Small Craft Advisory 846, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 AM AKST Mon Nov 6 2017

.ANALYSIS AND UPPER LEVELS...
The two main features currently impacting Alaska are a deepening
upper level low over the Southern Gulf and an amplifying upper
level ridge centered over the Western Coast. Further west an upper
level trough is digging south and east across the Western Bering.
A 150 kt jet at 250 mb is streaming north out ahead of this
trough while taking a sharp turn south over the top of the ridge
over northern Alaska. A 110 kt jet is diving south across Central
Alaska and down into the Gulf in between the upper level ridge to
the west and trough to the east.

Water vapor imagery shows dry upper level air diving south on the
front side of the ridge while being pulled into the low over the
Gulf. This is leading to rapidly clearing skies over southcentral
behind the low as the last few showers over Prince William Sound
taper off as the system pushes out of the area. Some fog and low
clouds are still lingering along the Chugach Front Range and over
the Cook Inlet but strengthening off-shore flow will rapidly clear
out the remaining low level moisture in the area this morning.
Strong off-shore flow is already developing across the coastal
regions in Southcentral as the upper level jet moves overhead,
including Valdez where winds are already gusting above 40 mph.

&&

.MODEL DISCUSSION...
Models are in good agreement regarding the intensity and placement
of the overall synoptic scale features across the region but are
struggling regarding specific details, especially with the outflow
winds across the Gulf Coast. The high resolution models
(WRF, HRRR, and NAMnest) are handling the strong gap winds much
better than the regional models like the GFS, NAM, and EC,
showing strong winds mixing down over Valdez today.

&&

.AVIATION...
PANC...Lingering fog and low stratus hung up along the Chugach will
diminish this morning as offshore flow takes over, keeping
conditions VFR through the day. Strong northerly winds will pick
up later in the morning through the evening, with gusts expected
to reach over 30 kts in the afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Radiational cooling last evening into the overnight hours allowed
surface-based inversions to form over most inland locations of
Southcentral (as seen in the 12Z Anchorage sounding). The inversion
has been strong enough to keep winds from mixing down to the surface.
However, mountain-top observations along with the Talkeetna wind
profiler show strengthening and lowering of stronger winds. As
arctic air continues to advect down from the north today and
surface pressure gradients tighten, expect the winds to mix out
the temperature inversions and bring widespread gusty winds to
Southcentral.

The strongest winds will be along the coast as the arctic air
pushes over and down the coastal mountain ranges. The High Wind
Warning for Valdez up to Thompson Pass looks on track with
favorable conditions for mountain wave activity to produce strong
erratic wind gusts. Did shorten the warning (now ending at 6pm)
since the upper jet is progressing eastward more quickly than
originally forecast.

Winds will die down overnight tonight for much of the area as an
upper level ridge builds overhead. The presence of deep Arctic
air in the Copper River Basin combined with persistently tight
pressure gradients surrounding the basin will keep gap winds going
a little longer along the Matanuska River, Thompson Pass down to
Valdez, as well as out of the Copper River Delta. Temperatures
will drop quickly where winds die down.

Benign weather will then settle in through Tuesday night under the
influence of the upper ridge. A weak short-wave trough crossing
Southwest Alaska Tuesday night will skirt Southcentral, likely
bringing no more than a few clouds. As surface inversions set back
up Tuesday night expect will see fog redevelop, so have
introduced it into the climatologically preferred locations of
northern Cook Inlet and the Copper River Basin.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A ridge along the Kuskokwim Delta persists through tonight
then shifts east as the next front pushes to the west coast. Fog
will be the main concern across the Southwest through Tuesday
morning as a stable air mass traps moisture at the surface.
Northerly flow along the Kuskokwim Delta and Bristol Bay tapers
off this evening as the ridge axis moves overhead which will allow
patchy fog to become widespread across the region. Dense fog is
expected across the Kuskokwim Delta where the air mass is most
saturated.

Fog conditions improve on Tuesday as a front moves inland from
the Bering on an eastward track. At the onset of this event
temperatures will start out warm enough to support rain, however
precipitation changes over to snow as colder air advects along the
front. This system may bring a few inches of snow across the
Kuskokwim region, however the front diminishes rapidly as it out
runs the surface low which tracks across Kamchatka. Additional
shortwaves keep higher chances of snow in the forecast through
mid week.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A front across the western Bering and Aleutians quickly swings to
the central waters this afternoon. A brief period of storm force
winds develop across the central and northern Bering along the
frontal boundary, elsewhere gusty gale winds and rain can be
expected. Westerly flow and cold air advection follow the front
with mixed precipitation continuing across the Bering as an upper
level low over Kamchatka sends disturbances eastward.

&&

.LONG TERM FORECAST (Days 3 through 7: Wednesday through
Sunday)...

Offshore flow along the northern Gulf Coast will subside Tuesday
night as a transient ridge moves overhead. To the west however, an
approaching upper-level wave and a strengthening surface pressure
gradient will support increasing northwesterly outflow winds
along the Alaska Peninsula into the western Gulf Wednesday and
Thursday. Clouds and the chance for some very isolated snow
showers returns to Southcentral Thursday as a deepening trough
over the mainland sends a series of shortwaves southeast. Thursday
night, the trough will further amplify and a surface low will
develop in the northeastern Gulf. As the upper trough and the
surface low shift southeast, deep northerly flow will re-
establish itself over the Mainland. This setup supports the
potential for another strong outflow wind event along the northern
Gulf Coast towards the end of the week. In addition to the
wind potential towards the end of the week, expect temperatures to
rapidly cool Thursday night through Saturday over much of the
Mainland as an Arctic airmass is pulled southward on the backside
of the deep trough.

Gusty, showery weather will continue through Thursday over the
Bering Sea as the atmosphere continues to destabilize in response
to cold air advection. A ridge will build in Thursday night,
initiating a drying trend through the end of the week over the
Bering Sea.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 131.
MARINE...Gale 126-128 130-132 136 138-140 150 173 175-179 181.
Storm 185 351 352.

&&

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