Small Craft Advisory 776, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
423 PM AKDT Tue Oct 9 2018

.ANALYSIS AND UPPER LEVELS...
A mature closed low over the southern Bering Sea anchors a high
amplitude trough over the Bering Sea and North-Central Pacific.
An unstable air mass is associated with this low and is producing
showers and gusty winds, especially with a short-wave on the south
side of the low passing right along the Aleutian chain. An old
occluded front from this low is oriented east-west across the
northern Bering Sea, then southeastward and over Southwest Alaska.
This is producing areas of rain from Bristol Bay to the Kuskokwim
Valley and Kuskokwim Delta.

On the front side of the upper trough a weak surface low and
associated frontal system are moving slowly northward across the
western Gulf of Alaska, with light rain spreading as far north as
the eastern Kenai Peninsula. This low is weakening as heights rise
ahead of an intensifying low south of the Alaska Peninsula. This
second low is in a good spot ahead of a strong short-wave
rounding the base of the upper trough and near the left exit
region of strengthening jet streak. This low will be the primary
weather producer for the southern mainland of Alaska over the next
24 to 36 hours.

&&

.MODEL DISCUSSION...
Models remain in good agreement with large scale features over
the next couple days. The Canadian solution is an outlier on
Friday with phasing of upper short-waves and development of a
surface low which it ejects out of the trough and into the eastern
Bering Sea. While this solution is plausible will need to see
more models get on board before thinking about buying into it. The
primary forecast challenge is the very localized details of wind
and precipitation across Southcentral and Southwest Alaska tonight
through Wednesday as this major storm moves right across the
region.

&&

.AVIATION...
PANC...Clouds will continue to lower as a weak front approaches
from the south this afternoon and evening, but strengthening
southeasterly low level flow will keep ceilings safely in the VFR
category. Low level wind shear will become more and more likely
overnight into Wednesday morning out ahead of a deep low moving up
from the Pacific. Surface winds will remain out of the north-
northwest as southeasterly flow strengthens above this. The worst
period of shear will be just prior to frontal passage - approximately
15Z to 20Z. As the front pushes through, the Turnagain Arm jet
will bend strongly into the terminal then shift to up Inlet
(southerly) flow shortly later.

While downslope flow will maintain dry conditions for much of the
period, will probably see some light rain as a strong vorticity-
maximum tracks up Cook Inlet between 12Z and 18Z. This could
briefly lower ceilings to 5000 feet or lower, but would expect
ceilings to remain VFR.

&&

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

A warm front stretching across the northern Gulf will stall just
south of the coast this evening. Along the front, weak gales will
continue with rain falling primarily along the coasts and western
Cook Inlet. A strong southeasterly, cross-barrier flow aloft will
act to keep locations in the lee of the Chugach mountains on the
drier side through tonight.

The well-advertised stronger of the two surface lows continues to
trek northeast toward Kodiak Island this evening. ASCAT
scatterometer winds are indicating gale-force winds beginning to
wrap around the center of circulation as the low continues to
intensify. This system will move over eastern Bristol Bay
Wednesday morning. The low will then weaken as it heads over the
western interior Wednesday afternoon.

A secondary triple point low will develop along the decaying low's
frontal trough in the vicinity of Shelikof Strait overnight and
head up Cook Inlet Wednesday afternoon carrying what is left of
the front with it. Strong east/northeast gales to storm-force
winds will develop over the western Gulf tonight, especially along
both the eastern and western coast of Kodiak Island. Winds will
shift to a southerly direction, due to strong pressure rises on
the backside of the triple point, Wednesday morning as the front
moves inland and the low tracks up Cook Inlet. Very strong gap
flows are expected in favored locales, Turnagain Arm, Portage
Valley, Matanuska Valley, Knik Arm, and along the Copper River, to
name a few. Winds should diminish across most areas Wednesday
evening/night. As with the first low, a cross-barrier flow will
limit precipitation for areas in the lee of the Chugach
Mountains. However, precipitation is expected to increase for
thees locations as the low works its way up Cook Inlet Wednesday
afternoon/early evening and the overall flow shifts to the
southwest.

The longwave pattern driving steering these systems will remain in
place through the second half of the work week, with high
pressure sitting over the eastern Pacific and a low over the
Bering. The resulting southwesterly flow will keep clouds and
moisture over Southcentral. Another surface low will move from
the north Pacific into Bristol Bay late Thursday with more rain
and brisk southeasterly winds for Southcentral Thursday night
into early Friday.

&&

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

A strong low, the extratropical remnants of Typhoon Kong-Rey, is
currently centered a couple hundred miles south of Cold Bay. The
low center will make landfall along the Alaska Peninsula in the
vicinity of Chignik tonight, then track north along the west side
of the Alaska Range on Wednesday. This will bring widespread rain
across all of Southwest Alaska, along with strong and gusty winds,
especially through the gaps. Winds will be southeasterly ahead of
the low, then will turn out of the southwest behind it. Those
southwesterly winds may cause erosion issues along the Bristol Bay
coast from Port Heiden northward during the day Wednesday. As the
center tracks northwestward to the southern coast of Norton Sound
Wednesday evening, the associated rainfall will gradually end
from south to north across Southwest Alaska. Expect gale force
winds along the coast until long after the low passes.

The low will remain nearly stationary off the coast to the east of
St. Matthew Island on Thursday, as a secondary low develops and
moves ashore around Kuskokwim Bay Thursday afternoon. This will
bring another round of rain with gusty winds along the coast
during the day Thursday. Another disturbance tracks northward into
Southwest Alaska during the day Friday, though the models are
having a difficult time resolving the timing of this feature.
Regardless, it will mean a very unsettled weather pattern across
the area straight through Friday with any breaks in the
precipitation expected to be short. Impact-wise, those along the
coast who have south-facing exposures should be especially
cognizant of a very persistent onshore southerly wind flow through
Friday, which will likely cause localized erosion concerns at
each high tide.

&&

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

A vertically stacked low currently over the central Bering will
act as the focus around which numerous lows and impulses of energy
will cause their own localized steady rains and gusty winds. The
first such low will be the strongest of the lows to impact the
area through the period. It is centered south of Cold Bay and will
track into Southwest Alaska tonight, this will lead to broad
northwesterly winds across the eastern Bering as the low tracks
into mainland Alaska. Meanwhile another low centered off Kamchatka
will track southward into the western and central Aleutians on
Wednesday, bringing widespread small craft conditions to that
area. The low will track into the eastern Aleutians Wednesday
night, then north into Kuskokwim Bay on Thursday. These lows will
help bring the vertically stacked low further east with time,
keeping the central and eastern Bering very unsettled while the
western Bering gets a short break Thursday afternoon through
Friday morning. Another strong low moves into the western Bering
and Aleutians Friday afternoon and evening.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The mid to long term begins Friday with the models in overall
agreement with a low over the eastern to northern Bering Sea and a
ridge of high pressure extending into southeast Alaska. The
models have also come into good agreement with respect to the next
storm on tap for the western Bering Sea, moving into the western
Aleutians late Friday. This storm is taking a track along the
Aleutian chain to the western Alaska Peninsula through Monday
morning. This is where the models then begin to show more
significant differences in the solutions for it. At that point the
ensemble and WPC solutions will be used. The current forecast
have a slightly faster track for the low along the Aleutians, and
we will nudge toward this solution, primarily resulting in adjusting
the wind direction to better align with the WPC solution.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 119 120 125 131 136 137 139-141 155 160 173-177.
		 Storm 130 132 138 150.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...SA