Small Craft Advisory 879, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
509 PM AKST Thu Nov 8 2018

.ANALYSIS AND UPPER LEVELS...
The mainland of Alaska is currently under the influence of a
very baggy long-wave trough that extends from the Arctic southward
to the Aleutians. One embedded short-wave is currently departing
Southcentral Alaska, while, further to the west, a much deeper
system is impacting the Bering Sea and southwest mainland. The
upper jet is feeding into the long-wave, streaming across the
Aleutians. A secondary short-wave trough is digging on the
backside of the long-wave.

The system currently in the northern Gulf is weakening this
afternoon as it moves east, pushed by the building ridging ahead
of the next system. Despite light RADAR returns over Anchorage and
the Matanuska Valley all morning long, moisture was not able to
overcome the dry air at the surface, resulting in only
flurries/very light snow over eastern Anchorage and the Hillside.
However, closer to the actual surface low pressure, Cordova and
Valdez have experienced rain and snow, respectively.

The front associated with the Bering Sea system is affecting the
southwest coast; southeast winds are approaching gale-force.
Behind the front, a very convective-looking atmosphere exists with
plenty of cellular clouds, but not much has been reported along
the lines of gusty winds or showers.

&&

.MODEL DISCUSSION...
Guidance is in good agreement with the large-scale pattern for the
next few days. Consensus is that digging short-wave in the Bering
trough will become the new long-wave center, while at the same
time phasing with a piece of energy cut off in the sub-tropics.
The details are still hazy but there looks to be a strong front
coming into the region by the weekend.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

An area of low pressure continues to spin near Middleton Island,
bringing areas of rain and snow to locations east of a line from
the eastern Kenai through Paxson. Low-level dry air won the battle
overnight for locations to the west of here, as this mornings
sounding showed a wedge of dry air remained below 4000 feet. Look
for the precipitation tonight to gradually decrease in coverage
and intensity as it shifts east.

Our attention then turns to the next storm heading north towards
the eastern Aleutians/AKPEN. This storm will be rapidly deepening
as it heads north sending an occluded front towards the Gulf and
Southcentral. A deep moist fetch will precede the front, as the
moisture originates from near Hawaii. Current forecasts show
precipitable water values of 4 to 6 standard deviations above
normal. Needless to say, this system will have plenty of
precipitation with it. So the obvious next question is
precipitation type. Right now its too early to pin down who gets
what and how much, but, one thing is for sure. For locations near
the coast (if not further north) it'll warm up as the models show
850 mb temperatures increasing from the -8 to -10 range to +5 deg.
C in a 36 hour period. With such a strong 850mb front,
precipitation will be likely into at least the southern third to
half as Southcentral, even if the surface front hangs up near the
coast (looks like a strong barrier jet will develop, as an over-
running setup develops. Stay tuned !

&&

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

A very active pattern is underway as an occluded front is rapidly
moving inland across Southwest this evening. The front will weaken
as it moves inland, but expect a burst of snow for inland areas as
the front moves east tonight. Rain and show showers will follow
behind the front mainly along the coast as instability driven
showers move in with increasing onshore flow.

Ridging aloft will dry the region out Friday night before the next
round of active weather begins as a Pacific low moves a powerful
warm front into Southwest. This will bring threats for snow along
with a mix of rain, but some areas may see decent accumulations
east of King Salmon and west of the Western Capes Sat and into
Saturday evening. Along with E-NE winds of 35-50 MPH, this may
result in quite a bit of blowing snow as it falls which may
dramatically reduce visibility. Strong offshore winds will slowly
diminish Sunday as the front lifts north of the region but will
still remain gusty.

&&

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

A powerful storm force low is moving across the western Bering Sea
with the associated occluded front moving inland across the
Southwest Alaska coast. Seas near the core of the low are in
excess of 35 feet, but these winds and seas will dramatically
diminish by Friday morning. Thereafter, broad cyclonic flow will
develop over the Bering Sea with gales over both the western and
eastern Bering Sea before the next powerful storm force low drives
into the eastern Bering Sea on Saturday then moves over into the
central Bering by Sunday, reinforcing the stout northerly cold air
advection winds over the western half of the Bering.

&&

.LONG TERM FORECAST (Days 4 through 7/Sun night through Thu)...

The longer term forecast starts out in a very active pattern late
Sunday with a large and complex low centered in the Eastern
Aleutians with a secondary center along the front in the western
Gulf of Alaska. There is a cull between these lows but a tight
pressure gradient along the front curving out of the Gulf of
Alaska...into Southcentral and Southwest Alaska and then through
most of the Bering Sea. The main low will take a round about route
into the Gulf of Alaska through the middle of the week and then
into Southeast Alaska Thursday. While this is a rather complex low
and track, the good news is that there is fairly good agreement
on models, especially in the synoptic features, which brings
increased confidence in the main elements of the forecasts.

This pattern is bringing some very warm air to the southern mainland
and southeast Bering Sea for the start of the week. Therefore there
will be some windy and rainy areas which then will transition
back to snow showers with cooling temperatures aloft. The cold air
which was displaced to the northwest by this system will move
back over the region quickly in the Monday to Monday night
timeframe as the low further occludes and moves eastward. By next
Thursday, the mainland part of the state will once again be under
an upper level trough with a ridge over the central Bering Sea
leaving the area in generally dry pattern with seasonal
temperatures.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Surf Advisory public zone 155.
None. MARINE...Storms 185
412. Gales 175-181 351 411 412 413.

&&

SYNOPSIS AND MODEL DISCUSSION...ML
SOUTHCENTRAL ALASKA...PD/SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...EZ