Small Craft Advisory 756, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
517 AM AKDT Sun Oct 4 2020

.ANALYSIS AND UPPER LEVELS...

The pattern is amplifying once again this morning as a storm
system moves northward from the North Pacific into the Gulf of
Alaska with the main jet stream aimed at Southeast Alaska. This
storm is undergoing cyclogenesis in the left exit region of the
jet as winds in the core of the jet stream are currently 120-140
kts. Looking at water vapor imagery, this storm system has a
moisture source all the way south to around 30N latitude. Ahead of
this system is a potent upper level wave that is producing steady
rain currently for the Prince William Sound area including Valdez
and Cordova east to the Copper River Delta. This is also
generating upslope flow against the mountains north of Cordova
which is helping to squeeze out even more moisture in the
atmosphere.

A weak area of low pressure south of Nunivak island is producing
areas of occasional showers this morning.

Further west, the Bering Sea and Aleutian Chain remain quiet with
a deep upper level trough in place as the main jet stream remains
well south of the region. As a result, multiple upper level
shortwaves continue to move about the Bering and the Aleutian
Chain resulting in occasional showers from time to time. Most of
these showers are of the stratiform variety also when looking at
satellite imagery.

&&

.MODEL DISCUSSION...

Model guidance remains in good agreement on the synoptic features
for the next 3 days. For the incoming storm into the Gulf, all
models are now practically in lockstep with each other for the
low track and intensity, therefore, minor changes if any expected
this forecast package. The next storm system to impact the Bering
are all in agreement as well that a storm force frontal system
will impact the western Bering and western Aleutian Chain to
start the next work week. Models vary on exact placement of the
low to the west of the Bering Sea but agree on the timing of the
frontal system associated with this low. As a result, forecast
confidence is above average. The EC is the furthest east and
fastest with the low, GFS is slightly slower, and the Canadian NH
is slowest and furthest west.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

The strong low traversing the North Pacific over the last couple
days will reach the southern Gulf early this morning, passing
east of Kodiak by this afternoon. Models have been struggling with
this system, however, they have since to come into agreement that
it is slightly weaker and takes a more easterly path. The
significance of this is that, while we still have widespread gales
and small craft over the next couple days, southern Cook Inlet
will not see the gales or storm force winds mentioned previously.
Additionally, a very brief period of storm force winds are in
place for this afternoon as a barrier jet sets up south of Kayak
Island.

Rainfall for today will be confined to the coasts as the
associated front approaches, with moderate to heavy rainfall
expected for the PWS/eastern Kenai Peninsula area. After frontal
passage tonight, rainfall diminishes and becomes light. This more
easterly track will keep the Susitna Valley, Anchorage, and
western Kenai in drier conditions out through Wednesday. The low
exits towards the Southeast coast on Monday, diminishing marine
winds to widespread small craft through early Tuesday morning. A
broad upper-level trough with weak surface rotation sets up over
the region mid-morning on Monday after the low exits. Impacts
associated with this will be minimal and Southcentral (with the
exception of the southern Copper Basin) will dry out.

&&

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

Scattered showers continue to dissipate over the Y-K Delta and
Bristol Bay this morning as an upper level shortwave moves north.
However, some light rain may continue from Kipnuk to Hooper Bay
as a front stretches out and weakens over the area. An upper level
shortwave to the south of the AOR will pull the remnants of an
upper level low over the western AKPEN east this morning,
providing a chance for showers over the AKPEN today. On Monday a
upper level ridge is expected to build over the central Bering,
which will bring cooler air and clearer skies into the region
through Tuesday. Chances for precipitation will diminish to near
zero for most of the region through Tuesday, although rain and
snow are possible for the higher elevations of the AKPEN and the
Kilbuck Mountains.

&&

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

One more day of widespread northerly flow and showers are expected
on Sunday before a pattern change begins to take shape Monday
morning. At this time southwesterly warm air advection begins
over the western Bering, which will lead to ridging over the
central Bering by Monday night. Meanwhile, a large front is
expected to move east from the Kamchatka Peninsula, arriving at
the western Aleutians Monday evening. Storm force winds are
possible near Shemya Tuesday morning as the front moves east. The
front should weaken below storm force as it stretches out over the
central Bering Tuesday afternoon.

&&

.MARINE (Days 3 through 5)...

Bering/Aleutians

Early Tuesday southeast winds ramp to gale force across the
western Bering/Aleutians along with widespread rainfall due to an
approaching Kamchatka low. Wind/rain spreads into central
Bering/Aleutians mid to late Tuesday. Early Wednesday, the
Kamchatka low migrates towards the northwestern Bering, with
westerly small craft/low end gale force winds expected across the
western and central portions of the Bering/Aleutians. Eastern
areas of the Bering/Aleutians will see southerly winds increase
to near gale force along with plentiful rain. Late Wednesday the
low in the northwestern Bering weakens, with winds turning
westerly and decreasing to small craft levels. Thursday, this area
of low pressure continues to weaken near St. Matthew Island with
west- southwesterly winds at small craft advisory levels expected
across much of the Bering/Aleutians.

Gulf of Alaska

Light westerly flow is expected Tuesday and Wednesday. On
Thursday winds will flip to southerly, with some small craft level
winds expected across the Western Gulf.

&&

.LONG TERM FORECAST (Days 3 through 7/Tuesday through Saturday)...

A low centered in the northern Gulf on Tuesday morning will slowly
move eastward and out of the Gulf by Wednesday night. Guidance is
in good agreement with the placement and intensity of this low.
An upper level ridge will move over the Gulf and Southcentral in
the wake of this low on Thursday which will result in less cloud
cover and warmer temperatures. An upper level low will move off
Siberia into the northwestern Bering Sea Thursday morning. This
low will then move into the central Bering on Friday. There is a
high degree of uncertainty with respect to this low on Saturday as
there are significant differences in solutions with respect to
the placement and intensity of this upper low. The one area of
consistency on Saturday is the upper trough associated with this
upper low will extend over the western Gulf and into Southcentral.
This will support cooler temperatures across Southcentral and
Southwest Alaska next weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 121 125. Flood Watch 121
MARINE...Storm Warning 119 351 Gale Warning 120 131 352.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MV
SOUTHCENTRAL ALASKA...KM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
MARINE/LONG TERM...MK/ED