Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 180120
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
520 PM AKDT Wed Oct 17 2018

.ANALYSIS AND UPPER LEVELS...

A vertically stacked upper-level low spins over the eastern
Bering between Bristol Bay and St. Paul island. This low sits at
the base of a long-wave trough which encompasses much of the
eastern Bering and the western mainland coast. The jet stream, in
a similar fashion, dips down in a trough well south of the
eastern Aleutians. Due to weak upper level support, the low is
heading towards a weakening trend. Over the western Aleutians, a
weak ridging is occurring on the trailing edge of the trough. In
At the surface, an occluded front associated with the low in the
Bering continues to push on land over the western mainland with
convective style showers pupping up over the lower Kuskokwim
Valley and the AKPEN. A surface short wave embedded along the
leading edge of the trough continues to bring some rainfall to
Southcentral especially along Prince William Sound but it also is
moving into the MAT-SU. As weak low pressure persists through the
Gulf, Southcentral maintains a dense cloud deck while throughout
the offshore areas in the Gulf, patchy skies are dominate.

&&

.MODEL DISCUSSION...
Models have all now coalesced around the GFS solution with the
strong low moving through the Gulf of Alaska on Thursday night and
Friday. This is good news as the GFS has been the preferred
solution for this storm for the past few days. After that, the
models do not seem to have a good lock on the next storm which
will move out of the area south of the Aleutians toward Kodiak
Island. Models mostly differ on the track of the low with a
discrepancy between whether it will track east of Kodiak or into
Bristol Bay.

&&

.AVIATION...
PANC...VFR conditions will persist in spite of some showers moving
through the area. The gusty southeast winds will diminish over
night and become northerly for Thursday afternoon

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Tonight through
Saturday)...

Broad unstable cyclonic flow across Southcentral will continue the
threat of showers into Saturday, especially along the coasts. A
developing low over the NE Pacific Thursday rapidly develops
under favorable upper level left exit jet position. This system
heads to the western gulf south of the Kenai Peninsula Thursday
night/early Friday morning. It will bring widespread gales to the
gulf marine areas. Additionally, there will also be a batch of
easterly storms to the north gulf marine zones (119 120 131) just
ahead of the front Thursday night. This low then weakens and
lifts north across the Kenai Peninsula Friday afternoon and
eventually ends up in the interior Friday night. Winds should then
veer around to the south across much of the Gulf and Southcentral
Friday with a brief batch of southerly gales over gulf waters and
strong southerly winds over the Copper River continuing into
Friday night before weakening.

Unlike the Atmospheric River (deep southerly fetch of moisture)
associated with the last two system to hit the gulf, this weather
system will bring a quick shot of moderate to potentially heavy
rainfall to the gulf and eastern Kenai peninsula late Thursday
night into Friday afternoon. Precipitation will become more
showery across many areas after the low/frontal passage Friday
evening.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2/tonight
through Friday)...
The low over Bristol Bay will continue spin disorganized bands of
showers into Southwest Alaska. Showers will diminish as the low
weakens and slowly moves south, though not much change is expected
overall due to its slow progression. A second low will move into
the Gulf of Alaska Thursday and is anticipated to send an easterly
wave over the the Aleutian Range, spreading rain over the
Kuskokwim Valley and Bristol Bay Thursday night into Friday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2/tonight
through Friday)...
The vertically stacked, slow moving low currently over Bristol
Bay will continue to drift south. It will gradually dissipate as
it does so, weakening to a trough Thursday night. Its slow
progression means not much change in the weather for the Eastern
Aleutians. Rain showers will continue to be present on the north
side of the islands, with some rain shadowing on the south side.
Gusty winds through gaps will be possible mainly Thursday night as
the low moves south.

The Central Aleutians will be relatively quiet with a shortwave
ridge moving through, while an elongated area of low pressure will
enter the Western Aleutians early tomorrow and progress east
through Friday, weakening as it goes.



&&

.LONG TERM FORECAST (Days 3 through 7: Friday Night through
Wednesday)...

The primary impactful weather across Southern Alaska through the
long-term will be associated with yet another strong North Pacific
low. There is still some disagreement between each of the models
about how strong the low becomes, where it tracks, and the timing.
While all of the models show a strong low tracking near the
western Gulf Saturday and Sunday, the aforementioned disagreements
make it imprudent to specify impacts for any one particular area
at this time. However, mariners and those planning recreational
activities outdoors this weekend should keep a close eye on the
forecast.

Here are the aspects of the storm that can be said with some
confidence: The initial front brings the start of the rainfall
into Kodiak Island on Saturday, spreading northward through the
afternoon. The low looks strong enough at this point that gales
are likely, especially south and east of the low center. Storm
force wind gusts are a possibility. The low will once again be
able to tap into a feed of subtropical moisture, and take that
rainfall into the North Gulf Coast once again. Fortunately, unlike
this last event which caused localized flooding in spots such as
Seward, this time that moisture feed and associated heavy rain
looks to steadily track eastward, so no one area should pick up
excessive amounts of rain for a long time frame. The track of the
center of the low, while uncertain, looks to be further west than
previous lows, so the impacts to Prince William Sound with a
possible barrier jet look reduced with this event.

Behind the low during the Monday through Wednesday time frame,
uncertainty remains high, but the center of the low and its
steering upper low look to anchor themselves near Southwest
Alaska, which should generally keep onshore flow and moist
conditions through Southcentral. Some of the models suggest a
North Pacific low tracks into the Gulf during the time frame, but
there is zero consensus on that. There is somewhat better
agreement that colder air moves into the Bering by mid-week and
some of that sneaks into mainland Alaska for late next week. This
would allow Southern Alaska to experience more seasonable
temperatures for a change.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storms 119 120 131 351. Gales 125 130 132 138-140 178 179
352 414.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...EZ/RA
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
LONG TERM...JW


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