Small Craft Advisory 871, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
642 PM AKST Tue Nov 6 2018

UPDATE TO LONGTERM DISCUSSION

.ANALYSIS AND UPPER LEVELS...

An upper level ridge continues to drift eastward into the Yukon
Territory while behind it, a broad upper level trough positioned
over the Bering is beginning to move over the northwest part of
the state. Anchoring this trough is a 994 mb surface low situated
just west of Norton Sound. In response, southerly onshore flow is
present across most of Southwest Alaska, which is producing
scattered precipitation and low level stratus.

Looking southward, a North Pacific low has advanced into the Gulf
of Alaska. At the surface and ahead of the low, a leading front
with strong southeasterly winds is gradually lifting to the
northeast.

&&

.MODEL DISCUSSION...
Models are in good synoptic agreement into the latter half of the
week. It is worth noting that the low moving into the Gulf has a
subtropical connection and there will be warming at low levels due
to this. The different models are varying on how warm it should
get around and near the Gulf. The NAM was the preferred model
overall for this low.

&&

.AVIATION...
VFR conditions and northerly winds no more than 10 kt should
persist into Wednesday afternoon.

&&

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

A synoptic pattern shift occurs during the next 24 hours, with
the once dominant ridge across the mainland, shoving off to the
east as a progressive trough takes over. This change brings increased
chances for precipitation across the southern mainland along with
challenges in precipitation type and gale force winds for the
Gulf.

The storm system currently south of Kodiak Island moves a front
along the southern gulf to the northern waters late tonight.
The main issue causing difficulty in defining precipitation type,
is that this warm core system stalls along the eastern Kenai while
a colder core system advances from the west. Precipitation types
will be dependent upon the position of the boundary between them.

Wednesday night through Thursday isolated to scattered showers
will move inland as short waves move ahead of the gulf low. Mixed
precipitation will be possible across the diffuse boundary which
is expected to be stretched along the northern Kenai to the
Chugach Range. Even though there is still uncertainty with the
evolution of this system, accumulations are expected to be light.
The highest snow accumulations are expected along higher elevations
of the northern Kenai to Western Prince William Sound, however narrow
bands of precipitation are possible.

By early Thursday morning the colder air mass from the west
settles across the region changing precipitation types over to all
snow.

&&

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

A stalled front draped from Dillingham to Sleetmute will continue
to produce areas of light snow across the southwest interior
through the overnight hours. Closer to the coast, any lingering
precipitation will be in the form of rain showers as the warm,
onshore flow behind the front has increased surface temperatures
into the upper 30s. A deformation zone setting up between the
stalled boundary and a low moving north from the western Gulf will
act to enhance precipitation over the southwest interior later
tonight into Wednesday. The precipitation may begin as a mix of
rain and snow, but change over to all snow as the atmosphere
saturates and colder air begins to wrap into the back side of the
low. Light snow accumulations are possible for locations well
inland such as Sparrevohn and Sleetmute. Conditions then trend
colder and drier Wednesday night into early Thursday as the high
crests over Southwest Alaska. Areas of fog are possible overnight
Wednesday into early Thursday as the stable airmass settles
overhead. A second round of strong winds along the coast and areas
of light snow comes Thursday as another front swings through the
region. This front is fast moving and will weaken as it moves
onshore. Nonetheless, light snow accumulations (1 to 2 inches) are
possible for locations such as Dillingham and Bethel for Thursday.
Snow showers will linger through Thursday night before tapering
off on Friday. Behind the front, high surf and areas of coastal
erosion are possible Thursday night through Friday as prolonged
southwesterly flow with gales develops along the coast behind the
front.

&&

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

The first of two strong, Bering lows is currently north of Saint
Matthew Island and will continue to track to north-northeast away
from the forecast area. As it does, the gales and high surf
currently along the coast from Toksook Bay north will diminish.
Underneath the low, a cold and unstable airmass will continue to
produce a few scattered showers overnight from the Pribilofs on
north. Upstream, a ridge of high pressure currently building over
the western Bering and Aleutians will slide east, with the ridge
axis reaching the eastern Aleutians Wednesday. This ridge will
continue to track east and move over west coast Wednesday night in
response to the next Kamchatka low and associated front moving
into the western Bering. Southerly gales will develop along the
front as it slides over western Aleutians Wednesday morning. The
front and gales will propagate east over central Bering and
Aleutians Wednesday afternoon and evening and over the eastern
Bering Thursday morning. The Kamchatka low will also strengthen as
it tracks into the western Bering Wednesday night. As it does,
a large area of westerly gales will overspread much of the Bering
with a smaller area of storm-force situated on the southern
periphery the center of circulation. This low will be of similar
strength and take a similar track to the now departing low,
creating a prolonged westerly fetch across the Bering. Given
this, high surf and coastal erosion are possible from the
Kuskokwim Bay north to Hooper Bay late Thursday through Friday as
the center of circulation makes its closest approach.

&&

.LONG TERM FORECAST (Days 3 through 7)...

An active pattern will be on tap for the Bering, Aleutians and
Southwest AK Friday to Tuesday. The longwave pattern features
trough bisecting the Bering and a high amplitude ridge from SE AK
to the Yukon Territory. At the surface there is a low East of
Kamchatka and another low south of Kodiak. As time elapses, this
energy will shift eastward. In the beginning of the period there
is good clustering between the latest runs of the
GFS/ECMWF/Canadian Regional. The latest runs of the GFS and ECMWF
have converged compared to the solutions from 24 hrs ago. However,
given the vagaries models past 96 hrs, blending and hedging is
still required. Saturday looks like it will be driest day this
forecast period for the Anchorage metro area.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm 411. Gales 119 120 131 132 150 177-179 185 351 352
412 413.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
LONG TERM...PS