Small Craft Advisory 700, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 AM AKDT Thu Sep 17 2020

.ANALYSIS AND UPPER LEVELS...

A deep low has now made its way from the North Pacific across the
Alaska Peninsula and into Bristol Bay. A tap of Pacific moisture
that extends nearly to Hawaii is feeding into the low. This
moisture feed is boosting pockets of moderate rainfall extending
along the low's front, or roughly between the Y-K Delta and the
Prince William Sound. A dry slot curling into the low behind the
front is helping to clear out the rain rather abruptly for the
time being between Sand Point and King Salmon, but the break may
be short lived as moisture wraps into the head of the occlusion
just west of the surface low. Gale force winds are being generated
in spots between Sand Point and Kodiak Island where the low level
winds are mostly aligned with the storm's current heading. Knik
and Turnagain winds are gradually picking up as well, but they are
currently being driven down the Cook Inlet by the appreciable
north to south pressure gradient in place ahead of the approaching
system.

&&

.MODEL DISCUSSION...

Models are now in very good agreement with the track and strength
of the low as it merges with another weak low over the Bering
Strait and tracks inland just east of the Bering Coast. The
confidence in expected wind and precipitation impacts across the
region has greatly improved now that the system is ongoing. Still,
timing out precipitation end time and potential fog development
behind the front's passage could be tricky for Friday to Saturday
due to a nearly saturated air mass that may very well linger in
place without any obvious clearing mechanism behind the low.
Otherwise, synoptic agreement remains quite good for the current
model cycle through Saturday, and the forecast confidence remains
high through this period.

&&

.AVIATION...

PANC...Mainly VFR conditions are expected through most of the TAF
forecast period. However, periods of rain with low clouds may dip
ceilings below the 5000' threshold to MVFR and perhaps briefly
IFR/LIFR at times through Friday morning. A front will move
through Thursday afternoon with LLWS of 20 kts persisting until
the front moves through. Gusty Turnagain winds from the Southeast
should begin to move in behind the front and continue well into
Friday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today
through Saturday Night)...

A strong low moving north across Southwest Alaska today has a
leading front moving across Southcentral. The front is causing the
ongoing rain over nearly all of Southcentral except Kodiak Island.
Peak wind gusts as of this writing include 60 mph in Portage, 47
mph at McHugh Creek, and 35 mph both on the Hillside and up in the
Mat Valley. As the front lifts north today, strong winds will
persist along the Hillside, Turnagain Arm and Portage Valley. More
details on that are in the recently issued Special Weather
Statement. The winds will diminish this evening, though gusty
winds will also move into the Bowl and Palmer/Wasilla as well.
This is due to the front lifting northward, causing a southerly
push up Cook Inlet that will drive the Turnagain Arm and Knik
River Valley winds northward into the aforementioned populated
areas. The south to southwesterly winds will remain gusty through
the day on Friday as well in the Susitna Valley, Anchorage, and
along the Cook Inlet.

Heavy rain is also ongoing across many areas of Southcentral this
morning. As expected Seward has the greatest precipitation totals
so far, picking up nearly 0.7 inches of rain since yesterday
afternoon. The heavy rain will cause higher river levels in the
Seward area, and in the headwaters of the Yentna and Skwentna
Rivers in the Susitna Valley. More details on that are also in
the Special Weather Statement. The heavy rain will persist into
this afternoon in the aforementioned areas, then will gradually
diminish. The front is causing the heavy rain, but following
frontal passage, southwesterly onshore flow will allow upslope to
keep the rain going along the coastal Chugach mountains into
Saturday, though it is not expected to be particularly heavy by
then.

By Saturday, the upper level low over interior Alaska will
gradually drag a trough across Southcentral. This will lead to
widespread shower activity, especially over the mountains Saturday
afternoon during peak heating. However, even lower elevation areas
can't rule out a few showers. A colder air mass will move into
Southcentral Saturday night, which depending on the timing of a
strong upper level trough crossing southcentral, could be the next
chance for termination dust to occur on the Talkeetna and Chugach
Mountains.

&&

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

A 982 mb low pressure system is currently spinning just north of
Nelson Lagoon. This low is currently in the process of occluding
(when a cold front overtakes the warm front) this morning. As the
low occludes it will slow down as it continues moving to the west
and northwest. Areas across the Yukon-Kuskokwim Delta will be
raining a good portion of the day as this region will be in the
deformation band of the storm (where the precipitation pivots)
keeping the rain over one area for a longer time period especially
between Bethel and Hooper Bay. A band of rain will continue to
affect portions of the Middle Kuskokwim Valley this morning before
a dry slot moves into the region late morning into early
afternoon as the front continues northward. This dry slot is
currently bringing dry and relatively clear sky to King Salmon
south to Egegik early this morning. However, this will change
later on this morning around 9-11 am as the backside of the system
brings lower ceilings and rain showers to the region for the
remainder of the day. On and off rain showers will then be the
main theme for the forecast package as different pieces of upper
level energy rotate through the area.

Winds will also be breezy to windy today across Southwest Alaska
and the AKPEN due to the tight pressure gradient created with this
storm system. Winds will be strongest today as the system will be
in its mature stage. Winds slowly begin to subside Friday as the
system moves inland and weakens and the pressure gradient over the
area lessens.

Another fast moving system is expected to bring a quick period of
rain to the AKPEN north to around King Salmon Friday afternoon
through Saturday afternoon before quickly moving east into the
western Gulf of Alaska.

&&

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

A 982 mb low pressure system across the eastern Bering Sea is
creating a tight pressure gradient with higher pressure off to the
southwest across the North Pacific. This is creating breezy
conditions mainly with a northerly to northwesterly wind
component.

A fast-moving system is expected to quickly skirt across the
southern Bering and much of the Aleutian Chain beginning this
afternoon and continuing through Friday evening before moving
eastward into the Gulf of Alaska. This will bring a quick burst of
rain to much of the Chain with small craft advisory winds to the
southern marine zones south of the Chain.

The next system will begin to move into the western Aleutian Chain
and western Bering Friday evening. A 120-140kt jet is expected to
be racing across the North Pacific Friday evening along a
baroclinic zone with cold air at 850 mb to the north of -4 to -6
Celsius and 8-10 degrees Celsius along and south of the jet
stream. This will result in rapid cyclogenesis Friday night into
Saturday as the low moves into the Bering. At this time, this is
expected to be a gale force low with storm force gusts though a
storm force low with sustained storm force winds isn't off the
table at this time. This was held off on for now as the GFS
solution was the outlier and the strongest model as of now. This
system will also tap moisture from the tropics and bring a fairly
decent rain to much of the Chain Friday night through Saturday.

&&

.MARINE (Days 3 through 5: Saturday through Monday)...

By Saturday, a strong low pushes into the Bering Sea, bringing
gale force winds to the coastal areas of the Western/Central
Aleutian Chain and small craft elsewhere. Models are in
disagreement on the exact strength and location of this low,
but do agree on the general track as it travels eastward toward
Southwest Alaska. The Canadian tracks slightly more northeast
toward Chignik, whereas the GFS and EC are more in agreement
tracking it more toward Dutch Harbor. It looks like it then
travels to the Gulf by Monday, but there is uncertainty on both
the specific track and the strength of the winds over the Gulf.

&&

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

A strong low pushes into the Bering Sea on Sunday. Widespread
rainfall and gale force winds along the Aleutian Chain are
expected in association with this low. A low amplitude ridge out
ahead of the low moves over the Southwest region at this same
time, providing somewhat drier conditions, although the
possibility of rain can't be ruled out completely. This ridge
moves with the low and will be over Southcentral by Monday. By
Tuesday, the low will travel into the Southern Gulf, with the
heaviest rainfall confined to Kodiak Island and the Coastal areas.
Gale force winds are also possible for the Southcentral Coast and
Northern Gulf.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 119 125 130 131 132 138 139 150 155 160
165 180.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AS
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...KM

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