Small Craft Advisory 629, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
503 AM AKDT Sat Aug 31 2019

.ANALYSIS AND UPPER LEVELS...
A front that moved through Southcentral overnight brought
widespread light rain with rainfall amounts averaging under
0.05" for inland location and 0.20" along the coasts. Kodiak was
one of the higher reporting stations with 0.93". The leading edge
of the front is now stretching from the northern gulf to the
Susitna Valley. Behind the front across the southwest...winds are
light and patchy fog is developing across the region.

To the west...a mature low pressure system is maintaining its
strength as it tracks east along the Central Aleutians.
Scatterometer winds from satellite showed 50 kt northerly winds
just south of Adak early this morning. Gusty southerly flow is
increasing across much of the southern Bering with approach of
this storm system. On the backside of the low...gusty north flow
is noted along the western Bering/Aleutians.

&&

.MODEL DISCUSSION...
Models are in good agreement through the beginning of the next
week. Only minor adjustments were made during the morning package
to increase winds with only minor edits made for precipitation.
The main concerns continue to focus around strong mountain gap
flow and rain amounts with this next storm system.

&&

.AVIATION...
PANC...VFR conditions are expected through Sunday afternoon. Winds
will generally be light in the wake of the front. The main
concern for winds will be along the next front that moves through
late Sunday. A low level jet increases by mid Sunday morning
through the Turnagain Arm...and will be monitored as wind shear
may need to be added.

&&

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

The front and upper trough lift north across Southcentral today
with rain tapering off to showers as the front passes north and
east. A developing storm center south of the central Aleutians
this morning intensifies as it moves to the coastal Southwest
Alaska Sunday evening. Its associated occluded front moves over
the western Gulf of Alaska late tonight then moves stretches from
the Susitna Valley, across the Kenai Peninsula, to the southern
Gulf of Alaska Sunday afternoon. The front then weakens as it
heads to the Copper River Alaska/eastern Gulf Sunday night. Strong
and gusty winds and rain will accompany this front. Strong
southeast winds in the low to mid levels (925mb- 850mb) will keep
the brunt of rain confined to the coasts and upslope areas over
western Cook Inlet and western Susitna Valley. Areas in close
proximity and to the lee of the Kenai, Chugach, and Talkeetna
Mountains will receive very little rain through Sunday. These
areas will likely see an up tick in rain potential on Monday as an
upper vorticity trough moves into the area and the low-mid level
flow shifts to a more southerly direction.

The second major impact of this system will be winds. Southerly
gales are expected to develop over the Gulf along the leading
edge of the front as it moves eastward tonight/Sunday. Strong
pressure gradients will develop over much of Southcentral tonight
through Sunday night the as the low and front progress eastward.
This will give rise to very strong Gap flows, especially along
Turnagain arm and through Portage Valley. Although not as strong,
winds will be fairly brisk across the along the Copper River and
the Knik Valley. These gap flows will remain fairly strong through
Monday morning before slowly diminishing.

&&

.FIRE WEATHER...

The current system lifting northward across the southern mainland
will likely bring wetting rains to many of the active fires today.

The next storm will be much larger and stronger. Gusty east to
southeast winds will be the big story across the Bristol Bay area
tonight and Sunday and across the Kenai Peninsula Sunday
through Sunday night. Winds will remain light across the Susitna
Valley corridor. Rainfall will depend heavily on proximity to
mountains as downslope flow plays a key role with this system.
The Swan Lake, McKinley, and Deshka fires will all see fairly
strong downslope flow and will likely remain mostly dry with
frontal passage. Bristol Bay has a better chance of seeing some
rain.

Behind the front, flow will shift to south to southwest on
Monday. The combination of this shift in flow and upper-level
short-waves aloft will bring a good chance of wetting rains to all
of the aforementioned areas.

&&

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

Clouds and scattered showers will linger across the Kuskokwim
Valley this morning as a trough lifts north. Behind this feature,
a weak onshore flow and diminishing cloud cover has allowed for
areas of fog to develop across Bristol Bay and the Kuskokwim Delta.
These areas of fog will dissipate around mid-morning prior to the
next frontal system advancing toward the Southwest coast. This
front is attached to a deep 968 mb low south of Adak, and will
signal an abrupt shift in the weather pattern as it brings
widespread rain and strong southeasterly winds to the region
beginning this afternoon. This front will move inland tonight and
lift north across the interior by Sunday as the main low moves
just south of Nunivak Island. As the low lifts north, the strong
southeasterly winds will shift to the west, allowing showers to
continue across the region through early next week.

Coastal impacts will be a growing concern over the next several
days as this system moves slowly northward. Prolonged
southeasterly winds of 35 to 45 mph along the coast this afternoon
through Sunday night could increase waves and push water up along
the northern coasts of Bristol and Kuskokwim Bays. Vulnerable
villages along these areas could experience periods of coastal
erosion and flooding, especially during high tides. Likewise,
areas along the southern and eastern shores of both bays could
experience periods of coastal erosion and flooding Monday into
Tuesday as the low lifts north and a prolonged fetch of westerly
winds of 25 to 35 mph impacts the Southwestern coast.

&&

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

A deep 968 mb low spinning south of Adak, and its associated
occluded front, continue to produce a wide swath of gale-force
winds across the central and western Aleutians as well as a small
area of storm-force winds wrapping around the back side of the
low. As this low crosses into the Bering tonight, its front will
sweep across the eastern Bering with southeasterly winds in excess
of 40 kts and seas approaching 25 to 30 feet along the Pacific
side of the eastern Aleutians and AKPEN. Gale-force winds and seas
approaching 15 to 20 feet will continue into early next week
across the eastern half of the Aleutian Chain, as well as the
Pribilofs and eastern Bering, as the low begins to weak and lift
north of Nunivak Island.

&&

.MARINE (Days 3 through 5: Sunday through Tuesday)...

The most significant weather feature continues to be the broad
gale force low in the Bering Sea on Sunday and Monday. The track
of this low is the same as it has been, heading northeastward
across the Bering before drifting northward to the Arctic. The
gale force winds on the southside of the low are slightly further
north and east of yesterday's forecast, but a wide swath continues
to cover the Eastern Bering and down through Unimak Pass on
Sunday. Monday the winds will diminish and by Tuesday winds are
expected to be westerly and sub-gale force.

The front associated with this low is still expected to bring
gale force winds in the Barren Islands on Sunday. By Monday, the
winds will have weakened in the western Gulf, but a barrier jet
will bring minimum gales to the north Gulf Coast and into Prince
William Sound. These should rapidly diminish on Monday, leaving
Tuesday sub-gale force across the region.

&&

.LONG TERM FORECAST (Days 3 through 7: Sunday through
Friday)...

Sunday weather looks most interesting for Southcentral, with a
second wave moving up from the southwest. This looks like the best
chance for cross-barrier flow, so downsloping will limit
precipitation and bring gusty winds to the eastern sides of the
mountain ranges. By Sunday night and Monday morning, the upper
level flow will become more directly southerly, which will allow
for increasing chances of much-needed rain across Southcentral.
Copper River Basin will be the driest area in this flow, although
they should expect to see rain to their south and over the
Wrangell Mountains. Although not unseasonably strong, this will
be the first such storm to move our way this fall, so expect a
significant change in the weather.

West of the Alaska Range, the low is forecast to move northward
through the eastern Bering, towards the Arctic. This should bring
gusty southeasterly winds to the coastal areas and the Aleutian
Islands. Again, this isn't unseasonably strong, but a significant
change from what we have been seeing. Of more concern is the
prolonged westerly flow across the Bering beginning Monday and
ending Wednesday. Right now models are not indicating problems
with storm surge for the west coast, but this bears watching as we
head into early next week.

After Tuesday and Wednesday, models are in surprisingly good
agreement for the end of the week. The long wave trough remains in
place, which will keep an active weather pattern with low pressure
systems moving through the forecast area through the end of the
week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storms 176 177
 GALES...130 131 132 138 150 155 160 165 170-175 178-181 185 185
NONE. FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...LH

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
246 PM AKDT Sat Aug 31 2019

AKZ155-161-020100-
Kuskokwim Delta-Bristol Bay-
Including the cities of Bethel, Hooper Bay, Nunivak Island,
King Salmon, Dillingham, Naknek, and Pilot Point
246 PM AKDT Sat Aug 31 2019

...ELEVATED WATER LEVELS AND MINOR COASTAL EROSION POSSIBLE
MONDAY MORNING THROUGH TUESDAY NIGHT FOR GOODNEWS BAY AND
AREAS NORTH...

Sustained southwest winds 20 to 30 mph will begin Monday morning
along the Kuskokwim Delta coast. Winds will increase to near 35
mph by Monday afternoon and last through Tuesday night.

Expect elevated water levels of 1 to 4 feet above normal high
tide for Goodnews Bay and areas north. Increased wave heights may
also cause minor coastal erosion. Residents and those with
interests along the coast are encouraged to move property away
from the shore.