Small Craft Advisory 740, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
507 AM AKDT SAT SEP 17 2016

.ANALYSIS AND UPPER LEVELS...
The main low over the area is over the southwest coastline and is
tracking to the east. Shortwaves rotating around this low are
bringing showers to areas around southern Alaska. Out west ridging
is building in over the western Bering and is also tracking east
behind the low. The gradient between these two features combined
with some weak cold air advection is helping to bring strong
northwesterly winds to the eastern Bering.

&&

.MODEL DISCUSSION...
The models are in good agreement with the main synoptic features
through the mid term. This leads to a high confidence forecast for
the timing and positioning of these features.

&&

.AVIATION...
Isolated VCSH continues through early afternoon with prevailing VFR
conditions. Confidence in ceilings diminishes by late afternoon as
weak up-inlet cold advection along with decent instability and
moisture may combine to produce showers capable of reducing
ceilings to MVFR at times, mainly 01z to 09z. Northerly offshore
flow will allow for slow clearing trend to begin after 12z Sunday
morning.

&&

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

The last of a series of upper level disturbances is moving up
Cook Inlet and bringing showers to the region again today. This
disturbance will stall in the Inlet as the upper level low begins
to drop southeastward and into the Gulf of Alaska tonight.
The ridge building in behind it will bring offshore flow to
Southcentral Sunday and Monday before the next system moves in
quickly on Monday night or Tuesday. Clearing skies over
Southcentral Alaska Sunday night in particular will bring the chance
for below freezing temperatures in the areas that tend to run
colder.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Sat and Sun)...
An interesting set-up this morning as the upper-level low is
effectively pivoting around itself. This is evident on the Bethel
radar and on satellite. While instantaneous rainfall rates are not
all that impressive, the system is moving rather slowly. This is
allowing storm total estimates to climb up over 2.5" so far.
Thankfully, most of the rain is falling out over the Yukon Delta
and away from population centers. As another shot of energy
accelerates down the west side of this low and into the base of
the trough, it will serve to keep the low tracking towards Kodiak
Island by tonight. So the area of moderate rain will track right
along the west coast through the day before crashing into the
north side of the Alaska Peninsula overnight. It will bring some
gusty northwest winds with it as a ridge over the Bering starts to
push over the Alaska Peninsula and serves to tighten the surface
pressure gradient. The period of strongest winds looks to be from
late this evening through late morning Sun. Winds will accelerate
through some of the gaps of the Alaska Peninsula resulting in the
strongest gusts over marine zone 150 (west of Kodiak).

For Sun, a transient shortwave ridge will move through the area.
This should lead to a break from the precipitation, especially
with some dry northwest winds. But it will also bring enough
stability to set the stage for dense fog Sun morning. As the fog
burns off Sun afternoon, the remnant of a warm occlusion will just
arrive over the coast of the Y-K Delta. This will shift the flow
back to southwest and start to usher some milder air back in for
Mon.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Sat and
Sun)...
The Bering will be the first place to show signs of the pattern
shift that will start next week. But first, from the Central to
the Eastern Bering today, cool northwest flow will hang on for one
more day. This will once again result in some showers and gusty
winds. But the first in a series of fronts is already pushing
through the Western Bering. It is bringing some light rain and fog
to Shemya. But more importantly is bringing a shift to southerly
winds and much warmer subtropical air all the way to 60N. This
will set the stage for another developing low to phase west of the
Western Aleutians and bring a stronger front (with storm-force
gusts) into the area by this evening. The low will then begin a
slow eastward track across the Bering and push the front steadily
towards the west coast by early next week. This system will have
enough upper level support and low level thermal gradient to bring
a swath of gale-force winds with it. It will eventually be the
front that brings a change to much more active weather to Southern
Alaska next week.

&&

.LONG TERM FORECAST (Days 3 through 7)...

From previous discussion...

Beginning the long term period on Sunday evening, there will be a
rather vigorous low and associated warm front crossing from the
western Bering Sea eastward with a shortwave ridge ahead of it
moving into southern Alaska. With a digging shortwave across the
southeast Gulf of Alaska moving toward the Panhandle, this will
incite a weak offshore flow pattern with drying conditions across
the mainland.

The story quickly turns to the dramatic amplification of the
pattern as a northeast Pacific ridge amplifies ahead of the Bering
trough. This will allow for a deep southerly subtropical fetch to
form, and this will be aided by remnant tropical energy from
former Typhoon Meranti. There is high certainty in this general
pattern amplification, but there remains less certainty in the
details, and in these highly energetic patterns, the devil is in
the details. What is pretty certain is that a moderate to strong
gale force front will move into southwest Alaska Monday and Monday
night bringing moderate rainfall along with it. This will shift
east into Southcentral Tuesday and Tuesday night as the front
straddles the western Alaska Range. Orientation of the
trough/front is still unknown, but it is likely heavy rain will
begin enveloping most of southwest Alaska as well as Kodiak Island
and the Kenai coastline as subtropical energy reinvigorates the
front. There should be enough subtropical moisture to also spill
rain over into interior locations west of the Copper Basin, but
there will be a downslope shadow which will limit rainfall to much
lighter amounts. Rainfall total potential is impossible to nail
down at this juncture, but the possibility of 5-10" is possible
for locations along the gulf coast by Thursday. In terms of
stronger lows forming along the front, much uncertainty exists
here with the ECMWF showing a more weak and progressive frontal
system whereas the GFS and Canadian both spin-up powerful lows
which drive inland. At this juncture, it will be noted that the
potential exists for strong lows to impact the mainland, but for
now, an ensemble approach was favored which will highlight gales
moving into the northern Gulf of Alaska and eastern Bering Sea.
Should a stronger low become certain, powerful winds are possible
for inland locations sometime Tuesday or Wednesday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...150 155 165 175-178 180.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
AVIATION...CC
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
LONG TERM...JA