Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 210112
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
512 PM AKDT Fri Sep 20 2019

.ANALYSIS AND UPPER LEVELS...
This afternoons CIMSS Layer precipitable water satellite loops
shows a strong tropical feed of moisture into a low pressure
system that`s currently located in the northern Gulf. This storm
is also located in the base of a long wave trough that extends
from western Alaska into our neck of the woods.

Further west, a zonal flow has developed across the Bering Sea
and Aleutians, which an area of low pressure crossing northern
portions of the Bering, and another low poised to enter the
western Aleutians.

&&

.MODEL DISCUSSION...
The numerical guidance remains in good agreement through the next
48 hours, before starting to diverge for the start of next week.
In general, they all show another storm moving into the Gulf of
Alaska. However, the European (ECMWF) is slightly faster than the
remainder of the models. Earlier runs also had more discontinuity
in strength, track, and timing. The new datasets from the 12z run
have come into better agreement, but, there are still some
important differences.

In general, they (the models) agree on the track and strength of
a fairly strong storm as it crosses the Aleutians and into the
AKPEN. After this point is where things begin to diverge, as they
disagree on when a new low forms along the triple point, along
with its strength and track. This affects the location of where
the heaviest rainfall and stronger winds will be located. Overall,
the active weather pattern will continue.

&&

.AVIATION...
PANC...Rain will persist through about 18Z Saturday. While
visibility may be briefly reduced in heavier bands of rain, expect
VFR ceiling/vis to prevail for most of the time. Drier air will
then begin to filter in after 18Z Saturday, bringing an end to
the rain and allowing ceilings to rise.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Sunday)...
The wave train of lows with breaks in between continues for
Southcentral through this weekend. The low that caused all of
today`s rain is moving toward eastern Prince William Sound. The
center of the low is expected to loop back to the west tonight,
then will drift to the southeast through the day Saturday. As the
low retrogrades west across Prince William Sound tonight, light
rain with periods of moderate rain is expected to continue for
Anchorage, the Matanuska Valley, and points east across the Gulf
Coast and the Copper River Basin. Anchorage will be on the western
edge of the rain, so any slight eastward deviation in the track
of the low could result in Anchorage staying much drier, and as
such forecast confidence in the duration and where the heaviest
rains will set up tonight is low. It appears likely, however, that
the rain will remain nearly stationary with somewhat heavier rain
over East Anchorage, the Hillside, and Eagle River. This is
expected to help keep the river levels high, and the Urban and
Small Stream Flood Advisory for Anchorage continues as all that
water flows down the streams.

On Saturday, as the low turns back to the east, the rain will
diminish as the low weakens and will shift eastward with time. The
very slow movement of the low will make this a gradual process.
The further west you go, the more likely you`ll see some sunshine
later in the day Saturday. Mostly clear skies will move across the
Kenai Peninsula, Anchorage, and the Susitna Valley by Saturday
evening, with lingering clouds in the Matanuska Valley, Copper
River Basin, and points east. The mostly clear skies will be
associated with a cooler air mass, the coolest of the season so
far. Thus, those aforementioned areas where the skies clear will
be threatened with near freezing temperatures away from the water,
including the interior Kenai Peninsula, east Anchorage, and the
Susitna Valley. Temperatures were nudged upward a few degrees for
the Matanuska Valley and especially the Copper River Basin where
clouds are expected to remain for at least some portion of
Saturday night. By the predawn hours Sunday morning, even the
Matanuska Valley skies should be clearing.

A mix of sun and clouds is expected for the day Sunday for much of
Southcentral, with more clouds near the coast. The next strong low
to enter the Gulf will move in Sunday afternoon, with rain breaking
out over Kodiak Island by mid-afternoon, along with strong easterly
winds. The rain will move up the coast into Sunday evening and
through the overnight Sunday night. Currently it appears as though
the usual rain shadowing areas including the Western Kenai Peninsula,
Anchorage, and the Matanuska Valley will be largely rain shadowed
Sunday night with much less rain for those areas.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3; Tonight
through Monday)...
Scattered showers and clouds from King Salmon north along the
Alaska Range will diminish overnight as a trough to the east
weakens and a ridge builds over the region. The clearing
conditions and weakening northerly winds will again allow to
overnight lows to drop near the freezing mark. There is also the
potential for patchy fog overnight tonight into early tomorrow
morning, especially from Bristol Bay north to the middle and upper
Kuskokwim Valley. Clouds will be on the increase over the Y-K
Delta beginning Saturday as the next frontal system advances east
toward the coast. Widespread rain showers will move over the coast
by late morning, moving slowly inland through the day as the
front and its associated surface low weakens and lifts northeast
to Norton Sound. A deformation zone developing between this
feature and a deepening low over the central Aleutians will allow
precipitation to persist across Bristol Bay Saturday night into
Sunday. More widespread precipitation and gusty southeasterly
winds are then expected across Bristol Bay and the AKPEN late
Sunday as the low tracking just south of the Aleutian Chain pushes
an occluded front toward the southwest coast. The rain lingers
into Monday with the gusty winds turning to the northeast as the
low passes just south of the AKPEN and continues its trek east
into the western Gulf.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3;
Tonight through Monday)...
Westerly gales and widespread ares of rain showers will develop
overnight across the Northern Bering this evening as a low
tracking east deepens. Areas of gale-force winds and waves above
10 feet will continue through Saturday from Saint Matthew Island
and the Pribilofs east to Kuskokwim Bay before diminishing
Saturday night as the low weakens and lifts into Norton Sound.

Farther south, a ridge stretching across the eastern half of the
Aleutian Chain will continue to slide east as a developing north
Pacific low approaches the western half of the chain. By Saturday
night, this system reaches the central ALeutians with gales
developing along its occluded front. The surface low continues to
strengthen Sunday as its front sweeps toward Bristol Bay and the
AKPEN. widespread gales and areas of moderate to heavy rain are
likely along the front with northerly storm-force winds developing
along the backside of the low across the central Aleutians. This
area of storm-force winds moves east from Dutch Harbor to south of
Cold Bay by Sunday night as the low moves just south of the
AKPEN. The northwesterly winds the weaken to gales and eventually
small-craft winds on Monday as the low tracks into the western
Gulf.

&&

.MARINE (Days 3 through 5)...
An occluded storm force low will continue moving eastward into
the Gulf Monday. Broad north/northwest flow at sub-gale magnitude
will prevail over the Bering Sea, though gales are likely to
persist south of the AKPEN through Monday night, as the backside
of the projected low will be over the AKPEN/western Gulf. In the
Gulf of Alaska, there continues to be uncertainty regarding the
progression of the low Monday. The last several runs of the GFS
have spawned strong secondary lows in the Gulf. Though we`re not
buying into these mesoscale features just yet, there is moderate
confidence in gales somewhere in the Gulf Monday...but uncertainty
remains with location and duration. The low will exit to the east
Tuesday into Wednesday, and winds over the Gulf will diminish,
though stronger gap winds will likely persist a bit longer over
the Barren Islands. Meanwhile, a strong low will approach the
Bering. This will likely bring gales, if not storm force winds to
the western Aleutians/Bering by Tuesday, spreading to the central
Bering Wednesday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A relatively progressive pattern will continue as a low tracks
eastward into the Gulf Monday, bringing widespread precipitation
to the Gulf/North Gulf Coast Monday. Meanwhile, broad northwest
flow will persist over the Bering, bringing showers as cold air
passes over warmer waters. Brief upper ridging will develop over
the Bering in the Tuesday to Wednesday time frame, and as the Gulf
low exits to the east, conditions will trend drier. That said, a
jet streak along the eastern edge of the ridge will maintain
cloudy conditions and scattered showers over the southern mainland
Wednesday.

Looking farther west, a strong low will approach the western
Bering/Aleutians roughly in the Wednesday to Thursday timeframe.
This low will originate from Tropical Storm Tapah, which is
currently 300 miles east of Taiwan. As per usual, models are
struggling with the evolution of this low. Strong winds and a good
shot of rain are likely over the western and central Aleutians
Thursday into Friday. The wet pattern looks to extend into
Southcentral next weekend, but it`s too early to have much
confidence in details.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Urban and Small Stream Flood Advisory 101.
MARINE...Gale Warning 130 131 132 138 173 174 176 180 185.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...MM


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