Flood and High Wind in Alaska, September 21 to 28, 2016

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
500 PM AKDT TUE SEP 20 2016

.ANALYSIS AND UPPER LEVELS...
Two high amplitude features dominate the weather across the region
with a ridge currently encompassing the Gulf of Alaska and the
mainland, and a low over the Bering. The powerful front associated
with the Bering low is currently moving into western Gulf of
Alaska towards Kodiak Island. Due to the strong pressure gradient
associated with the front, gap winds are already occurring across
the Kenai Peninsula, Anchorage Bowl and Matanuska Valley ahead of
the front. An upper level jet stretching between the ridge and low
is currently supporting southwest flow at mid levels and is
helping to support weak mountain waves and gusty winds across the
Anchorage Bowl this morning. The jet is also helping to transport
copious amounts of moisture northwards and is expected to support
heavy rainfall at times across the Western Kenai beginning this
evening.

&&

.MODEL DISCUSSION...
While models have locked on well to a solution for the eastward
progression of the low and relative stubbornness of the ridge,
models still disagree slightly on the progression of the associated
front across the gulf of alaska overnight. These differences are
fairly small, but enough to change how strong winds are across
local gaps tonight, and how far down the local mountainside winds
will travel. Models also appear to be lagging the event slightly,
so there is a chance that strong winds occur faster than expected.
For Wednesday night into Thursday the question remains how quickly
the low level coastal jet intensifies along the northern Gulf and
how far it extends into Prince William Sound.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Strong winds and rain develop rapidly this evening as the Bering
storm system lifts a triple point low just southeast of Kodiak
Island north toward the Cook Inlet. The impacts from this storm
shift toward the east into Thursday as another surface low develops
in the Pacific and races toward the northeast Gulf coast.

The main concerns with this afternoon's forecast package were the
various wind and flood products. With the front moving across the
area into Wednesday, flood advisories are now in effect for both
Tuxedni Bay along the southern Inlet and for streams and rivers of
the Seward area. The high wind warnings that were previously issued
were kept intact for portions of Anchorage Bowl, Turnagain Arm,
Portage Valley, and Cordova area. However, models are starting to
trend toward a faster push of the front into the interior, which
may taper winds down earlier than expected. The exception to this
is Cordova where strong winds will linger. The pressure gradient
along the Northeast Gulf will be re-enforced by the next surface
low moving up from the Pacific.

Heading into Thursday, the Bering storm sends a strong shortwave
from the western Gulf to the northeast Gulf which then lifts the
surface trough towards the the central interior. This spreads rain
across the Copper River and back toward Prince William Sound and
then to the areas further west and north. The wave will also
advect a colder air mass into the Gulf bringing higher instability
along the newly developing Pacific low. Added the mention for
isolated thunderstorms across the Gulf waters as the shortwave
moves to the coast.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A strong front currently stalled over the Southwest mainland
today will quickly hasten it's pace eastward as a triple point
low ramps up south of the Alaska Peninsula this evening and pushes
the front into the Gulf region. Forecast concerns remain focused
around the rapid development of the above mentioned triple point
low, which models continue to show quickly intensifying through
later this evening as it rides northward along the front into
interior Bristol Bay. In addition to enhancing moderate to heavy
rainfall over much of the area, this feature will also bring the
potential for strong southeast winds, especially through Kamishak
Pass and over parts of the Bristol Bay interior as the pressure
gradient rapidly tightens ahead of the system.

Forecast thinking with respect to these winds is largely unchanged
from the previous forecast packages, with winds peaking this
evening before quickly diminishing later tonight after the passage
of the low. Conditions will begin to improve by Wednesday morning
as the system quickly departs to the north, with broad onshore
flow ahead of the sprawling Bering low supporting a more benign
cloudy and showery regime through Thursday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The large vertically stacked low in the central Bering will
remain the dominant feature impacting the weather over the region
through Thursday, as it slowly fills and tracks southeastward to
the vicinity of Nunivak Island through Wednesday night and
approaches the Western Capes by Thursday. This will support
widespread shower activity and Small Craft Advisory level winds,
with a smaller area of embedded gales today on the south and west
side of the low center. A pool of cold air aloft associated with
the low center will also continue to enhance instability and bring
the chance for a few thunderstorms to the central Bering and
Pribilof Islands through tonight. The potential for thunderstorms
continues to spread south and eastward as the low approaches the
southwest coast, extending the potential for thunderstorms over
the Eastern Aleutians/Bering and Alaska Peninsula on Wednesday and
Thursday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning the extended period forecast Thursday evening,there
will be expansive upper troughing stretching from the western
Bering Sea to the southeast coast of Alaska. The remnants of a
strong low will be moving north toward the northeast Brooks
Range/North Slope leaving behind it a broad area of showers over
southern Alaska and the coast. On Friday, a digging arctic trough
will move into Southwest Alaska then move east through Southcentral
Sunday and Sunday night. This will be a weak low, but it will help
to organize precipitation along and west of the Alaska Range then
across most of Southcentral. Colder air behind this low along with
the development of shortwave ridging should allow for seasonably
cool temps to develop Sunday night and Monday.

Thereafter, all the global models continue to show some level of
continuity with the large scale pattern even though they differ
with the details. Sometime Monday, another weakening piece of
upper level potential vorticity will split off the polar low and
dig through the eastern Bering into the western Gulf of Alaska.
This west to northwest flow pattern will favor precipitation
remaining west of and along the Alaska Range. A progressive and
fast west to northwest flow pattern will persist into late week as
the model solutions show a noisier pattern as the flow becomes
lower amplitude. Currently, the main storm track will generally
remain over eastern Siberia into the far western Bering Sea where
the active Polar Jet looks to remain through the extended.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 101 125 135. Flood Advisory 125 161 171.
MARINE...Storm Warning 119 120 125 128 129 130 131 132 139 141.
         Gale Warning 121 127 136 137 138 150.

&&

SYNOPSIS AND MODEL DISCUSSION...EN
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...JA