Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 211225
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
425 AM AKDT WED SEP 21 2016
.ANALYSIS AND UPPER LEVELS...
A storm-force front moving through the western Gulf continues to
bring widespread clouds and rain to much of the Gulf Coast and
the mainland. Downslope prone areas such the Anchorage Bowl and
Matanuska Valley are remaining mostly dry from strong cross-barrier
flow. Ahead of this front, strengthening pressure rises are
beginning to bring near hurricane-force gap winds from the
Portage Valley into Turnagain Arm as well as increasing winds
along the higher elevations around the Anchorage Bowl. Just south
of the Gulf along the front, you can begin to see signs of a wave
developing as it moves northward with the jet. This development
will continue and help to keep active weather to the Gulf into
Thursday. Behind the front the weather quickly becomes much
quieter from Bristol Bay westward in a much drier air mass with
weak cold air advection. This has allowed skies to clear rapidly
from Kodiak Island into the Southwest Mainland with much lighter
winds. Over the Bering, cold air advection associated with a cold
upper low over the Bering is bringing widespread rain showers from
the Pribilofs into the Eastern Aleutians, which will continue to
spread eastward towards the southwest coast today.
As has been the case over the past few days with this system, the
models all have the same general synoptic-scale idea, but
struggle with some of the finer scale details that are having a
significant impact on forecast confidence. The biggest challenge
is coming from timing and track of the wave that is rapidly
developing south of the Gulf that will begin to move to near
Prince William Sound by this afternoon or evening, but there
remains significant spread in the exact track of it. As a result
changes were pretty minimal to the wind forecast through this
evening, especially near Prince William Sound where the largest
uncertainty in the forecast exists. Once this low and associated
front make landfall by Thursday afternoon, forecast confidence
begins to improve as we move into a much quieter pattern with
better model agreement.
PANC...Expect winds to begin to slowly subside through the morning
and early afternoon after frontal passage around 14z to 15z. VFR
cigs should predominate until this afternoon when a more
persistent rain and MVFR cigs may begin to take over and last well
into the overnight. Confidence is lower than usual beginning this
afternoon onward due to forecast uncertainty with low track into
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The main focus in the short term will continue to surround the
storm-force front and it`s associated winds and rain. High winds
will continue to increase through the early morning hours along
Turnagain Arm and the Anchorage Hillside before diminishing as the
front begins to weaken this afternoon. Downsloping will also limit
most of the rainfall from reaching the ground throughout the
Anchorage Bowl and Matanuska Valley until this afternoon, when
weakening cross-barrier flow should allow for steady rain down to
the surface. Another concern from this system surrounds how long
heavy rain will continue along the Gulf Coast, and especially for
the Seward area. Early morning radar data is indicating that the
back edge of the heaviest rainfall is racing to the east; however,
the heavy rain will likely redevelop in the late afternoon/early
evening as the next wave approaches from the Gulf. As is mentioned
in the model discussion above there are significant differences in
the timing of this wave, but there is general consensus that heavy
rain will redevelop to the west when the wave approaches the
northern Gulf. This wave will also have an impact on how long the
storm-force winds that are developing along the North Gulf
Coast/Prince William Sound will persist. Recent satellite trends
are indicating a very small break behind the front and the
developing low, so it appears that the gradient should remain
tight enough to keep at least gale-force winds through Wednesday
night, however this may continue to be as strong as storms as some
of the models are indicating.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The southwest mainland will shift to a much more benign pattern
today as the intense North Pacific wave riding along the front
into the Kuskokwim Valley departs to the north. This will yield
widespread cloudy and showery conditions over the area through
Thursday night in persistent onshore flow ahead of the sprawling
Bering low as it gradually approaches the coast. This will
generally keep the heaviest shower activity along the coast, with
a slight chance for a few thunderstorms along the immediate coast
of the Kuskokwim Delta this afternoon and spreading to the
Bristol Bay coastline on Thursday as the pool of cold air aloft
associated with the low center shifts eastward towards the
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The large low near Saint Matthew Island will remain the dominant
weather feature through Thursday night as it continues to fill and
make slow eastward progress towards the southwest coastline. This
will keep widespread showers and Small Craft Advisory-level winds
over much of the Bering for the next few days in broad cyclonic
flow around the low. There will also continue to be a slight
chance for thunderstorms over a large swath of the eastern Bering
in this timeframe as cold air aloft moving over the relatively
warm waters continues to generate modest instability.
.LONG TERM FORECAST (Days 3 through 7)...
Main forecast update from Friday and Friday night was to add the
mention of slight chance of thunderstorms to Gulf offshore and
outer coastal zones as the combination of colder air aloft
combined with extremely warm SSTs will make for unstable
conditions over these areas.
From previous discussion...
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.
PUBLIC...High Wind Warning...101 125 135.
Flood Advisory...125 161.
SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB