Small Craft Advisory 410, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
515 PM AKDT Sun May 20 2018

.ANALYSIS AND UPPER LEVELS...
There is a large upper level low centered southwest of the Gulf of
Alaska. Extending from this low is an upper level trough and its
associated surface front over the northern Gulf and approaching
the coast. Most of the precipitation with this front is falling
along the coast, with downslope drying limiting precipitation
inland. There is a small upper level low over the northeast Bering
that is diving south towards the Gulf low. This low has a weak
trailing surface feature. There is a blocking ridge over western
Canada which is forcing systems from the Gulf low north into
southern Alaska. There is another ridge over the western Bering
and western Aleutians. There is an upper level low centered south
of the Kamchatka Peninsula that is moving towards the northeast.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement through the short term
portion of the forecast (Tuesday afternoon). The exception is
with how they handle the low in the Gulf. There are differences in
the timing of the front tonight into Monday morning, with the
Canadian and ECMWF a bit faster than the NAM and GFS. The Canadian
and ECMWF also divide the surface low into two centers on Monday,
while the NAM and GFS have consolidated (but elongated) low. These
factors only make minor differences in the forecast, but enough to
have the forecast confidence near normal rather than above.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist into Monday
afternoon. Late Monday afternoon will likely see the gusty
Southeast winds returning over the terminal.

&&

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

Persistent upper-level cyclonic flow will continue through the
short term forecast period. At the surface, a vertically stacked
983 mb low will drift slowly toward the Gulf. As it does, this
system will steer a series of warm and occluded fronts through
the Gulf and into southcentral Alaska. The first front will arrive
tonight, enhancing rainfall and producing a weak barrier jet for
coastal locations east of Cordova. A second front pushing onshore
Monday morning will produce another period of small craft and
gale-force winds along the marine zones. A third, weaker system
will move east to west through the northern Gulf Tuesday morning.

Farther west, the advancing low pressure system will strengthen
the east-southeast flow aloft and at the surface, resulting in a
general offshore flow for the lower Mat-Su Valleys south through
the Cook Inlet. The downslope effect from this set- up will help
to keep conditions mainly dry for areas in the lee of the
Talkeetna and Chugach ranges, although a scattered rain shower
cannot be ruled out as moisture from the fronts moves over the
Chugach Mountains. Turnagain winds are expected to strengthen
slightly Monday afternoon before diminishing early Tuesday
morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Showers will diminish in coverage overnight across the southwest
mainland as the upper low south of the region tracks slightly
farther east into the Gulf of Alaska, allowing warmer and drier
offshore flow to set up for Monday. This will in turn lead to
increased thunderstorm potential Monday afternoon into Monday
evening. Guidance is in good agreement on a thermal trough setting
up from the Kuskokwim Valley into Greater Bristol Bay and
providing a focus for convection as increased sunshine helps to
destabilize the atmosphere. Coverage is expected to be greatest
across the Lower Kuskokwim Valley, but isolated thunderstorms
will be possible from eastern portions of the Delta to the
northern shore of Bristol Bay. Convective activity will then wind
down overnight as weak transient ridging noses into the region
from the southwest. Flow will then transition back to onshore for
Tuesday, with difluent southwest flow aloft touching off another
round of showers, albeit with diminished thunderstorm potential
given a decrease in instability and the loss of a coherent forcing
mechanism compared to Monday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The forecast begins with relatively benign conditions in place
across the Bering through tonight beneath an upper level ridge.
Northerly flow and an advancing shortwave on the western
periphery of the mainland trough will be sufficient to maintain a
showery regime through Monday afternoon from the Akpen to the
Eastern Aleutians. A front will enter the Western Aleutians Monday
morning and bring a period of rain and gusty Small Craft Advisory
Level winds to much of the region as it moves east through the
middle of the week. The parent low will then move into the western
Bering by Tuesday and keep a more showery regime in place into
towards the end of the short term.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A progressive pattern will continue through next weekend, with an
upper low slowly tracking northward through the Gulf through
Thursday morning and another low tracking eastward across the
Bering/Aleutians through Friday. Models remain in fair agreement
with this cooler, cloudier, and unsettled pattern lingering over
much of the area into next weekend.

A triple point low will develop over the Gulf along the
approaching front by Thursday, and continue its progression
northward into Friday morning. This system will bring rain and
gusty winds to the surrounding Gulf areas as the front pushes
into the coast and further inland by friday night. The strong
southeasterly flow associated with these systems will favor
slightly drier conditions over the downsloped areas along the lee-
side of the Chugach range. Conditions will become more showery
overall and switch to an off-shore flow pattern by Saturday
morning, as the parent low tracks into the eastern Gulf through
next weekend.

Out West, the low over the western Bering will bring wet and
windy conditions to much of the Bering/Aleutians through Friday,
as it tracks eastward and crosses the AKPen into the northern
Pacific. High pressure will build into the western Bering and
Aleutians Friday, and continue to build eastward over much of
the area by Sunday morning.
&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 119.

&&

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