Small Craft Advisory 906, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
509 PM AKST Thu Nov 28 2019

.ANALYSIS AND UPPER LEVELS...
A deep vertically stacked low a few hundred miles south of Kodiak
Island is lifting northward as a short-wave trough just upstream
continues to dig southward. A nearly stationary frontal boundary
extends northward from this low to Shelikof Strait and Cook Inlet,
with Arctic air to the west over Southwest Alaska and warmer air
over the Gulf and Southcentral Alaska. Strong low level convergence
and upper level deformation is leading to moderate to heavy
precipitation across the western Gulf on up to the Cook Inlet and
the western Susitna Valley. As the low lifts northward flow out
ahead of it is backing toward the southeast which is pushing the
heavy precipitation westward across the Aleutian Range into
Bristol Bay and across the Alaska Range into the Kuskokwim Valley.
Precipitation is gradually ending from the western Kenai to
Anchorage and the Matanuska Valley. Heavy snow continues over much
of the Susitna Valley, with the heaviest snow shifting to the
western valley. Snowfall reports received thus far indicate
anywhere from one to three feet of snow have fallen across the
Susitna Valley. Rain and snow also persist across the Prince
William Sound area, through the precipitation is generally
lightening up as the deeper upper level moisture shifts westward.

On the back side of the low, cold air advection and tight pressure
gradients are leading to storm force winds along and near the
southern Alaska Peninsula, with gusts approaching hurricane force
through bays and passes.

Meanwhile, another deep low is occluding as it lifts northward
across the western Bering Sea. A strong jet on the back side of
the system is driving a cold front eastward across the Aleutians,
with a band of storm force winds right ahead of the front.

&&

.MODEL DISCUSSION...
Models are in decent agreement with large scale features through
Saturday. There are some differences in the thermal profile for
the western Kenai, Anchorage, and Matanuska Valley on Saturday
which will determine precipitation type. The NAM is the coldest
solution (mostly snow) while the GFS is the warmest (mostly rain).
Suspect the answer will lie somewhere in between as neither of
these models have verified very well with low level temperatures
in recent storms.

A large model spread does develop with handling of a fast tracking
low from the eastern Aleutians toward the Gulf Saturday through
Sunday. The ECMWF and Canadian solutions are more amplified and
deepen the surface low and track it more slowly. The GFS/NAM are
weaker and faster. For now, favor the NAM/GFS solution based on a
fairly zonal jet stream. However, will closely watch the trends in
upcoming model runs since the different solutions would result in
significantly different weather across the southern mainland of
Alaska.

&&

.AVIATION...
PANC...VFR conditions will persist. Strong and gusty southeast
winds will persist down Turnagain Arm and along the Chugach
Mountains. However, the down Inlet pressure gradient should mostly
keep the southeast winds at the surface just to the south over
northern Cook Inlet. It's a close call, but it looks like
southeast winds stay just outside of the terminal. Otherwise,
with a northerly surface wind, low level wind shear will persist.

Low ceilings have settled in with warm air overriding a cold
surface. This will be tough to erode through the night with
abundant low level moisture. Some patchy fog will also be possible
through the morning hours.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

Broad southerly flow aloft will continue to allow for additional
snowfall in the northern parts of Susitna Valley overnight
tonight. As midlevel winds become more southeasterly,
precipitation will eventually taper off over the Matanuska Valley
and Anchorage Bowl as a downsloping component enhances. Warmer
air moving into the southern Susitna Valley may allow any
lingering precipitation to briefly change to rain. Given the
recent rainfall over the southern Kenai Peninsula, there are
elevated water levels along the Anchor River and a Special Weather
Statement has been issued.

Rain is expected to continue through Friday across the northern
Gulf coast. By Friday night, an upper level wave moving over
Southcentral from the west may bring another round of
precipitation to Southcentral, though there is high uncertainty
regarding how much cold air will be in place (for snow) and how
far north warm air is advected (making rain). There is good
confidence that unsettled weather will continue through Saturday.

&&

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

A cold front remains stalled along the west side of the
Alaska/Aleutian Range. Meanwhile, south flow ahead of a
vertically stacked low centered roughly 400 miles south of Kodiak
Island will continue to pump moisture northward. A deformation
band is stretched from the upper Kuskokwim Valley to the interior
Bristol Bay area. This will support moderate snow from Sleetmute
east through Friday afternoon. Winter Weather Advisories remain in
effect for interior Bristol Bay (east of King Salmon) and the
middle Kuskokwim Valley (Sleetmute east). An upper ridge to the
west and north flow in the lower levels will keep areas farther
west (including the Kuskokwim Delta and Bristol Bay) generally
cold and dry. Late Friday into Saturday, a low in eastern Siberia
will steer a weakening front into the Southwest Mainland. Winds
will veer to westerly along the front, and some snow is likely.
However, this weak front will pass over quickly, so snow
accumulations should be relatively light.

The forecast becomes more challenging Saturday into Sunday. Models
show a North Pacific low developing along the jet stream Saturday
and drifting northeastward. There is high uncertainty with the
expected track, and this low will have the potential to bring heavy
precipitation in its periphery. The 12Z operational GFS has the low
skirting along the AKPEN and eastward, which would mostly contain
the heaviest precipitation to the AKPEN/Alaska Range. However, the
12Z ECMWF and many of the 12Z GEFS members bring the low northward
towards Bristol then inland. This could bring snow and blowing
snow impacts to much of the Southwest mainland. Right now, we
went with a blend of the operational runs and kept precipitation
mainly in Bristol Bay into the Alaska Range. Be sure to monitor
the forecast for this time period.

&&

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

A storm force low in the western Bering is shifting northward and
pushing a front across the Aleutians. Southerly winds have already
hit storm force (50 knots) in Shemya and have started to
diminish. The swath of storm force winds should stay near the low,
thus primarily in the northwestern Bering, but the front will
bring precipitation and southerly gales as it crosses the
Aleutians. It should be a fast-moving system, clearing the Bering
by early Saturday morning. Expect isolated to scattered showers
over much of the Bering Saturday behind the front. The forecast
becomes more challenging Saturday into Sunday. A North Pacific low
is expected to develop along the jet stream Saturday and drift
northeastward, bringing a swath of heavy precipitation and
possible gales in its path. There is significant model discrepancy
with the forecast path, with the solutions ranging from the low
skirting along the south side of the AKPEN eastward, to the low
taking a north turn and approaching Bristol Bay.

&&

.MARINE (Days 3 through 5)...

Bering/North Pacific/Bristol Bay: On Sunday, low pressure east of
the Pribilof Islands will move towards Bristol Bay bringing small
crafts and gales to the region. This is a very progressive
pattern. Another low will quickly enter the Bering. The models
differ on the intensity of the winds associated with this second
low, but at the very least, expect gusty winds and enhanced seas.
The current thinking is that this second low will cause westerly
small-craft winds with embedded gales to develop from the western
Aleutians Monday through the AKPEN by Tuesday. The result will be
one area of low pressure over the central Bering and a second
near Shemya on Monday moving east. It is possible this second low
paralleling the Aleutian Chain deepens well enough to produce an
area storm force winds around the core. There is still quite a bit
of uncertainty with these systems, however, so the evolution of
these lows will be closely monitored.

Gulf Of Alaska: A surface low will move into the Cook Inlet with a
frontal in the Gulf Of Alaska and the North Pacific. Small crafts
and gales will accompany this system Monday and into Tuesday.

&&

.LONG TERM FORECAST (Days 4 through 7)...

An active weather pattern will continue from the weekend into
early next week as the axis of the upper-level trough slowly
shifts from the Bering to the Alaska mainland. As it does, the
pattern becomes slightly less amplified, allowing a series of
fast-moving shortwaves track from the Aleutians chain and Bering
east to the Gulf. First up is an area of low pressure over Bristol
Bay / AKPEN late Saturday moving inland Sunday with a secondary
low spinning up in the northern Gulf near the eastern Kenai
Peninsula, moving into Prince William Sound by late Sunday. There
is still quite a bit of uncertainty with this, but guidance is
beginning to convergence on this solution. If true, this would
bring a wintry mix of precipitation into Southcentral Sunday and
Monday as colder air brings to work into the region. Temperatures
and precipitation type will be the biggest challenges with this
system, but right now it appears that there increasing chances for
snow in and around Anchorage by the start of next week.

Farther west, a low over the Bering and a developing low over the
western Aleutians Monday will track east to the Kuskokwim Delta
and AKPEN, respectively, by Tuesday. Again, there is some
uncertainty with the strength of these systems, but it does
appear more likely that rain and gales will impact the Chain
early next week while the Bering low produces some snow for
coastal Southwest Alaska.

As these systems and the upper-level trough slide east, a
northerly flow will develop over the state ushering in colder and
drier air across the Southern Mainland by mid week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Warning 145.
		 Winter Weather Advisory 152 161.
MARINE...Storm Warning 150 155 411.
		 Gale Warning 412 413 172-178 181 185 351-352 119 120 131 132 138.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...PS/TM