Flood in Alaska, May 11 to 14, 2018

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 PM AKDT Thu May 10 2018

.ANALYSIS AND UPPER LEVELS...
There is a "double barrel" upper level low, with one center over
the Alaska Peninsula and another over the central Aleutians.
Associated with the peninsula low is a strong 973 mb surface low
just nudging into western Bristol Bay from the south. Winds are
fairly strong across southern Alaska and the Bering/Aleutians, but
especially through gaps in the terrain such as Turnagain Arm.
There is a front over the northern Gulf, with a strong barrier jet
along the coast. There are also hints of a triple point low
trying to develop over the southern Gulf. Rain and showers cover
much of southern Alaska, with downslope drying over Cook Inlet,
the Copper River basin, and the Bristol Bay area.

&&

.MODEL DISCUSSION...
The numerical models are in fairly good agreement today. They do
diverge on the location of the low as it moves north along the
west coast. There is also some disagreement on the track of a low
that moves into the Gulf on Sunday. Both of these are fairly
minor, and the forecast confidence is a bit above normal.

&&

.AVIATION...
PANC...Breezy conditions will continue into the late evening then
diminish. Showers will then develop, but conditions are most
likely to remain VFR. Some MVFR conditions are possible,
especially during the day Friday, but should be brief and thus are
not included in the TAF. Winds will increase again late tonight,
and continue through the day, and should be fairly strong.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The front pushing through the Gulf is bringing strong easterly
winds along the north Gulf coast that are extending through Prince
William Sound and Turnagain Arm. These winds will increase through
the evening as the front approaches with areas in the northern
Gulf seeing 50 kt tonight. This system will also bring heavy rain
to the north Gulf coast but areas around the western Kenai
Peninsula through the Matanuska Valley will stay protected from
much of the rain with the strong easterly winds.

As the front moves inland early Friday morning winds will weaken
and shift to the south. This will allow for rain to move up Cook
Inlet and spread from the western Kenai through Anchorage and into
the Matanuska and Susitna Valleys by Friday afternoon. Southerly
winds and rain showers will then continue, but diminish, through
Saturday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Another round of wet and cloudy weather is on deck this forecast
period. At the surface there is an occluded front south of the
AKPEN. Both the radars at Bethel (PABC) and King Salmon (PAKC)
have been active today. As moisture continues to get wrapped
around the surface low it will directly move across portions of
southwest AK. Once the surface low move north of the AKPEN it will
move into Bristol Bay and then Hooper Bay and then towards Norton
Sound. That type of low track is going to keep pumping moisture
into southwest Alaska. Anticipate wet weather tonight, tomorrow
and Saturday.

&&

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

There is an occluded front south of the AKPEN. The latest
ASCAT/advanced scatterometer pass has a swath of gales south of
Chignik and the surface winds across the Bering are predominately
out of the north. Over Bristol Bay, the winds are definitely out
of the east. Aloft there is a broad area of cyclonic flow across
the Bering that extends south of the Aleutians. There are two 500
MB lows along the Chain, one near Atka and the other near Nelson
Lagoon. Expecting the low over Atka to move south while the low
over Nelson Lagoon will move north. The weather will generally be
unsettled and wet along the Eastern Aleutians and the Eastern
Bering tonight and tomorrow. Weak ridge will build in from the
west Friday and Saturday.

&&

.LONG TERM FORECAST (Days 3 through 7: Sunday through Thursday)...
Thinking is largely unchanged for the extended portion of the
forecast as diffuse longwave troughing holds over the eastern
Bering and promotes a generally cloudy and showery regime across
the southern half of the mainland, particularly for Southwest
Alaska. Guidance has held steady in the last 24 hours with a low
pressure system tracking quickly through the eastern Gulf on
Sunday, which would keep the bulk of the significant impacts east
of Prince William Sound. Weak ridging looks to build over the
eastern half of the Gulf in the wake of that system, which would
perhaps help to keep the Southcentral region a little bit drier
than Southwest for some stretches next week. Despite that, the
threat for some shower activity would remain possible through much
of the week as the area remains in southerly flow on the western
periphery of the ridge. Meanwhile, guidance has trended slightly
farther south with the next low passing south of the Aleutians in
the first half of next week, which would tend to keep conditions
a little more benign across the chain and the Bering Sea in
general. The front associated with that low then looks to move
north and approach the Akpen by Wednesday and then potentially
start to impact Southwest AK and the western Gulf by Thursday,
though details are still a little more uncertain getting into that
time period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 119.
  Gale Warning 120 125 128 130 160 165 180.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
LONG TERM...CB