Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 161309
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
509 AM AKDT Tue Oct 16 2018

.ANALYSIS AND UPPER LEVELS...
A powerful storm force low is moving inland along the North Gulf
Coast this morning. Satellite imagery depicts the broad baroclinic
leaf structure over the state, ahead of a negative tilt shortwave
trough. A coupled upper level streak aloft is also playing a role
in the rather impressive precipitation rates along the coast.
MIMIC TPW satellite imagery is showing a well developed
atmospheric river which is tapping 25N with precipitable water
values of 200-250 percent of normal. Buoy observations in Prince
William Sound are supporting storm force winds with gusts to near
60 knots while 24 hour rain rates are around 4" in places such as
Cordova. In other words, this is a heckuva impressive storm
system.

Over the Bering Sea, an occluded and stacked low is acting as the
"anchor" low for the fierce low farther east. As a whole, the
general low circulation stretches from the far islands along the
Aleutian Chain to the Panhandle of Alaska--over 2000 miles.

&&

.MODEL DISCUSSION...
The numerical models are in excellent agreement with the pattern
through Wednesday night. Large discrepancies begin to emerge
Thursday as a powerful storm force low moves into the Gulf of
Alaska from the Central Pacific Ocean, an area lacking in
observations and a pattern which is hard for models to capture.
The ECMWF is the weakest and farthest east while the American
models are depicting storm force winds over most of the Gulf. For
now, a blend of all models will be favored until better continuity
develops.

&&

.AVIATION...
PANC...Moderate-heavy rain will taper off to intermittent showers
this morning as the front moves inland, which will bring brief
periods of MVFR conditions as heavier rain bands push through.
Southeast winds will move into the terminal in conjunction before
slowly diminishing overnight. Otherwise, VFR conditions will
prevail.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Today through Thursday)...
The storm force front that is making landfall over the North Gulf
Coast will bring continued rains to much of the Gulf coast and
into Southcentral this morning. The heaviest rainfall will occur
along the front over the Cordova area, as well as along the
western edge of the low in the Eastern Kenai Peninsula where
1-1.5" of additional rainfall is possible for areas including
Portage and Seward. Further north over Southcentral (including the
Anchorage Bowl) rain will become heavier this morning as the Gulf
low approaches from the south. This will also bring gusty winds
from Turnagain Arm eastward into the Copper River as a broad and
tight pressure gradient throughout the Chugach. With no down inlet
flow to protect the area, these winds will spread into south-west
Anchorage as well as the Matanuska Valley, producing gusts to
around 30 mph.

While most of the widespread and heavy rain will depart the area
by late this afternoon as the Gulf low and associated front
dissipate, a generally cloudy and showery pattern will continue through
Thursday as little movement is expected with the persistent upper
trough over the Eastern Bering. This will likely result in
temperatures remaining above normal (especially overnight) with
little chance of widespread freezing temperatures to the Anchorage
Bowl and Matanuska Valley as is common for this time of the year.
As a result the Anchorage airport will likely set a new record
for the latest first freeze, which currently stands at today set
in 1969.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Today through Thursday)...
The upper level low in the eastern Bering will continue to bring
unsettled weather to Southwest Alaska through midweek. Showers
currently mainly along the coast will rotate further inland and
increase in coverage through this afternoon, remaining abundant
through Wednesday as a series of additional reinforcing shortwaves
spin through. The offshore low will start tracking slowly back to
the south Wednesday night and Thursday, allowing shower coverage
to gradually diminish. A front associated with a strong low
tracking north into the Gulf will begin spreading precipitation
back into the area from the east across the Alaska and Aleutian
Range around Thursday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Today through Thursday)...
The large eastern Bering vertically stacked low currently
centered roughly half way between the Pribilof Islands and Cape
Newenham will slowly fill as it remains in that general location
through tonight. The low will begin tracking slowly back to the
south on Wednesday, crossing the Alaska Peninsula Thursday morning
before weakening into a remnant trough. Upper level ridging
building in over the western Aleutians tonight and Wednesday will
continue east to reach the central Bering Wednesday night and
Thursday. The next incoming frontal system will reach the western
Aleutians Thursday.

&&

.LONG TERM FORECAST (Friday through Tuesday)...

At the onset of the forecast period, the synoptic pattern
features a low near Kenai, a second low south of the Aleutian
chain and a low near Kamchatka. Anticipate a very wet and progressive
pattern. There is good general consensus between the
GFS/EC/Canadian but the best performing model is thought to be the
GFS for this forecast period. Expect back to back lows to bring a
hefty amount of precipitation to Kodiak, Prince William Sound and
the Anchorage Bowl. Even though the third week of October is
considered to be part of the cold season for Alaska, the
temperatures with both storms will remain well above freezing in
the low elevations. Higher elevations on the Kenai and other
portions of Southcentral can expect a fresh blanket of snow this
forecast period but significant accumulations not expected. The
entire forecast period, the GFS has above normal temperatures at
500 mb from Kenai to the Yukon Territory to Juneau.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 121.
MARINE...Gale 119 125 173-177 185.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...PJS



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