Small Craft Advisory 644, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
426 AM AKDT Mon Aug 20 2018

.ANALYSIS AND UPPER LEVELS...

The widespread rain that fell from Cook Inlet to Cordova yesterday
has tapered off slightly as a shortwave trough continues to track
north into the interior of southern Alaska this morning. Plenty
of moisture, however, is still funneling into the southcentral coast
as evident on radar with showers persisting across Prince William
Sound and redeveloping through the Susitna Valley. This is due to
the establishment of an atmospheric river, a deep southerly fetch of
moisture with sub-tropical origins. This is being steered by a
stalled low over the western Gulf and a ridge of high pressure
stretching across the Alaska Panhandle. Satellite imagery this
morning already shows the next round of moisture quickly moving
north through the Gulf as the next shortwave advances.

Further west, a decaying occluded front continues to cover the
Southwest in clouds with a few lingering scattered showers. A
shortwave ejecting from the upper-level low well south of Sand Point
is pushing another round of low clouds and rain across the northern
half of the Alaska Peninsula this morning.

Over the Bering, a warm front is producing clouds and rain across
the central Bering, Aleutians, and Pribilofs. Winds are relatively
light around this front, with some southerly gusts to 20kts noted
in the buoy reports. Satellite imagery and observations reveal
areas of fog ahead of the front, mainly over the northern Bering
and Pribilofs and a widespread deck of low stratus behind the
front from the western Bering south to Shemya and Adak.

&&

.MODEL DISCUSSION...

Models are in good synoptic agreement throughout the forecast
period. The main forecast challenge will be with the timing of
the heaviest bands of precipitation across the Southcentral coast
as the shortwaves, and eventually the upper-level low, move
onshore from the gulf. Precipitation amounts across the eastern
Kenai Peninsula and Prince William Sound will also be a
challenge, but there is decent model consensus of a bullseye of 4
to 6 inches of rain from the eastern half of Prince William Sound
to the Copper River Delta through Tuesday evening. A general 2 to
4 inches of rain are possible from the western half of Prince
William Sound to the eastern Kenai Peninsula.

&&

.AVIATION...
PANC...Brief LIFR conditions are possible through 14z as a low cloud
deck drifts off the Inlet over the airport complex. After this time,
VFR conditions should persist with light winds. Periods of light
rain are likely through 18z with ceilings dropping to 4,500ft.

&&

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

The main area of heavy rain pushed east of Cordova overnight, but a
secondary pulse in the atmospheric river will push it back westward
today. Therefore there will be periods of heavy rain in the eastern
Prince William Sound area by this afternoon.

The upper level low, which is currently in the vicinity of the
Alaska Peninsula will then begin to move northeastward this
evening and move over Prince William Sound Tuesday before exiting
the region into the Yukon Tuesday night. As this upper level low
moves through the region, it will bring widespread rain to
Southcentral Alaska with some periods of heavy rain possible
across much of Southcentral Alaska tonight.

All this rain will raise water levels in the rivers and stream
across the region. The main flooding concern is in the Cordova
area where there is a flood Advisory in effect through Tuesday
evening. Other rivers along the Gulf Coast and Prince William
Sound will also see some noticeable rises, but none are expected
to reach flood stage at this time except possibly in the Cordova
area.

&&

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

A wet and abundantly cloudy weather pattern will dominate the
Southwest this forecast package. Back to back lows will steer
plumes of moisture into the region. Currently, there is a low
south of the eastern Aleutians and AKPEN with wrap around
moisture over most of SW AK. The latest satellite imagery
estimates that the TPW/Total Precipitable Water is 120-150 percent
of normal over Southwest. The same satellite image also pings
into a secondary moisture gradient upstream which is progged to
move eastward impacting the region as the forecast period unfolds.

&&

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

A decaying low south of the eastern Aleutians will continue to
push eastward. All the while, winds in the boundary layer will be
from the southeast and wrap lingering moisture into the eastern
portion of the chain. A second front will follow in its wake. This
storm has already matured and will move steadily across the
Bering and the Aleutians. A reinforcing shortwave will slide
around the back side of the upper low bringing a second shot of
precipitation with this storm. In a nutshell, expect cloudy and
wet weather for the Bering and the chain.

&&

.LONG TERM FORECAST (Thursday through Sunday)...
Beginning Thursday, models are in good agreement regarding the
overall synoptic pattern as an upper level ridge over Eastern
Alaska is quickly replaced by a broad upper level low over the
Bering Sea. An embedded shortwave trough will lift NE across the
AKPEN and into southcentral during the day on Thursday bringing
another round of rain to the region, with the focus looking to be
along the Gulf Coast and the Susitna Valley. Unsettled weather
looks to continue into Friday as the flow becomes more zonal after
the passage of the shortwave.

Uncertainty creeps into the forecast by next weekend as model
solutions differ on a potential pattern shift with the building of
an upper level ridge. The EC shows a rapidly amplifying long wave
ridge building across the western mainland into southcentral
Saturday through early Sunday with an upper low digging south
across the western and central Bering at the same time. The ridge
pushes east rather quickly as the upstream low moves into the
mainland by Sunday. The GFS has trended in building an even higher
amplitude ridge and keeping it somewhat stationary over the west
coast through early next week. This would bring a prolonged period
of nice weather across most of the mainland, while the EC
solution would mean only a day or two of 'summer' like weather.
Ensemble and WPC solutions are taking a middle ground approach
building a somewhat muted ridge over the southern mainland for the
weekend. Either way, there is decent agreement that at least
portions of southcentral and eastern Alaska will see a break in
the rainy/unsettled weather next weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood advisory for the Cordova Vicinity.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
LONG TERM...KVP