Small Craft Advisory 799, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
422 PM AKDT Sat Oct 21 2017

.ANALYSIS AND UPPER LEVELS...
Satellite imagery this afternoon depicts a very well defined NPAC
jet stream stretching from Japan to the west coast of Washington.
A cold upper level arctic trough is centered over the Y-K Delta
with cold arctic air spilling out over the Bering Sea into the
North Pacific. A complex surface low is moving inland across the
Southeast panhandle with this system representing the leading edge
of the NPAC jet stream. Closer to home, weak southerly flow aloft
is bringing in higher values of potential vorticity aloft which
is helping to create a broad area of upper level ascent over
Southcentral. This is helping to create an area of convectively
enhanced showers over the North Gulf Coast and Prince William
Sound. These showers have been moving inland across Portage/Whittier
and west into Turnagain Arm and Pass...and even into the
Anchorage Bowl. Snow showers are also impacting the coast of the
Kuskokwim Delta in association with the arctic trough.

&&

.MODEL DISCUSSION...
The models are in overall good agreement with the large scale
pattern late into next week, and are generally pretty locked into
a deep upper 930 hpa low over the Bering Sea as the remnants of
Typhoon Lan phase with an extratropical low. However, the models
are in very poor agreement with a series of small scale embedded
shortwave troughs features lifting north along the trough axis
embedded within the low level cold air. The first of these systems
is expected to move into Bristol Bay and north into the Lower
Kuskokwim Valley Sunday night into Monday morning. The ECMWF and
Canadian are both faster than the stronger NAM and GFS solutions.
For this forecast, the GFS was favored for that low followed by a
blended ECMWF and GFS approach as it moves east into Southcentral
Monday evening.

Finally, another low is expected to move up toward the coast
Tuesday morning and afternoon. The GFS and ECMWF are both in good
agreement, however, with this pattern of low amplitude lows moving
north, confidence remains rather low at this point even with
agreement among the global models. For now, this forecast will
highlight increasing snow chances along the coast with chance snow
threats for inland locales.

&&

.AVIATION...
PANC...Light snow this afternoon/evening will drop ceilings to
near 3000 feet along with brief MVFR visibility reductions before
snow ends late this evening. Another weak low will clip the area
Sunday afternoon, bringing a slight chance for snow showers to the
terminal. Winds are expected to remain below 10 knots.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Anchorage's first official measurable snow of the 2017-2018 snow
season occurred this afternoon. A wave moving through around a
strong, cold upper level low centered near Hooper Bay tracked up
the Inlet. As it gathered moisture from a weak surface low over
Prince William Sound, the snow moved down Turnagain Arm and into
Anchorage. As the wave and associated lift move northward tonight,
the light snow activity will gradually shift into the Matanuska
and Susitna Valleys this evening through the first half of the
night, then dissipate before sunrise.

The pattern of weak waves moving northward into Southcentral
continues on Sunday as a second wave moves northeastward from
south of Kodiak Island Sunday morning into Prince William Sound,
then dissipates as it moves into the eastern Copper River Basin
Sunday night. Another round of snow is possible with this wave,
particularly for Valdez, Cordova, and the southern Copper River
Basin Sunday afternoon and evening, where a couple inches could
accumulate.

The third wave in the train moves into Kodiak on Monday morning,
bringing with it more cold air, so snow is the expected
precipitation type there. As it lifts northward, it will interact
with a separate wave tracking northward through the central Gulf.
The details of how this interaction takes place are still very
unclear and vary between the models, but generally speaking snow
is possible along portions of the Gulf Coast Monday afternoon,
with some potential it may spread inland beyond that. Stay tuned.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Low pressure along the Kuskokwim Delta remains nearly stationary
through Sunday then merges with a fast moving low which races
north across the Alaska Peninsula to the Kuskokwim Sunday night
into Monday. At the Same time, an anchored upper level low along
Nunivak Island will swing short waves along the southwest mainland
which keeps snow showers ongoing through Tuesday. The highest
chances for snow on Sunday are expected along the coasts of
Kuskokwim Delta and Bristol Bay as new energy glides around the
low from the Bering Strait. The focus for the highest snow
accumulations will be along the low that races in from the
North Pacific, therefore increased snow chances along the eastern
Alaska Peninsula, eastern areas of Bristol Bay and along the
western Alaska Range Sunday night into Monday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
Low pressure along the eastern Bering will bring widespread snow
showers through the weekend with a mix of snow and rain along the
Alaska Peninsula and Aleutian chain. The main forecast challenge
surrounds the extratropical system Lan, which moves a front into
the western Aleutians Sunday night. Heavy rain and max storm force
gusts are expected along the frontal boundary which moves north
and east to the northern and central Bering through Tuesday. This
system will occlude late Monday night with the surface low rapidly
deepening along the western Aleutians on Tuesday. There is still
uncertainty on the strength and tracking of this system and should
be monitored for updates.

&&

.LONG TERM FORECAST (Days 3 through 7)...
An active pattern is expected to evolve during the long range
forecast through the middle of next week, with the focus being on
the remnants of Typhoon Lan entering the Bering and upper level
disturbances over the Gulf tracking into Southcentral.

The moisture rich and storm force remnants of Typhoon Lan will
push into the western Aleutians/Bering Monday evening in the form
of a warm front, also bringing some warmer temperatures and rain
to the area through mid-week. Models remain in good agreement up
until this point, but confidence begins to wane as the models
begin struggling more with the track and timing of the system
central and eastern Bering through Thursday. This leads to lower
confidence on the timing, placement, and strength of any potential
wind threats and precipitation over the Aleutians and Bering as
the leading front passes through. This system will eventually
spread gusty winds and rain/snow over the southern Mainland
Wednesday morning through the end of the week.

The other features we are monitoring are a series of upper level
waves tracking through the Gulf and into Southcentral Monday
evening through Wednesday. Given the cold air already in place
over the area, these waves bring potential for snow to make it
into Southcentral, with potential for moderate to heavy snowfall
along the Northern Gulf coast, Susitna Valley, and Copper river
Basin areas. The only caveat to that is that the models are
struggling with where and when these smaller systems develop and
when they eventually track.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...TP

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
152 AM AKDT SUN OCT 22 2017

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

0406 PM     SNOW             1 ESE POTTER MARSH      61.05N 149.77W
10/21/2017  M1.9 INCH                           AK   TRAINED SPOTTER

0733 PM     SNOW             1 N RABBIT CREEK        61.10N 149.82W
10/21/2017  M1.5 INCH                           AK   NWS EMPLOYEE

0753 PM     SNOW             2 NNW GLEN ALPS         61.12N 149.71W
10/21/2017  M3.0 INCH                           AK   TRAINED SPOTTER

            NEAR OMALLEY TRAILHEAD, 1400 FEET ELEVATION

&&

KO