Small Craft Advisory 649, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
452 AM AKDT Sun Sep 8 2019

.ANALYSIS AND UPPER LEVELS...

An upper level wave that has tapped into Gulf moisture that has
been transported north with a dying front continues to produce
rain across portions of Southcentral this morning. The rain has
ended in Anchorage, and continues to move east and weaken from Prince
William Sound north through the Talkeetna Mountains. Cloudy
conditions persist behind the rain, though fortunately any low
cloud cover has been localized. The low ceilings have been more
common across Southwest Alaska. The area is between rainfall
producing disturbances, so while the radar is quiet now, it will
become increasingly busy later today. More on that below.

The axis of a longwave trough is over the central Bering,
extending southward across the central Aleutians. The trough is
producing rainfall ahead of it, so wet weather is ongoing across
the eastern Aleutians and the Alaska Peninsula, and will spread
northeastward with time towards Southwest Alaska. A secondary low
is approaching the western Aleutians, but almost all of the
impactful weather associated with that disturbance will remain
well south and west of the Aleutians.

Ridging over the Yukon with the Bering trough is leading to broad
upper level southwesterly wind flow between the two systems over
most of Southern mainland. This flow is keeping conditions wet
over all of southern mainland Alaska, with the notable exception
of the Copper River Basin.

&&

.MODEL DISCUSSION...

Model agreement remains excellent for the short-term. Agreement
has also improved regarding the future track of the remnants of
Typhoon Lingling, which will move into the Bering this week.
Though the small details are still very uncertain at this time,
the model agreement does increase confidence of the eventual
impacts of the storm, most importantly coastal erosion. More on
that in the long-term discussion below.

&&

.AVIATION...

PANC...With the steady rain over, the early morning hours will
feature chances of fog and low ceilings around the airport through
sunrise. VFR conditions are expected with light winds through the
day. Tonight, more low-level moisture moves in, which will likely
result in periods of drizzle for much of the overnight. The
drizzle will diminish by sunrise Monday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Sun and
Mon)...
After a soggy Sat, things will slowly dry out today. Healthy
rainfall amounts fell across just about all areas west of the
Richardson Highway. Many locations from the Swan Lake Fire up
through Anchorage and into the Southern Susitna Valley saw around
a half inch of rain or more. Seward received nearly 2 inches of
rain as the southerly flow favors that area. Much of the Copper
River Basin did not (and will not) see any rain from this system
as high pressures holds court.The front will drift east over the
Talkeetna Mountains and the Prince William Sound as it falls apart
today. Low level moisture in its wake will likely produce
widespread low stratus and some fog through at least the morning
hours. As the front weakens, winds will slacken and become light
at the surface across Southcentral AK today.

Tonight now looks to feature some more light rain for areas
around Cook Inlet than originally thought. Energy associated with
our current front runs into high pressure that remains over the
Eastern Copper River Basin. As the high surges back to the west,
it forces the remaining energy back west over the Eastern Kenai
Peninsula and the Anchorage Bowl. Meanwhile, a broad trough from
the Eastern Bering impinges on the Alaska Range. This should leave
any remaining low level moisture with no place to go, creating
some low-level convergence. We have increased PoPs tonight into
Mon morning from Whittier up through Talkeetna to account for this
shift in the pattern. It looks to be just some steady light rain
(or even drizzle) so we are not expecting QPF amounts to be nearly
as high as Sat's event.

By Mon, the broad trough from the Bering pushes across the AK
Range. This should make for a cool, cloudy, and showery day across
all areas save the Copper River Basin. High pressure holds on for
one last day out there leading to dry conditions with some breezy
southeast winds.

&&

.FIRE WEATHER...After wetting rains across much of the area Sat, the
Copper River Basin continues to hold out as the primary fire
weather concern. Areas east of Eureka will continue to be dry
with above normal temperatures through Mon. However, we are not
expecting these values to approach critical thresholds. For the
remainder of the area, afternoon humidities will remain very high
in the 60-70% range and there will be another likely shot of light
rain tonight into Mon. A cooler and showery pattern looks to
establish over the area through at least Tue.

&&

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

Isolated to scattered showers continue over the Kuskokwim Delta,
lower Kuskokwim Valley and Bristol Bay areas behind a decaying
front that has pushed into Southcentral. Expect a swath of
stratiform rain and south winds this afternoon as a low over the
central Aleutians steers a warm occluded front towards the
Southwest mainland. As the front lifts northward Monday,
precipitation will transition to showery in nature and winds will
become southwesterly. Precipitation will diminish Tuesday.

&&

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

Precipitation and small craft advisory level southwest winds will
persist into this evening over the eastern Aleutians and into the
eastern Bering, along a front associated with a low situated over
the central Aleutians. The low will lift north tonight, and weak
ridging will build into the western Bering Sea. The ridge will
shift eastward Monday, as a gale force low approaches from the
Kamchatka Peninsula. Southerly gales and widespread precipitation
will move into the western Bering/Aleutians by late Monday, and
spread eastward Tuesday.

&&

.MARINE (Days 3 through 5: Tuesday through Thursday)...
Bering Sea and the Aleutian Coastal Waters: A strengthening low
moving east across the Bering is expected to produce gale force
winds both in its leading front, and also associated with the low
itself. The leading front will be over the central Bering Tuesday
morning, and will reach Southwest Alaska Wednesday. Widespread
gales associated with the low itself will envelop most of the
western and central Bering through at least Wednesday, with
reducing chances for gales on Thursday. There is a low chance of
storm-force winds both with the leading front and the low as
well from Tuesday through Wednesday as well.

Gulf of Alaska: A front associated with the northern periphery of
a low will briefly impact the far southern Gulf Tuesday night with
possible gale-force winds. The front is expected to weaken by
Wednesday, diminishing the threat. On Thursday, the leading front
associated with the aforementioned Bering low may produce gale
force winds near the Barren Islands through the Kamishak Gap in
southern Cook Inlet, but gales are not yet anticipated over the
open waters of the Gulf.

&&

.LONG TERM FORECAST (Days 4 through 7)...
For the extended period we remain in a very active pattern
Wednesday through Sunday. The models are in good agreement
Wednesday with a low approaching the southern Gulf of Alaska and
moving that off to the east through Wednesday night.

The bigger issue is with the storm in the northwest Bering Sea,
the remnants of former Typhoon Lingling Wednesday that is
forecast to move to the eastern Bering Sea Thursday and Friday.
Model solutions differ in the intensity as the storm moves into
the eastern Bering Sea Thursday night. The position of the storm
Friday is coming into better agreement in the latest model runs
with the same general solutions of a main center off the Yukon
Delta coast and a secondary frontal low in the Kodiak Island area.
The models are also coming into better agreement with a weaker
storm center than previous runs. One area of uncertainty regards
the interaction of the Bering Sea storm with the extratropical
remnants of Typhoon Faxai as they become ingested into the low
Tuesday evening. Regardless of the model solutions, this storm
will have abundant tropical moisture entrained into it from the
former typhoon that should result in very good precipitation
efficiency near the low pressure center and ahead of its
associated front as it moves from the central Bering Sea and
Aleutians into southwest and south central Alaska. The latest
model runs have not had good continuity in regard to this, thus
confidence is a bit lower in the current solutions. Previous
solutions supported a bombogenesis scenario, but the latest
solutions are a bit weaker with central pressures only dropping
into the 970s to 980s MB range instead of the 950s MB range.

With the models in decent agreement regarding the Bering low
stalling out over the northern Bering Wednesday through Friday,
coastal flooding concerns are increasing along the Bering coast for
this time frame, particularly from Kuskokwim Bay northward through
Hooper Bay. A long time period of southerly onshore flow is likely
to produce higher than normal tides, even if the strength of those
winds will be variable.

Continuing into late Friday and Saturday, the models diverge with
respect to the location of the remnants of this large low as well
as the central pressure lending to little if any confidence in the
model solutions for Friday and Saturday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...JPW