Small Craft Advisory 854, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 AM AKST Wed Nov 8 2017

.ANALYSIS AND UPPER LEVELS...
Current analysis shows an upper level ridge axis over the ALCAN
border continuing to move off to the east while a broad upper
trough is located over the northwestern part of the state. A weak
shortwave trough is moving east across southcentral but is rapidly
weakening as the upper level support pulls northeast and runs into
the ridge to the east. Satellite imagery shows a thin and nearly
stationary trough with associated cloud cover extending south into
the southern Gulf which is quickly shearing apart. With the
exception of a few high clouds, skies are mostly clear across
the eastern domain leading to cold temperatures this morning. In
fact, temperatures in the Copper River Basin have dropped into
the negative teens.

Out west, zonal flow is dominant across the Bering though a few
weak embedded shortwaves are leading to an area of rain and snow
showers moving east across the Kuskokwim Delta.

&&

.MODEL DISCUSSION...
Overall, models are in good agreement through the much of the
short term. The biggest differences revolve around the system
pushing through southern portions of the state on Thursday,
especially as the frontal boundary makes its way across the Alaska
Range and into southcentral. The GFS shows the precipitation band
falling apart quite a bit before reaching populated areas across
southcentral by Thursday afternoon while the NAM and EC show the
front maintaining more of its structure leading to slightly higher
snowfall amounts over the region.

&&

.AVIATION...
PANC...VFR conditions and light winds will continue through
the TAF period.

&&

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

The short term in Southcentral will see a progressive pattern
with a weak trough moving quickly to the east with zonal flow
over the area today. A sharp trough will move in behind it
tomorrow, and then another ridge will be moving into the area
Friday.

The biggest forecast question will be if any snow will be
associated with Thursday's upper level trough. While the trough
should have enough lift associated with it, the moisture will be
somewhat limited. It is also moving eastward at a pretty good pace
which also is a limiting factor. However, newest model runs are
continuing to increase POPs and QPF values somewhat and most of
the elements are there for a quick shot of snow tomorrow afternoon
in the Anchorage and Mat-Su areas as well as the Kenai Peninsula.
Even so, amounts are not expected to be very high with the
limited moisture and speed of the trough.

After this trough a low will develop along the northeastern Gulf
coast, ushering in gusty northerly winds along the Gulf coast on
Friday and continuing into the weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An upper level disturbance propagates eastward across the
Southwest mainland through this afternoon bringing light snow to
the Kuskokwim Valley and Bristol Bay. As the weak front associated
with this system nears the Alaska and Aleutian Ranges this
afternoon, precipitation may mix with or change over to rain as
temperatures moderate to near or just above freezing.

The next system to impact the region moves in this evening from
the Bering bringing rain and snow into Thursday. A front
organizes across the central Bering as a strong short wave moves
from the base of an Arctic low this afternoon. The front moves
across the Kuskokwim Delta this evening and continues on an
eastward track. Locations along the southwest will be warm enough
initially to support rain as a warm wedge of air slides overhead;
however, a colder air mass advects along the front changing
precipitation over to snow. There is still some uncertainty on
snow amounts as this is a rapidly developing system over the
Bering where observations are sparse.  If the front should move
across slower than expected higher amounts would be possible
across west facing slopes.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

Gusty westerly flow prevails across the Bering today as a nearly
zonal pattern is squeezed between low pressure over the Arctic and
a ridge just south of the Aleutians. The low swings a strong short
wave, currently across the western Bering, toward the Southwest
mainland. Gusty gale winds and rain are expected to develop along
the front which will stretch from north to south over the entire
Bering as it passes on an eastward track. Cold air advection
across the northern waters will change precipitation over to snow
along and north of Saint Matthew Island this afternoon. The front
moves across to the Alaska Peninsula tonight and then into the
western Gulf Thursday morning. A few showers will be possible
along short waves moving around the ridge which builds across the
Bering in the wake of the front. A warmer subtropical air mass
associated with the ridge modifies temperatures above normal into
the beginning of the weekend keeping light precipitation as rain.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The numerical models are in very good agreement for the long term
portion of the forecast, therefore the confidence today is higher
than normal. The basic pattern is anchored by a long wave ridge
that is negatively tilted from about 170 degrees west longitude
near the Aleutians to west of 180 in the northern Bering Sea. A
downstream long wave trough will reside over eastern Alaska into
the Yukon territory. Several short wave troughs top the ridge and
drop northwest to southeast across the state during the forecast
period. The strongest of these drops through on Sunday. The WPC
guidance uses a nearly equal blend of the GFS and ECMWF. The
forecast incorporates a considerable portion of the deterministic
solutions along with the ensemble means, especially in the early
periods.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales: 130 131 165 170 175 177 179 180 181 185 412 413 414.

&&

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