Small Craft Advisory 408, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
453 AM AKDT Sun May 20 2018

.ANALYSIS AND UPPER LEVELS...
The complex low pressure system aloft is looking slightly more
organized as the Aleutian shortwave being propagated by the jet is
strengthening as it moves toward the Gulf of Alaska. The system
looks to have taken control of the long-wave so to speak, becoming
the center of circulation. There are four or five short-wave
troughs now emanating from the long-wave, and will be the foci
for active weather in the short term. The western Canadian ridge
is still in place, bending the jet into the Gulf of Alaska,
however it does appear as if it is giving way to the east,
allowing southern Alaska to be more under the influence of the
incoming long-wave trough. Way out west, it looks like our
blocking high is no more and integrating into the ridge of a
developing system south of Kamchatka, which would signal a
reorganizing longwave pattern.

For sensible weather, what is there to say that I haven't been
saying everyday for the last week? Cloudy? Check. Showery? Check.
Diurnal gap winds? Check.

&&

.MODEL DISCUSSION...
Guidance is in great agreement with several of the facets of the
forecast that were touched on above. The low south of the Alaska
Peninsula will indeed become the long-wave center and slide into
the Gulf, and the blocking high weakening will allow for the
long-wave pattern to become slightly more progressive. Minor
differences still exist with short-wave and frontal timing with
the Gulf system, but overall synoptically the story stays the
same, a low in the Gulf, clouds and precipitation around it.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Broad upper level cyclonic flow will continue to steer surface
lows/fronts north across the Gulf through early this week. The
net result will be generally cloudy and wet conditions along the
coasts with sporadic showers/rain with some sunshine inland. As
surface features move north across the Gulf they will bring
periods of small craft to gale force winds along coastal marine
areas. The combination of surface ridging along the North Gulf Coast
and a strengthening thermal trough over the interior will give
rise to localized southerly winds along the Copper River, out of
the Knik River, and over west Anchorage during the afternoon and
evening hours through Monday. Elsewhere, Turnagain Arm winds will
increase this afternoon then decrease again overnight tonight.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...

The radars at Bethel and King Salmon continue to be active early
this morning. Expect lingering showers across portions of
Southwest today. The NAM model has pinged into some unstable
pockets this afternoon, but the NAM is often more bullish with
convection. More confidence in thunderstorms on Monday. A warming
trend will begin which will contribute to a more buoyant airmass
being in place and instability. The diurnal heating combined with
remnant dynamics from the upper low will result in thunderstorm
activity Monday afternoon and evening.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...

Looking at the latest Himawari satellite image, there is a train
of storms from the AKPEN to the Korean Peninsula. Looking at the
500mb charts on the models, there is a closed low near St Lawrence
Island and a second closed low south of Cold Bay and multiple well
defined shortwaves embedded in the pattern. There will be a break
in activity late tonight as a ridge axis noses its way over Attu
and the Western Aleutians. But with this progressive flow, the
next low will barrel into the region quickly shifting the ridge
axis eastward.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The overall progressive pattern will continue through much of
next week, as one upper low sets up over the Gulf of Alaska
through mid-week and is quickly replaced by the next upper level
low/trough tracking in from the west mid-week into next weekend.
The initial system will allow several systems to rotate into the
southern mainland through mid-week, bringing precipitation and
some gusty winds to the coastal regions of southcentral Alaska
over the weekend and into next week. However, 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.

The low and associated front will begin to impact the western
Aleutians Monday morning and steadily track eastward through
Wednesday, covering much of the Bering and Aleutians with wet and
windy conditions. The low begins to stall over the central and
eastern Bering Wednesday night, and eventually begins to track
into the AKPen by Friday. Models are showing a triple point low
forming south of the AKPen Thursday morning, with an associated
front ahead of it tracking through the Gulf region Thursday
through Friday. This will bring a repeat scenario of wet and
windy conditions along the Gulf Coast with downsloping conditions
along the lee-side of the Chugach. The models continue to agree
that at least through next weekend, the persistent pattern of
cooler, cloudy, and unsettled weather will remain in place.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 119 120 130 131 138.

&&

SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
LONG TERM...TP