Small Craft Advisory 619, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
405 AM AKDT Sun Aug 12 2018

.ANALYSIS AND UPPER LEVELS...
The main weather feature today and tomorrow is the strong low
north of False Pass and King Cove along with the deep fetch of
tropical moisture associated with it. The surface low is in a data
sparse area but the models have all pinged into a minimum
pressure near 976 mb. This extra-tropical cyclone has strong jet
support and the wind field at the surface is asymmetrical. The
latest ASCAT/advanced scatterometer pass has detected areas of
gale force winds both south and north of the Aleutian chain.

&&

.MODEL DISCUSSION...
The models have initialized well this morning comparing surface
observations and the latest ASCAT/advanced scatterometer data when
compared to the analogous time in the 00z runs of the GFS ECMWF
and NAM. Models are in good agreement through Monday with the
synoptic pattern over Alaska, the Bering and the North Pacific.
Very confident that a wet pattern will impact Southwest Alaska and
portions of Southcentral.

&&

.AVIATION...
PANC...VFR conditions may give way to MVFR ceilings. Winds will
increase today. A tight pressure gradient will set up and result
in gusty winds at PANC. The max gust at this time for PANC is 37
knots for this afternoon, but it is possible that amendments will
be needed for PANC today. Its an active pattern, and the TAFs have
been very challenging as the remnants of typhoon Shanshan have
moved across the region.

&&.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A wet and windy day is in store for Southcentral Alaska as a major
storm over the eastern bering Sea tracks to Southwest Alaska, with
a frontal system marching eastward across the Gulf and Southcentral.
The storm is moving faster than originally thought, so have
adjusted forecast to move up timing of peak winds by around
3 hours. Some of the strongest winds will be in the Anchorage
area, a result of both a strong Turnagain Arm Jet and overall
strong low level winds. Forecast strength of winds still looks on
track, strong for this time of year, but not to the point of
damaging.

Heavy rain will be favored today along south to east facing
slopes of the Prince William Sound and the western Susitna Valley.
As the frontal system exits to the east tonight deep south-
southwest flow will develop with a series of upper level short-
waves tracking underneath the low center sitting over Southwest
Alaska. This change in flow will focus rain across the Mat-Su
Valleys, Talkeetna Mountains and Denali Highway corridor, as well
as northern to eastern Prince William Sound. This rain will
persist across these areas through about Tuesday. With many rivers
and streams in the Susitna River Basin already elevated from
flooding last week, there is a good chance some of these may once
again experience some minor flooding as we head through the next
few days.

&&

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

Gusty winds and precipitation will continue across the Alaska
Peninsula and Southwest today as a strong 977 mb low increases
its forward momentum and races across the eastern Bering. The
associated occluded front will quickly traverse the area during
the day Sunday with the low moving onshore near Kipnuk by Sunday
evening. A slight risk of thunderstorms remains within the middle
Kuskokwim Valley Sunday afternoon ahead of the front. Gusty
southerly winds ranging from 40 to 60 mph out ahead of the low
will shift to the northwest by Sunday evening as the low tracks
onshore and persist through Monday evening before diminishing in
intensity. High surf and coastal erosion remains a threat along
the shores of Kuskokwim and Bristol Bays Sunday evening through
Monday evening with water levels 2 to 4 feet above astronomical
high tide expected. A Special Weather Statement has been issued
detailing this event. A secondary trough diving south through the
eastern Bering will bring another round of precipitation across
the southwest mainland and AKPEN Monday night into Tuesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

THe big story remains the strong 977 mb low positioned 100 nm
southwest of St. George this morning. This low, which brought
steady rainfall and southerly wind gusts around 65 mph to the
central and eastern Aleutians through the early morning hours will
continue to quickly track through the eastern Bering today. As it
does, rainfall will diminish in coverage and intensity and winds
will shift from the south to the west and northwest. Strong winds
on the back side of the departing low will keep continue to
produce gusts of 50 to 60 mph from Dutch Harbor east and over the
Pribilofs through the evening hours before slowly tapering off
from west to east. Lingering wind gusts tomorrow will remain
confined to the far eastern Bering as the low weakens over the
Kuskokwim Delta. A moist , northwesterly flow over the eastern
Bering will continue through Tuesday as a weak trough moves in
from the north. Farther west, high pressure will begin to build
over the western and central Aleutians and remain in place through
Tuesday. Stable air underneath the ridge will keep low stratus
and areas of fog over the the western Bering and Aleutians. The
next low begins to trek toward the western Aleutians on Tuesday.

&&

.LONG TERM FORECAST (Days 4 through 7: Wednesday through Saturday)...

The long-term forecast is fraught with uncertainty straight from
Wednesday on. In general, a trough or upper level low will be over
southern Alaska to start out the forecast, this should promote
isolated to scattered shower development during the day Wednesday,
especially through the mountains. The GFS opens the upper level
low into a trough quickly, the EC maintains a much deeper upper
level low that remains nearly stationary over Bristol Bay into
Friday, while the GEM moves the low into the northern Panhandle by
Thursday afternoon. Such wildly differing solutions mean
speculating on many specifics anywhere in the long-term would be
pointless, as they're highly likely to change in the coming days.

A couple broad generalities that can be surmised are that there
are no deep, big systems producing widespread rain anywhere in
southern Alaska until possibly Saturday. Thus, it appears likely
that most areas in both Southwest Alaska and Southcentral see a
generally dry period through Friday. The persistent longwave
trough over Southern Alaska should mean daily afternoon showers,
especially in the mountains, through Friday. Those recreating
outdoors should plan on the potential for brief showers. Winds
should be generally light through the period, and temperatures
will remain seasonable as well.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...STORM 155 160 165. GALE 351 352 130 131 132 137 138 139
140 150 170 172 179 180 181 185 412 414.

&&

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

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1235 PM AKDT Sun Aug 12 2018

AKZ145-132000-
Susitna Valley-
Including the cities of Talkeetna, Willow, and Cantwell
1235 PM AKDT Sun Aug 12 2018

...Streams Along the Parks Highway Expected to Be Near Bankfull
Through Monday Morning...

Recent heavy rains have elevated water levels to near bankfull on
many streams along the Parks highway. The Talkeetna River, Willow
Creek, Montana Creek, and other streams flowing out of the
Talkeetna Mountains are particularly likely to have Bankfull
conditions. As heavier rains taper off today, expect water levels
to start to drop by late Monday morning.

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
252 PM AKDT Sun Aug 12 2018

AKZ155-161-131300-
Kuskokwim Delta-Bristol Bay-
Including the cities of Bethel, Hooper Bay, Nunivak Island,
King Salmon, Dillingham, Naknek, and Pilot Point
252 PM AKDT Sun Aug 12 2018

...HIGH SURF AND COASTAL EROSION POSSIBLE ALONG KUSKOKWIM BAY AND
BRISTOL BAY COASTS TONIGHT THROUGH MONDAY NIGHT...

High surf and coastal erosion is possible along the Kuskokwim Bay
and Bristol Bay coastlines starting tonight night and continuing
through Monday night.

The storm system from the remnants of former-typhoon Shanshan has
moved over Southwest mainland Alaska and will continue to weaken
as it moves slowly inland. As the system tracks inland, High-end
gale-force winds along the back end of the system will bring high
surf and a small storm surge to Kuskokwim and Bristol Bays
beginning tonight and persisting into Monday night.

The storm surge is expected to reach up to 2 ft above the
grassline and high surf will break on top of this surge. This
brings the potential for erosion on some beaches.

Persons with property near the shore should take precautions to
move it farther away from the water if possible.

Keep up with the latest weather forecasts at: www.weather.gov/afc