Small Craft Advisory 533, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 PM AKDT Sun Jul 8 2018

.ANALYSIS AND UPPER LEVELS...
Low pressure in the eastern Bering Sea has caused a shift from
sunny conditions to cloudy and rainy skies across Mainland
Alaska. Satellite imagery shows dense cloud cover over most of
Mainland Alaska and the Bering Sea. Rain is falling over
Southcentral, and along the western coast which can be observed
in radar imagery, surface observations and web cameras. The
vertically stacked low has moved near Norton Sound with the
long-wave trough extending over the eastern Aleutians and Alaska
Peninsula. The jet stream at 30,000 ft, has continued to move
east, curving to the north over the AK Pen. Because of its
placement within a favorable part of the jet, the low deepens and
the effects are becoming more widespread. The expansive nature of
this low has extended a strong front eastward into the Gulf of
Alaska. As the front pushes into the Gulf of Alaska, weather has
made a significant switch to windy and rainy conditions. While
gusty winds started yesterday in the usual areas of Turnagain Arm
and Anchorage, those winds have ramped up due to an increased
pressure gradient along the North Gulf Coast. Offshore areas of
the Gulf of Alaska and the Barren Islands are also seeing an
intensification in wind. Southerly flow is bringing in moisture
to Southcentral aiding in creating the dense cloud cover and rain
over southern Mainland Alaska.

&&

.MODEL DISCUSSION...
Even with the major pattern shift created by the Eastern Bering
low, the very slow movement of it has allowed for very good
synoptic agreement to continue through Tuesday. As forecast
confidence remains good throughout the area into Tuesday, with
only minor changes needed to the forecast which continues to
favor higher resolution models. Afterwards the models are still
struggling some with the next system which quickly move into the
Bering behind the current low, but are beginning to come into
better agreement with timing. Agreement is still poor enough
however to limit details in long term forecast which continues
to rely on ensemble guidance.

&&

.AVIATION...
PANC...Little relief is expected from the gusty southeasterly
winds until late Monday morning when the front will dissipate
over the North Gulf Coast. This will bring an end to the strongest
winds that are expected to occur this evening, however a
persistent pressure gradient along the Chugach will allow for
south-southeasterly winds to continue for most of the day Monday.
Otherwise VFR conditions with periods of light rain are expected
this evening which will likely bring ceilings near but likely not
below 5000 ft.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
On the latest satellite image, the cloud shield from this low
pressure system spans from Valdez to the Central Bering. The
radars at King Salmon, Kenai and Middleton Island have all pinged
into precipitation. The upper low that is sustaining this system
is still over Kuskokwim Bay. There are multiple embedded
shortwaves. The upper level low will move very slowly and expect
cyclonic flow to persist over the region through Wednesday.
Expect more clouds and unsettled weather. There will be breaks
between the precipitation waves but generally expecting a wet
pattern.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Visible satellite imagery this afternoon reveals an impressive
expanse of cloud cover encompassing almost the entirety of Alaska
and the Bering Sea due to the rather large low pressure system
situated between the Pribilofs and Nunivak Island. Over the
southwest mainland and AKpen, rain will continue to increase in
coverage and push farther inland tonight as the second in a series
of shortwaves moves east in tandem with the low. Behind this
trough, winds have begun to shift to the southwest. It is this
wind shift and a continuation of the prolonged onshore flow that
will bring higher seas and high surf to the Kuskokwim and Bristol
Bays. Storm surge model runs this afternoon have trended surge
values about one foot higher than this time yesterday. The highest
surge is still expected to coincide with low tide, thus keeping
coastal impacts to a minimum. Still, isolated areas of coastal
erosion are possible over the next 48 hours. A special weather
statement highlighting hazard remains in effect through Tuesday.

The cloudy and rainy weather will remain through Wednesday as the
surface low and the upper-level trough both drift inland and
weaken. Colder air wrapping in underneath the low (850mb
temperatures between -1C and +1C) will keep surface temperatures
10 to 15 degrees below normal. The colder air aloft may also help
to trigger an isolated thunderstorm or two embedded in the shower
activity around the Bethel area Tuesday afternoon out ahead of
the core of low pressure. A ridge tries to build in across the
AKpen for late Tuesday into Wednesday as the next system begins
to dig south through the central Bering. Although the chance this
will diminish the chance for precipitation across southwest
Alaska, clouds should continue their hold over the region as the
onshore flow persists.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Over the eastern half of the Bering, a large low pressure system
will continue to spin as it slowly works from west to east. Expect
widespread small-craft and gale-fore winds, showers, and cool
weather through Tuesday with this system. As the main trough axis
slides over the west coast late Sunday into Monday, a trailing
shortwave will dive south through the central Bering. This will
bring another round of showers to the central and eastern Bering.
Colder air from the Arctic will also advect across the Bering
with 850 mb (5,000ft MSL) temperatures dropping to near -5C for
Monday, moderating to -3C as it reaches the eastern Bering for
Tuesday. This will keep surface temperatures around 10 degrees
below normal through midweek.

Over the Western Bering, warm air advecting from Siberia will
slide over the cooler ocean waters, bringing widespread stratus
and fog. By Monday, a low will spin up in the western Bering and
drop south through the western and central Aleutians. This could
bring some light rain to Atka and Adak overnight Mon into Tue and
help reduce the areal coverage of fog.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast periods begins Wednesday evening with a
deamplifying upper trough over Southern Alaska. The upper flow
will be "flattening" out so clouds and leftover showers will still
be possible. Arctic upper level energy moving into the Bering Sea
will be in the process of phasing with a migratory cutoff low
over the North Pacific. There will, therefore, be a reprieve in
the wetter weather late Wednesday through Thursday before the
Bering low moves inland Friday and Saturday, bringing another
round of wet and cloudy weather to Southern Alaska. This trough
will move east through Sunday with the possibility of ridging
aloft building in for the beginning of the next work week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 175 185.

&&

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