Areal Flood Advisory 21, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
540 AM AKDT MON SEP 19 2016

.ANALYSIS AND UPPER LEVELS...
The main low sits over the western Bering with its front draped
across the central Bering. Both are tracking to the east with the
front bringing up a long fetch of warm moist air from the North
Pacific. This is translating into rain for the Aleutians as it
passes. Strong winds are also associated with the front which is
showing as a wide swath of gale force winds moving with it. To the
east ridging is over southwest Alaska and it is also moving off to
the east.

&&

.MODEL DISCUSSION...
The models are in good agreement with the placement and intensity
of the main low as it tracks across the Bering this week, but they
are still struggling with the formation of a triple point low that
looks to form on the front associated with this system. The 00Z
runs are in better agreement than the 12Z runs were but there are
still several issues with impacts as a result of these
differences, one of which is timing and intensity of rain for
areas of Southcentral Alaska. The other impact is for winds for
areas in and around the north Gulf coast, including areas of
channeled terrain. These differences are making it difficult to
nail down exact timing. However, confidence is still high in high
amounts of rain for the Eastern Kenai Peninsula and Western Prince
William Sound, as well as for high winds for much of Southcentral
Alaska.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Today should be a rather quiet weather day around Southcentral
Alaska. The storm in the Western Bering Sea in still on track to
send a very strong front through Southern Alaska Tuesday night
into Wednesday. This system will be stronger than the one from
early last week and there is a possibility of winds reaching to
100 mph threshold along the higher elevations of the Anchorage
areas as well as Western Turnagain Arm. The atmosphere is also
setting up to allow some of the strong winds to reach lower
elevations in the Anchorage and Eagle River areas Tuesday night
into Wednesday morning which could bring wind gusts up to 65 mph
to the Lower Hillside and portions of East Anchorage as well.
Other portions of the Anchorage bowl will likely not see winds
quite as strong but the presence of leaves on the trees increases
the chances of broken limbs causing power outages. This front will
likely bring high winds to the Cordova area on Wednesday prompting
the High Wind Watch for there.

The other big concern is the possibility of flooding along the
North Gulf Coast. The Seward area looks to be the most vulnerable
where models are showing very heavy rainfall Tuesday night into
Wednesday morning. Models are showing around 5 to 7 inches (NAM
model is indicating 8 inches) of rainfall occurring in a 12 to 18
hour period which could cause flooding in the Seward area.
Therefore a Flood Watch has been issued for that area. While rain
will taper off Wednesday morning, river levels would likely remain
in flood stage well after the rain ends prompting the Flood Watch
to be in effect until Thursday morning. This is a very preliminary
view of this event, but it bears close watching and updates will
be available as details become more clear.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A strong front will begin to push inland from the eastern Bering
Sea later today, bringing an extended period of strong winds and
moderate to heavy rain to much of the southwest mainland to start
the work week. The initial forecast concern associated with the
front will be the potential for high surf and erosion along the
coast of the Kuskokwim Delta as the front moves ashore accompanied
by high-end gale-force winds out of the south, with the worst
conditions expected from early afternoon through late evening
before before conditions rapidly improve as the front departs to
the east. The High Surf Advisory has thus been extended through 10
pm this evening with the issuance of the morning package.

The front should clear the Kuskokwim Delta region by late
tonight, with rain spreading eastward into the Kuskokwim Valley
and Bristol Bay regions. Attention then turns to a triple-point
low developing south of the Alaska Peninsula late tonight and
riding northward along the front into Bristol Bay on Tuesday
night. In addition to steady rain, this feature has the potential
to produce strong southeast winds through Kamishak Gap and along
the western slopes of the Aleutian Range from Tuesday afternoon
into Tuesday night. Current forecast thinking is that gusts will
top out in the 60-65 mph range in these areas, with winds in the
40 to 50 mph range extending well inland to interior Bristol Bay
and the Kuskokwim Valley. The track and evolution of this
developing wave will bear close monitoring, as small changes in
the details could have large implications on the wind forecast in
the next 24-48 hours.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The front extending eastward from the large, mature low over the
western Bering will keep the most significant weather impacts
over the southeast Bering through tonight as rain and southerly
gale force winds with storm force gusts spread eastward from the
Eastern Aleutians to the Akpen. The front will move off to the
east by early Tuesday, leaving widespread showers and gusty
southwest winds over a large swath of the Bering as the low move
eastward towards Saint Matthew Island. A core of cold air aloft
associated with the low center could generate enough instability
to touch off a few thunderstorms over the waters of the northwest
Bering through Tuesday.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Updated forecast grids overnight to account for a little quicker
progression of the front through the Gulf of Alaska Wednesday,
which matches with latest model trends, with the EC quickest and
the GFS and NAM comprising a solid middle ground for the
Wednesday/Wednesday night time frame. We also trended higher with
winds from Prince William Sound to Passage Canal to Turnagain Arm
to Anchorage for the Wednesday morning time frame at the peak of
what is shaping up to be a significant fall storm. Otherwise, no
significant changes beyond Thursday morning. For Cook Inlet the
main change was to go with the southerly winds for the day on
Wednesday with pressure rises behind a secondary low that moves
north into the Susitna Valley through the day.

Previous long-term discussion...

The models continue to struggle with the rapidly developing
triple point low near Kodiak Island late Tue-Wed, and as a result
forecast confidence hasn't change significantly over the Cook
Inlet region. In particular for the waters of south Cook Inlet,
guidance is indicating anything from northerly to southwesterly
gale force winds depending on the low position, so forecast
confidence in this area is very low on Wednesday. Further to the
east, there is much better agreement however with the associated
front moving into Prince William Sound, allowing us to bring
widespread storm force winds to that area through Wednesday. For
Turnagain Arm and higher elevations of the Anchorage Bowl,
confidence has improved enough for a High Wind Watch to be issued
with the afternoon forecast package as all guidance now indicates
a significant pressure gradient setting up on Tuesday evening.
Another concern from this system surrounds the potential for
persistent heavy rain and flooding along the Gulf Coast. In
particular for the Seward area, there remains some uncertainty as
to how long the heavy rain will last, however even the quickest
solutions are bringing at least 4-5" of rain in 12 hours, so
appears likely that this will be a significant rain event,
possibly heavier than the system from last Sunday. Its still a
little too far out at this point to determine exactly what
response this heavy rain will have on area rivers and streams, but
this should become more clear in the coming days.

Looking into later this week, the models have all trended
significantly weaker with next low that will rapidly develop and
intensify in the gulf on Thursday. As a result it now appears that
this system will not become as significant as initially thought a
few days ago, but it will still bring active weather to the Gulf
and south Mainland into Friday. This low will likely bring a
prolonged period of gusty winds and rain to the Gulf Coast, and in
particular Prince William Sound that looks to last into late this
week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Watch 101 125 135
         Flood Watch 125
         High Surf Advisory...155.
MARINE...Gales...130-132 138 139 141 150 155 160 165 170-172 173
                 178-181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...CC/DEK