Small Craft Advisory 854, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
422 PM AKDT TUE NOV 1 2016

.ANALYSIS AND UPPER LEVELS...

An active jet stream is keeping the region active weatherwise
today. An upper level trough persists through the central Bering
south and east into the North Pacific just south of the Gulf of
Alaska. With an upper level high remaining in place over the
northern Alcan border region (and building west across northern
Alaska), a southeasterly jet streak continues to push moisture
over a pair of frontal boundaries from the Gulf of Alaska through
Southwest Alaska. The southern frontal boundary is the strongest
of the two features, and is producing fairly decent rainfall near
Iliamna late this morning into early afternoon.

Meanwhile, behind this front and extending west through the
remainder of the Bering region, a dynamic post-frontal environment
exists with alternating regions of warm and cold advection. With
such a high-energy dynamic pattern in place, the small-scale
systems are producing small areas of heavier rainfall rates as
they move quickly across the region. These areas will serve as the
main focus for the forecast for the next couple of days.

&&

.MODEL DISCUSSION...

The models all are in very good agreement with the synoptic
pattern. The main focus will be on the smaller-scale post-frontal
systems which will likely continue to overperform with regards to
rainfall rates with the current dynamic environment in place.
Over the southern mainland, the front will continue to sharpen as
it moves north tonight, ushering in a post-frontal (and more
unstable) environment.

The main challenge for this forecast package will be adding
emphasis (especially within the POP/QPF and wind/gusts grids) to
the smaller-scale systems that tend to be underestimated by
models within the larger-scale post-frontal environment in place.

&&

.AVIATION...

VFR conditions are expected through Wednesday afternoon. However,
there is some uncertainty between 09z and 15z as a frontal system
moves through tonight. Models are trending more aggressive in
trying to bring a very brief period of rain or rain showers and
thus lower ceilings. It is still too early to say if there could
be an hour or two of MVFR ceilings at this point in time.
Turnagain Arm winds may also bend into the airport very briefly
with frontal passage, but this is also low confidence and is not
indicated in the current TAF.

&&

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

A frontal boundary remains draped across the northern Gulf of
Alaska but has been rapidly dissipating throughout the day. A
decent barrier jet developed from Middleton Island northward as
expected and will dissipate just before midnight as the front
continues to fall apart. Rainfall has been plentiful across the
eastern Kenai Peninsula with rainfall amounts ranging from 0.75
inches to 3 inches in upslope observation sites at higher elevations.
Rainfall is expected to linger through at least Wednesday morning
with rainfall amounts ranging from another quarter of an inch to
three quarters of an inch over Prince William Sound and the
eastern Kenai Peninsula. Otherwise, it has been quite dry for
interior locations as downsloping has been fairly dominant. That
will likely change for a brief period (08-12Z) tonight/tomorrow
morning as the front tries to push over parts of Southcentral
Alaska and downsloping weakens. Rainfall amounts will likely be
less than 0.05 inches but could create some slick roads again
across Anchorage and the Matanuska Valley as seen the past several
mornings. For Thursday into Friday, a showery regime sets up as a
longwave trough encompasses much of the mainland and extends into
the North Pacific. The majority of the showers will be observed
across the northern gulf coast as moisture upslopes along the
coastal mountains while the interior once again looks dry.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Broad complex low over the Bering will continue to bring cloudy
skies and weak offshore flow to the southwest Mainland through the
work week. A frontal system associated with this will bring
continued widespread rain northern Bristol Bay this evening. To
the south along the Bristol Bay coast, clearing behind the rain
may cause for development of patchy fog/stratus overnight as
skies begin to clear. Subsidence however will be relatively weak
overnight, which should prevent any development from becoming too
extensive. A weak wave will then develop behind the front on
Wednesday, spreading rain from western Bristol Bay into the
Kuskokwim Delta that should continue into Friday morning.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A broad low complex centered over the northern Bering will
continue to bring clouds and showers throughout the Bering into
late this week. To the east of this low, a weak wave will continue
bringing rain from the Pribilof Islands southeastward into the AK
pen, where it will continue through Wednesday with otherwise light
winds. On the other side of the low, strong cold air advection
over the western Bering/Aleutians will bring widespread gale force
winds with rain showers into Friday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The Bering Sea, and the Aleutians remain under a showery weather
regime as a gale force low tracks into the central Bering Eastern
by Thursday. This system weakens before phasing in with another
low pressure system which tracks into the Western Aleutians by
Friday. By Sunday the storm track takes this synoptic feature in
the North Pacific Ocean as it evolves into a complex low south of
the Eastern Aleutians. The Bering will have a brief break before
the next gale force low, and associated weather front enters the
Western Aleutian/Bering by Monday morning. This brings widespread
rain/stratus with gale force winds to the aforementioned region.
Meanwhile, the Southcentral region continues with drier conditions
with high pressure in the interior, and weak onshore flow
accompanied by warmer temperatures through upcoming weekend. By
Sunday evening a storm force low develops just east of Kodiak
Island. This brings storm force southeasterly to easterly winds to
the northern Gulf heading into Monday morning. Look for widespread
rain to occur mainly along the northern Gulf, and the coastal
communities.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...GALE...119 170 177 178 185.

&&

SYNOPSIS AND MODEL DISCUSSION...CC
SOUTHCENTRAL ALASKA...MC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...PD