Small Craft Advisory 1042, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
519 AM AKST Fri Dec 30 2016

.ANALYSIS AND UPPER LEVELS...

Eastward moving upper ridges are over the Bering and Alcan this
morning, with upper troughs along the west coast of mainland AK
and over the Kamchatka peninsula. At the surface, a trough of low
pressure is over the Kenai peninsula and northwestern Gulf of AK.
Low pressure centers are over the extreme northwest Bering and
south of the western Aleutians. A decaying weather front with
significant snowfall is moving rapidly across southcentral AK.
Frontal systems are also associated with the northwestern Bering
low, as well as the low aimed at the western Aleutians. All of these
systems are bringing gale and storm force winds to the Aleutians
and Bering, as well as the northwestern Gulf of AK.

&&

.MODEL DISCUSSION...

Models remain in good synoptic agreement, with timing differences
due to the way mesoscale events are being handled. The forecasts
utilize the GFS, with some NAM for details in the east.

&&

.AVIATION...
PANC...IFR conditions will persist early this morning. As the
front continues east conditions will become marginal by mid
morning. Lingering clouds will finally scatter out late in the
afternoon and remain VFR with light winds into Sat morning.

&&

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

Widespread snowfall continues this morning across much of
Southcentral, but will begin to slowly taper from west to east as
a cold front continues to push to the southeast. An upper level
jet of 140 kts is supporting a strong potential vorticity anomaly
and impressive downward vertical motion behind this system. Gusty
winds are expected to form as this potential vorticity anomaly
pushes southward across the North Gulf coast, with the strongest
gusts expected across Prince William sound. A combination of
cooler behind this front and clearing skies will help
temperatures to drop tonight. Surface high pressure will build in
through Saturday and support clear skies and gusty offshore winds.

&&

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

A cold upper level trough swinging into Southwest Alaska this
morning will bring increasing west to southwesterly winds and
falling temperatures. Isolated to scattered snow showers will
track onshore through the morning then taper off this afternoon.
Snow showers in the mountains will be more numerous and
persistent especially along west facing slopes. An upper level
ridge will build in over Bristol Bay and the Southwest Alaska
coast tonight and then shift inland Saturday. Ridging will
continue to amplify over mainland Alaska through the weekend,
stalling incoming Bering frontal systems along the Alaska Peninsula
and Kuskokwim Delta coast.

&&

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

A rapidly strengthening low to the southwest of the western
Aleutians will track north along 170E deepening to around 950 mb
off of the Kamchatka Peninsula Saturday. The associated front will
merge with another frontal wave well to the south of the central
Aleutians combining into a rather strong front over the central
Aleutians tonight and then surging north across the Bering
Saturday. This front will bring a large area of storm force
southerlies to the central Aleutians and Bering as well as high
winds to the Pribilof Islands. The front will stall and weaken
over the eastern Bering Saturday Night and Sunday as it runs into
the large upper level ridge amplifying over mainland Alaska. The
next low will cross the Aleutians near Kiska on Sunday and
continue north through Bering along 180.

&&

.LONG TERM FORECAST (Days 3 through 7/Friday Night through Wednesday)...

Forecast guidance managed to take a shift from previous days.
Over the weekend a ridge of high pressure will build in across
most of south central Alaska. Guidance settled on a slower
progression of the ridge and the weekend across the area is
expected to be met with bouts of clouds and sunshine. A weak
wiggle in the flow is showing some signs of bringing light snow to
the Western Alaska Range Sunday evening and into Monday, but the
current trend keeps backing off on those chances.

The blocking ridge across southcentral will lead to a continued
storm trajectory across the Western Aleutian chain. The first in a
series of storms will slide north along the date line and occlude a
front that will stall out across the Central Aleutians. The ridge
of high pressure will slowly build to the west and eventually span
much of the state leading to a quiet mid week across most of the
state.

It looks like there will be a break between systems early next
week which will allow winds and seas to abate, but the pattern
will remain favorable across the Western Bering for strong systems
moving up from the Pacific as we head through next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisories/Warning 101 111 141 145
High Wind Watch zone 195.

MARINE...Storm Warning...170 172 173 174 175 176 179 180 181 185 Gale
warning...119 120 121 125 130 131 155 165 171 177 178.

&&

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...ELN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...SS