Small Craft Advisory 420, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 AM AKDT FRI MAY 20 2016

.ANALYSIS AND UPPER LEVELS...
A stationary cold upper level low sits over the Northern Bering
bringing southerly flow and rain to Southwest Alaska. This flow
is aided by the jet coming over the southern part of the state
from the south. Out east a ridge over the Gulf weakens while
making way for a North Pacific low to move towards the AK Pen.
Another low is south of the Western Aleutians and is tracking
east.

&&

.MODEL DISCUSSION...
The models are in good agreement in the short term but are
struggling with the timing of a front moving over Southwest Alaska
Saturday night. The NAM is much faster with bringing the front in
than the other models which led to a preference for the slower
solution.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The main weather challenges for the next couple of days are gusty
winds and rain along two frontal passages. The first system
currently draped along the Kenai Peninsula and northern Gulf
pushes inland through the morning spreading light rain across
Southcentral. Gusty winds across channeled terrain will increase
later this morning as a surface ridge, with an axis along the
northeastern Gulf and Prince William Sound, is squeezed by an
approaching trough. A tighter pressure gradient is expected,
therefore, winds were increased during the morning package for
Turnagain Arm and higher elevations and through the Knik River
Valley. Rain will taper off from south to north through the
evening...however wind gusts continue through late tonight, after
the second front moves through.

The second front moves into the southwestern Gulf later this
afternoon. The timing of this front takes a similar fast track
through Southcentral into Saturday morning. There is a significant
difference with the second front as the upper low near the Alaska
Peninsula lifts north at the same time. The low shears apart as it
progresses east across the Gulf and southern mainland. There is
still some uncertainty in the timing for winds tapering off as
models differ slightly on the track of this system. Rain activity
is expected to diminish across the southwestern Gulf Saturday
afternoon with lingering activity across the northern Gulf and
southern mainland as the upper wave shifts slowly east Saturday
night.

&&

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

A cold core upper level low will continue to reside just off the
west coast of Alaska (near Saint Matthew Island) through Saturday
before it finally begins to move closer to shore. 850 mb temperatures
within this closed low range from -4 to -7 degrees Celsius and has
been the culprit for the cooler temperatures seen across Southwest
Alaska. This period of cooler temperatures, which is actually
near normal temperatures for this time of the year, looks to
persist through the weekend as the low remains anchored west of
the mainland. A shortwave and its associated surface low that are
currently in the process of developing south of the Alaska
Peninsula will rapidly intensify and shift northward this morning
and bring a significant amount of precipitation to much of the
southwest mainland. Looking at the current forecast grids,
rainfall amounts through Saturday ranging from three quarters of
an inch to nearly an inch and a half will be possible across the
Bristol Bay area and the Lower Kuskokwim Valley. The Kuskokwim
Delta should miss out on much of the rainfall with only a tenth to
half of an inch of rain expected through Saturday. Fog is likely
to develop during periods of lighter rainfall as the surface and
boundary layers both remain stable and saturated under this
stratiform air mass.

&&

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

A stacked surface low will continue to reside over the northern
Bering Sea through the weekend with the low becoming more
expansive and weaker as the weekend progresses. This broad low
pressure should help inhibit fog development across much of the
Bering Sea as the low keeps the atmosphere well mixed but
scattered rain showers will likely be seen across the region. An
area of low pressure near Shemya will track east today and move
into the Bering Sea Saturday morning. The low will be weakening
during this transition so no strong wind patterns are expected
with this system. However, a nice core of heavy precipitation
will likely make its way across the Central Aleutians on Saturday
and Eastern Aleutians late Saturday.

&&

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

Beginning the extended period saturday evening, the trough which
will have brought active weather to southern Alaska will be
weakening and moving east, with another split upper trough over
the central Bering Sea. Weak offshore flow over Southcentral
Alaska will allow for a gradual drying overnight while more rain
moves into Southwest Alaska. Eventually the flow pattern will
become more westerly and progressive with a series of fast moving
shortwave troughs moving across Alaska. This will keep
temperatures moderate with intermittent periods of clouds/rain
mixed with nice weather in between each individual system through
most of next week. No major warm-ups, cool-downs, or significant
wet or dry periods are expected.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...130 131.

&&

SYNOPSIS AND MODEL DISCUSSION...dk
SOUTHCENTRAL ALASKA...kh
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...mc
LONG TERM...ja