Small Craft Advisory 292, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
527 AM AKDT Thu Apr 6 2017

.ANALYSIS AND UPPER LEVELS...

The movement of a strong vertically stacked low located south of
Kodiak Island has slowed to a crawl this morning as spokes of
energy pinwheel around the center of the low into Southern Alaska.
For a wrapped up low such as this, the wind field around it
remains rather impressive. Top gusts at Middleton Island
approached 60 mph yesterday with many other stations from Kodiak
northward along the coast into Prince William Sound seeing gusts
above 40 mph. Channeling through passes including Turnagain Arm
have kept a steady breeze going into the night at Ted Stevens
Airport.

On radar, Middleton Island imagery shows lines of showers moving
into Prince William Sound with a typical downslope shadow west of
the Kenai and Chugach Mountains. On occasion a shower or two will
survive the downsloping to wet the ground in Anchorage but most
locales have stayed dry. It's not difficult to see why looking at
the 00Z sounding yesterday, with all of the lowest levels of the
atmosphere below 800 mb remaining very dry.

Over Southwest Alaska, the low levels also remain dry, minimizing
the chances the showers on radar produce any precipitation that
reaches the ground there as well. Much of the area got well into
the 40s again today, with coastal locales in the Kuskokwim Delta
holding on to sub-freezing temperatures as cold northwest winds
stream across the broad expanse of sea ice.

For the eastern Bering, the refrigerated air sweeping southward
across the sea ice is likely enhancing the tunneling effects of
the bays and passes of the Alaska Peninsula as that air sweeps
towards the Gulf low. Gale force winds with higher gusts have been
common along and downwind of those passes, namely at places like
King Cove and False Pass. The maximum wind gust today at King Cove
hit a very impressive 79 mph.

It really isn't until one makes it to the central Bering that one
escapes the reach of cloud cover and wind associated with the
Gulf low. However, the area of high pressure located there is
weakening as another front pushes eastward across the western
Bering and Aleutians. Shemya has been reporting steady light snow
much of the day and through the evening as a result of the front
and the relatively moist air mass behind it, though temps in the
mid 30s have likely limited accumulations.

&&

.MODEL DISCUSSION...

For the most part, the pattern remains a fairly steady one for the
next two days as the low over the Gulf slowly weakens and a series
of lows move southeastward across the western Bering and
Aleutians. Since most of the features of interest are synoptic
scale, the models are in good agreement on their strength, speed,
and location through the next two days. Thus, as is typical the
challenges lie in the smaller scale details which are hardly ever
well resolved. The GFS is the model of choice for this package as
it represents a happy medium between the small differences in the
other models for all the lows affecting the area. It also
continues the trend in the models of bringing the Gulf low further
west into the Alaska Peninsula and perhaps Bristol Bay. Forecast
confidence is above normal.

&&

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

The current weather pattern will remain fairly stagnant through
the short term. Perpendicular flow persists across the Kenai
Peninsula, with IR satellite imagery this morning indicating a
broad mountain wave. The sounding this morning continues to
confirm that dry conditions will persist across the Anchorage Bowl
and other typically downsloped areas such as the western Kenai
Peninsula through the short term. While lapse rates are steep at
mountain top and radar indicates some spillover precipitation, the
sounding indicates a fairly deep layer of dry air. As a result,
no precipitation has been observed in the Anchorage Bowl this
morning. Stronger gusts will develop near Kodiak Island and
associated waters by this afternoon into evening, as cold air
advection persists. The most active weather will remain near
Kodiak Island in the short term as the low lingers and supports
steady precipitation and gusty winds into Friday. &&

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

Low pressure near Kodiak pushes a trough across the western areas
of Bristol Bay and the Kuskokwim today. Light rain develops along
the trough as it moves toward the eastern Bering. Isolated to
scattered chances for precipitation continue through the weekend
as low pressure persists south of the Alaska Peninsula. Diurnal
temperature trends will bring rain chances during the day and
snow overnight as the air mass moderates slightly cooler into
Saturday.

Across the Alaska Peninsula...strong northerly winds continue
through tonight as an inverted trough axis moves from the
southwest mainland to the eastern Bering and Bristol Bay. Gusty
winds through gaps are expected to gust between 40 to 60 mph which
will reduce visibilities between 3 to 6 miles at times. Winds
taper off for Friday, however snow continues through Saturday with
only light accumulations expected.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
An active weather pattern for the end of the week brings two
fronts across the western waters through Saturday. The primary
impacts with the short term forecast are gale winds and
precipitation along the frontal boundaries. The first front
currently extends across the western Bering to the central
Aleutians. The surface low driving the front pushes along the
western and central islands through the afternoon and fills
quickly this evening as it moves into the North Pacific. A colder
air mass will keep precipitation all snow along the western
Aleutians and a mix of snow and rain for the central Chain. The
second front moves to the western Chain late Friday with the
associated surface low moving across Shemya late saturday.

&&

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

Beginning the extended forecast period Friday night, there will be
a high latitude blocking high over the Beaufort Sea and a gale
force low over the Western Bering Sea. Southcentral will be under
the influence of weak upper level troughing centered over Western
Alaska with an associated surface low near the Alaska Peninsula.
With the main Pacific storm track remaining well south of the
mainland, the challenge will be the interactions of weakening
lows moving into the NE Gulf of Alaska then merging with the
preexisting trough across Western Alaska. It does seem that this
will keep at least some clouds across the inland areas with slight
threats for precipitation, more so Sunday and Monday. Expect
temperatures to be near to slightly above normal as the general
pattern remains onshore.

Middle of next week there is growing uncertainty with the general
large scale pattern, but models are beginning to hint at a
shifting storm track into the Bering Sea with an amplifying NE
Pacific Ridge. Both the latest deterministic GFS and ECMWF are
hinting at a series of Bering lows shifting a trough axis/front
into Western Alaska. Should this happen, expect widespread rain
and warming temperatures.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...GALES 130 131 132 136 137 138 150 155 177 178.

&&

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...ELN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...JA