disturbances look very well defined, with the best chances for
snow showers moving inland across the Kusko Delta and along the
mountains. The upper flow will become more northwesterly Wednesday
with a gradual drying. Wednesday night, high pressure builds in,
and with cold temps aloft in place, expect temperatures to plummet
back close to climatological norms.

&&

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

Broad westerly flow with areas of snow showers have set up over
most of the open Bering Sea. A low will move into the Eastern
Aleutians today and tonight, bringing snow (with some mix today)
and possibly some blowing snow for favored channeled terrain. A
series of low amplitude disturbances will merge and amplify over
the Pribilof Islands, bringing increasing threats for light
accumulating snow with convective enhanced graupel also possible.
The low over the Eastern Aleutians and the associated upper trough
quickly move into the NPAC by Wednesday morning with tapering
snow threats.

Attention quickly turns to a powerful storm force low which will
move into the Western Bering/Aleutians Tue night through Wed
night. The strongest core of warm frontal SE winds and cold air
advection SW winds will remain over that region with much weaker
gales as the front moves east toward the eastern half of the
Bering Sea by Wed night. Winds may gust to 70 MPH at Shemya, but
no significant high winds are anticipated at this point.

&&

.MARINE (Days 3 through 5)...

The period begins Thursday with a large intense low over the
western Bering Sea and another strong low southeast of the Gulf
of Alaska. Model guidance is in very good agreement for this
extended time frame lending higher confidence in the forecast for
the days 3 through 5 timeframe. Gale to storm force wind is likely with
the Bering storm Thursday and in advance of the front as it
progresses across the Bering Sea through Friday. Gales are likely
across the Gulf of Alaska Thursday, then expect benign
conditions, sub gales, throughout the Gulf of Alaska and adjacent
coastal areas.

&&

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

The main message for the longer term is the continuation of a
very active weather pattern.

A large intense low in the western Bering Sea Thursday with the
associated front extending through the Bering Sea across the
eastern Aleutians will continue along the north to northeast
trajectory. This storm and front will produce gale to storm force
wind in the Bering Sea and along the Aleutians. The Front looks
to make landfall along the Bering Sea coast Thursday as it weakens
significantly. All the models are in general agreement with a
secondary trough that develops into a front Thursday with the
formation of a frontal low Thursday night. This is where there is
some disagreement in the model solutions with the GFS supporting a
stronger low than the EC. At this point we will lean toward the
EC solution maintaining continuity in the forecast until we see
more of a model push for the stronger frontal low solution.
Following this Bering Sea storm another compact strong low will
be heading toward the western Aleutians Friday. We will maintain
continuity here and stick with WPC and the EC as we progress
through the weekend. Early next week the models converge back into
better agreement bringing a moderate low into the Seward
Peninsula region.

Over the Gulf of Alaska and southern mainland the main driving
factor will be the low southeast of the Gulf of Alaska and high
pressure over the mainland Thursday. This combination will
support colder conditions over the southern mainland along with
moderate outflow wind through gaps in the coastal mountains. At
this point we expect temperatures to cool around 10 to 20 degrees
over much of the southern mainland Thursday and Friday. Heading
through the weekend warmer air will be pushed across the southern
mainland from the Bering resulting in warming temperatures around
10 to 15 degrees. The models are in generally good agreement
Sunday bringing a weak low into the northern Gulf of Alaska
followed by a weak ridge of high pressure Monday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales 412 150 155 172 173 174 175 176 120 121 127 129 131
136 137 138 351. Storm 411 413 177 178 130 132 352.

&&

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