Small Craft Advisory 702, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
442 PM AKDT Thu Sep 17 2020

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low which was over Bristol Bay this morning
is tracking inland across Southwest Alaska this afternoon. The
associated occluded front is approaching the north Gulf coast,
leading to a barrier jet with storm force winds along the coast
and gale force winds in Prince William Sound. Additionally,
widespread gales are being observed in the wake of the low as
cooler more unstable air moves in and pressure rises. This is
primarily affecting coastal waters along the Alaska Peninsula
and Bristol Bay on into the western Gulf. Gusty winds also moved
across much of Southcentral today, ahead of the front. The highest
winds were observed through Portage Valley, Turnagain Arm, the
higher elevations of Anchorage, and along the Copper River. Gusts
of 65 to 75 mph were reported at multiple stations on the upper
hillside in Anchorage. This storm system is also bringing
widespread rain to Southwest and Southcentral Alaska, with the
heaviest rain along the coastal regions. Storm total rainfall
amounts thus far generally range from 0.5 to 1.5 in. While river
levels are elevated, flood risks are minimal.

&&

.MODEL DISCUSSION...

Models are in good synoptic agreement over the next few days.
Model agreement (and forecast confidence) has improved for the
fast moving low traversing eastward along the Aleutian Chain
Friday and Saturday. However, solutions diverge significantly
as the low crosses the Gulf, so there is uncertainty in the
wind/precip forecast for the Gulf coast and Southcentral.
Lastly, models are all trending stronger with the large low
crossing the Bering Sea Saturday through Sunday, so the
forecast will follow the same trend.

&&

.AVIATION...

PANC...VFR conditions will continue, though there could be a very
brief period of MVFR ceilings as batches of showers move through.
Gusty winds will persist through the forecast period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

A low has tracked well into the interior of western Alaska with
its associated front moving onshore along the Gulf Coast.
Consequently, gusty easterly winds over Turnagain and Knik Arms
will continue to diminish overnight into Friday morning. Rain will
taper off across Anchorage and the Matanuska Valley as well,
though precipitation will continue to remain widespread near
higher terrain through the weekend. An upper level jet will dip
well to the south of Alaska on Saturday brining much cooler
temperatures. While low temperatures overnight Saturday into
Sunday will generally be moderated by cloud cover, they are
expected to fall to near freezing over the Chugach and thus minor
snow accumulations at higher elevations cannot be ruled out.
Weaker flow will allow for a higher chance of showers in the
Anchorage area late Saturday into Sunday as well.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Sunday)

An upper low over Kuskokwim Delta has brought a lot of rain and
moisture west of Alaska Range, while isolated thunderstorms were
occurring south of Kuskokwim Valley this afternoon. Showery
clouds associated with this system will gradually taper off late
tonight, and cooler temperatures will linger over the region
tonight and Friday night. The next major system from the Aleutians
could bring more rainfall, but the timing may be a challenge as
models delay the forward motion of this expected system. Overall,
it will be a quiet forecast period with only isolated showers in
the coming days.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Sunday)

The next developing system will elevate seas and winds to gale
force conditions, but that may all change should this new system
intensify as it moves across the Bering and the Aleutians over the
weekend. As a result, anticipate another wet pattern to return
before the end of the week, with marine warnings to reflect the
expected situation.

&&

.MARINE (Days 3 through 5: Sunday through Tuesday)...

A strong low entering the Bering Sea on Saturday seems to be
trending stronger with the newer model runs. Thus, expect gale
force winds will more widespread in nature across the Bering Sea
and Aleutians.Saturday night through Monday. There is also the
possibility of a brief period of storm force winds along the
Aleutian Chain as colder feeds into the back side of the
storm Saturday night into Sunday. By Tuesday the low crosses
over the Aleutians and moves into the Northeast Pacific/Gulf of
Alaska. There is disagreement on the track, but with the latest
models generally farther south, suspect the bulk of small
craft/gale force winds may be confined to the southern Gulf/Kodiak
Island.

&&

.LONG TERM FORECAST (Days 4 through 7: Monday through Thursday)...

A strong low in the vicinity of the Eastern Aleutians/Alaska
Peninsula on Monday will continue southeastward into the
Gulf/Northeast Pacific by Tuesday. Thus, rain and wind over
the central to eastern Bering will shift south and eastward to
Kodiak Island and the Gulf. Cold air advection behind the low
will maintain robust northerlies across the eastern Bering and
Southwest Alaska coastal waters all the way through Wednesday.
Much of Southwest and Southcentral look drier on the northern
periphery of the low Monday through Tuesday, with steady rain
likely confined to the Gulf coast. However, it may not be
completely dry over Southcentral, as most models indicate a
short-wave trough moving through sometime Monday.

The forecast for late next week looks potentially "winter-like".
Most models show a trough dropping out of the Arctic mid week,
though there is disagreement on how far south the trough will
make it. The GFS is most aggressive in driving cold air (down
to -10 degrees C at 850 mb) down across Southwest Alaska, then
potentially interacting with energy being pulled from the
Gulf/Northeast Pacific up to Southcentral. At a minimum, could
be a round of mountain snow. Stay tuned, as forecast confidence
is quite low at this point in time.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 130 138 139 160 180.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...AP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...KM

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1113 PM AKDT THU SEP 17 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0855 PM     NON-TSTM WND DMG 1 W ANCHORAGE           61.16N 149.98W
09/17/2020                   ANCHORAGE          AK   NWS EMPLOYEE

            ESTIMATED 12 INCH DIAMETER TREE DOWN
            PARTIALLY IN THE ROAD WHERE SAND LAKE RD
            TURNS ON TO RASPBERRY HEADED EAST.

&&

AS