Small Craft Advisory 174, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
409 AM AKST Thu Feb 28 2019

.ANALYSIS AND UPPER LEVELS...

A large "bubble" of upper level high pressure, currently centered
over the central Gulf, continues to act as a shield of sorts,
deflecting any active weather trying to advance eastward into
mainland Alaska northward towards the Bering Strait and the
Kamchatka Peninsula. Thus, other than some persistent low stratus
around Kodiak, any associated cloud cover is from Dillingham west.
The area of low pressure responsible for that cloud cover is
centered well west of the Pribilofs, and producing high gale force
winds (40-45 kt) over the eastern Aleutians and north through the
Pribilofs, based on the 0648 UTC ASCAT pass. The low is associated
with a 115 kt upper level jet streak, which is located along the
leading edge of a long-wave trough, currently centered over the
western Bering.

Temperatures across the area are coldest under the clear skies
across the Copper River Basin, steadily warming as one goes west
across Southern Alaska. Above freezing temperatures persist across
the Kuskokwim Delta with gusty southerly winds ongoing. Under the
upper level high, Anchorage's sounding continues to show a strong
inversion between 900 and 925 mb, or 2,500 to 4,000 ft above sea
level. Above the inversion temperatures are near the freezing mark
while Anchorage and low elevation locations are in the single
digits and teens.

&&

.MODEL DISCUSSION...

The models remain in excellent agreement through the short term in
this current quiet weather pattern. As the upper level high is
finally shunted off to the south on Friday, the models begin to
disagree a bit on the intensity and location of the next round of
precipitation for Southwest Alaska by Saturday. This will be in
large part the result of how quickly the ridging breaks down over
Southcentral. Nevertheless these differences remain minor and are
not expected to result in any significant changes to the ongoing
forecast. The main forecast challenges the next couple days will
continue to be the strength of the gap winds (though downward
trends are expected to continue), the timing and location of any
patchy fog around the area, and any effect those two factors will
have on the temperatures. Overall forecast confidence remains very
high.

&&

.AVIATION...

PANC...Light winds will persist. There is a chance of patchy fog
in the 2-3 hours both before and after sunrise and both this
morning and again tomorrow morning. If the fog impacts the
airport, visibilities could drop well below the 1SM indicated in
the TAF for brief periods of time.

&&

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

The upper level ridge over the area will split apart. One portion
of the ridge will head north to northern Alaska today, and then
into the Arctic Ocean on Friday and Friday night. The other
portion will push south into the northern Gulf today and then into
the southern Gulf by Friday night. This will leave Southcentral in
a "col". Benign weather will continue, with mostly clear skies
and some local patchy late night and morning fog. By late Friday
night an upper level trough will begin to approach the area from
the west, bringing increasing clouds to Kodiak Island.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Today through Saturday night)

An area of high pressure at the surface and aloft continues to
keep interior portions of the interior dry, with even coastal
locations getting a brief respite from the series of fronts and
precipitation that has been consistently affecting those
locations the past few days. Speaking of which, another front will
reach the coast and extend down through the AKPEN by Friday
morning, bringing areas of snow and rain with it. Unlike the past
several fronts, this one will have some upper-level support, as a
vorticity (spin in the atmosphere) lobe rotating around the
Bering trough pinches of a piece of energy as it lifts north along
the coast.

While the main area of vorticity lifts north, the second piece
that was detached from it will form a new mid-level low in the
central Pacific, with what's essentially an inverted mid-level
trough extending north, connecting the two disturbances. This in
turn helps break down the ridge over southcentral Alaska, allowing
the front to progress inland and giving the interior a chance of
precipitation as well by Saturday morning, while continuing
through the day.

&&

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

A storm lifting through the central Bering will continue to
support sustained storm force winds ahead of an attached surface
front on the backside of the low. This front will sweep eastward,
reaching coastal southwest Alaska by Friday morning. As it does
so, winds will decrease below gale criteria (35 kt). Our
attention then turns to a strong low approaching Shemya from the
west Saturday morning, as another surface front reaches the
western Bering and Aleutians ahead of the parent low. This feature
will bring sustained storm force winds with seas increasing into
the 20 to 25 feet range, with widespread precipitation expected.
Winds will drop back to gales following frontal passage, except
near the approaching low pressure center, which looks to near
Shemya Saturday night, with sustained storm force winds likely
again with seas building towards 30 feet. Meanwhile, the surface
front will then bisect the central portions of the region.

&&

.MARINE (Days 3 through 5)...

Confidence is high for high-end southerly gales and storm force
gusts to develop over the western Aleutians overnight Friday into
Saturday along a cold front associated with a strong low moving
into the Bering. The core of the low center will track just west
of Attu Saturday night and thus strong winds and high seas will
persist there through much of Sunday. Otherwise, the
aforementioned front will continue to track west to east over the
weekend. Winds will diminish and seas subside as the front
approaches the strong blocking ridge that has been in place over
the mainland for several days. Sustained gales appear less likely
east of Cold Bay. Sub gales and lower seas are expected Monday
across the Bering in weaker westerly flow behind the departing
low.

Sub gales and generally calm seas continue over the Gulf through
Monday under a strong ridge.

&&

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

A cold front associated with a low over the western Bering is
expected to bring moderate precipitation to the Aleutians this
weekend. As there is southerly flow ahead of the front, this will
be in the form of rain or a rain/snow mix for many areas.
Agreement slightly deteriorates after Sunday, with the GFS
solution indicating a weaker low departing much more quickly just
to the west of Nunivak Island by Monday afternoon whereas the
ECMWF and Canadian models show a stronger low centered west of St.
Paul. The latter solutions would favor longer duration
precipitation for the eastern Aleutians and the southwest
mainland, where there could be the chance of accumulating
snowfall. In general all models show a weak ridge building in
behind the aforementioned low on Tuesday. Confidence is moderate
for a strong, broad low to move roughly over Attu Wednesday. The
associated front will bring strong southerly flow and moderate
rain and quickly (with strong westerly winds behind it) track west
to east across the Aleutians.

Fair skies expected over Southcentral Saturday under a ridge. A
weak shortwave trough will move over the region Sunday. Exact
timing is still uncertain (ECMWF slightly faster) however overcast
skies are likely and brief light snow cannot be ruled out. A ridge
will build in behind the front on Monday. ECMWF is more amplified
given the stronger low digging behind it as mentioned above. This
would lead to clearer skies as opposed to the less-amplified GFS
solution. The strong blocking ridge that has persisted over
Southcentral the past week will finally fully depart the region
Tuesday as the GFS and Canadian models indicate a possible snow-
producing trough moving over the region. While the ECMWF is not as
aggressive, it appears a more active pattern is likely mid-to-late
next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.

MARINE...Storm Warning: 185.
Gale Warning: 165 170 172 179 180 181.

FIRE WEATHER...NONE.

&&

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