Small Craft Advisory 374, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 AM AKDT Tue May 8 2018

.ANALYSIS AND UPPER LEVELS...
Water Vapor satellite imagery shows abundant subtropical moisture
streaming across the eastern Alaska Peninsula to Bristol Bay.
A mature closed low over the southern Bering and a ridge over the
eastern mainland is acting as a conveyor belt to pull this
moisture up from the North Pacific. The jet stream which helped to
amplify the ridge over the the eastern mainland, is blocking the
Bering low from moving east and it is now stationary just north of
the Central Aleutians.

At the surface, rain showers are developing along the central Gulf
and moving north as a disturbance aloft rides north ahead of the
storm system. Across the Bering...the occluded surface low pushed
an associated front to coastal Bristol Bay which extends west to
the Pribilof Islands. Showers are wrapping around the back side of
the system to the western half of the Aleutians. Winds are
currently strongest, with gale force strength across the eastern
and western Bering.

&&

.MODEL DISCUSSION...
Models are generally in good agreement through 48 hours, then
begin to diverge on the track of a north Pacific low which races
north to the Alaska Peninsula Wednesday night. Only minor changes
were needed through early Wednesday and then edits were made to
incorporate slightly faster progression of front that brings
another surge of moisture from the North Pacific through Thursday.

&&

.AVIATION...
PANC...VFR conditions are expected through Wednesday morning with
mid level clouds increasing this afternoon. Variable winds should
have a northerly direction by mid morning as the flow shifts down
inlet between low pressure advancing from the west and a ridge
holding along the gulf.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level short-wave trough extending from the large vertically
stacked low over the Bering Sea will approach from the south
today through tonight, then lift northward across the western
Gulf Wednesday with the tail end just catching Southcentral
Wednesday night. Similarly, a cold front at the surface will be
slow to lift northward. This will result in a prolonged period of
moderate to heavy rain for Kodiak Island and the western Gulf,
with 2 to 3 inches of rain expected along the east side of Kodiak
island over the next 36 hours.

Rain will also spread northward to the southern to eastern Kenai
Peninsula today and the west side of Cook Inlet, with strong low
level southeast flow serving to enhance rain along east facing
slopes. This flow will keep areas west of the Kenai and Chugach
Mountains mostly dry, with perhaps an occasional sprinkle. As the
tail end of the upper trough comes through Wednesday night, rain
will spread further inland across the Susitna Valley as well as
to the rest of Prince William Sound.

There won't be much of a break for Kodiak as a rapidly strengthening
storm moves up from the Pacific Wednesday night into Thursday,
bringing another round of heavy rain and strong winds.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3...Tue through
Thu)...Several pieces of energy will rotate around the massive
low and reinvigorate the front that is currently draped across the
area. Gusty southeast to southerly winds will continue across the
area due to the pressure gradient associated with the front.
Winds will slowly diminish on Wed as the low center drifts further
west. Precipitation will be showery this morning, except along
southeast facing terrain where upslope will keep things more
persistent. However, as the next shortwave rounds the base of the
trough and a some weak instability builds in this afternoon expect
precipitation to become more widespread. The overall pattern
though will still continue to favor terrain, leaving places in the
lee of the mountains (Iliamna, King Salmon, Bethel, etc) seeing
only spotty showers and very limited QPF. Another shortwave comes
around Wed afternoon, making for a repeat to the pattern.

Temperatures should continue to warm today, even under abundant
cloud cover. This is due to warmer 850 mb temperatures and a
generally warmer airmass moving into the region. The winds coming
off the terrain should also add another degree or two as the air
warms adiabatically.

Another compact low will approach the southern side of the Alaska
Peninsula from the North Pacific Thu. This will bring another
round of warm air, gusty winds, and rain to Bristol Bay by Thu
afternoon.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3...Tue through
Thu)...The parent low will continue to dominate the area with
showers and gusty winds. The sustained gale-force winds will hang
on both east and west of the low this morning. But by this
afternoon, winds should diminish to only small-craft level.
Instability associated with the low still looks promising. So we
have opted to continue the isolated thunderstorms over a fair
amount of the marine areas through today.

Tonight into Wed, it will just be a matter of watching the low
slide back to the west as it fills. As the low weakens, showers
and winds should continue to diminish. By late Wed, another
deepening low will move into the picture from the North Pacific.
This system will slide from south to north towards the Eastern
Aleutians. It will push a gale-force warm front across the Alaska
Peninsula and into the Eastern Bering by Thu. Expect another
round of moderate rain and warm air with this system.

&&

.LONG TERM FORECAST (Days 3 through 7: Thursday through Monday)...

Starting off the long-term forecast on Thursday, a dying front
along the north Gulf Coast will continue spreading heavy rains and
mountain snows to areas of western Prince William Sound and the
southern Kenai Peninsula coast, including Seward. While the
timing of an upper level wave is variable between the models,
there is a possibility that for a brief time that additional lift
may cause some precipitation activity to reach the lee of the
mountains into Anchorage and the Matanuska Valley. Meanwhile, an
area of low pressure will be approaching the southern Alaska
Peninsula Thursday morning, which in turn will make for windy,
rainy conditions from about Dutch Harbor eastward. The
precipitation will spread northward through the morning into
Bristol Bay, and reach the Kuskokwim Delta late in the day, as
the parent low turns westward. Interior Southwest Alaska shouldn't
see too much precipitation activity from this particular low
outside of some upslope showers along the Kuskokwim Mountains. On
Thursday night, a northward moving jet streak pushing into the
Gulf will cause a front and associated triple-point low to develop
over the western Gulf. This feature will ramp up the rainfall
rates across much of the north Gulf Coast as the night goes on.
The warm air mass over much of Southcentral Thursday will be
replaced by a somewhat cooler one, though the warmer air will
stick around longer in the Susitna Valley and Copper River Basin,
as the front should die out before reaching those areas.

On Friday, the front moves into the coast during the day, making
for continuous rain all along the north Gulf Coast for much of the
day, especially the morning. The front will reinvigorate the gap
winds through Turnagain Arm and the Knik River Valley once again.
There will be some potential once again Friday afternoon of
additional shower activity breaching over the mountains into
Anchorage and the Matanuska Valley. Meanwhile, the low over the
Bering largely stalls there, which will keep Southwest Alaska on
the wet side of things as well with persistent onshore flow,
though it should not be nearly as heavy or continuous as points
further east. A renewed push of cooler air with the front on
Friday will displace all of the warmer air eastward into Canada by
Friday night. Lingering shower activity will persist along the
north Gulf coast Friday night, but will not be nearly as heavy as
earlier in the day. Lowering freezing levels may allow some
accumulations through the passes as well. With the low lingering
just off the Bering coast, Southwest Alaska will also see
scattered to numerous showers continue through the overnight.

The instability brought on by the cooler air mass over the area
Saturday will keep showers going locally throughout southern
Alaska. The area should be between large-scale systems by then so
nothing organized is expected. A piece of upper level energy will
move over Kodiak Island Saturday evening, but a lack of any
temperature gradients in the area should preclude that feature
from generating much of its own precipitation. Sunday should be
the drier of the two weekend days with less lift due to building
upper ridging expected.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Warning 155.
MARINE...Gale 130 131 139 174 175 176 177.

&&

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
LONG TERM...JW