Heavy Freezing Spray Warning 93, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
400 PM AKST Sat Dec 10 2016

.ANALYSIS AND UPPER LEVELS...
A strong closed upper level high over the northern half of Alaska
is building southward toward Southcentral Alaska, with strong
subsidence and very dry air, as seen in Water Vapor Imagery and
upper air soundings. Tight pressure gradients combined with arctic
air sitting over the interior continues to produce gap winds
across Southcentral, strongest through Thompson Pass and the
Matanuska Valley. Cold and dry conditions dominate both
Southcentral and Southwest Alaska under mostly clear skies.

Meanwhile, a couple of surface lows and an associated occluded
front are elongating and weakening along the Western Aleutians,
with forward motion being stopped by strong ridging just ahead of
it. The central pressure of the low is quite unimpressive at
around 998 mb. However, the large pressure difference with the
surface high over the Bering Sea is leading to solid gale force
winds out ahead of the front.

Lastly, an upper trough over the Eastern Bering is stuck between
strong ridging both upstream and downstream. Weak lift along this
axis is producing some light precip, primarily along the southern
Alaska Peninsula.

&&

.MODEL DISCUSSION...
There are some minor differences with synoptic scale features
affecting the Bering Sea and Aleutians the next couple days,
but nothing which really affects the expected weather. One
noteworthy challenge for Southcentral is determining where gap
winds will persist and where they will die down. With such a
cold air mass in place this will lead to very large differences
in temperatures between windy and non-windy areas. Otherwise,
overall forecast confidence for the next couple days is high.

&&

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

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
With high pressure over mainland Alaska and weak low pressure over
the Gulf, weak outflow winds will continue but diminish through
Sunday. The high pressure will also keep skies clear around
Southcentral Alaska which will allow for temperatures to drop
overnight. This will help with fog formation the next couple of
days.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Partly cloudy skies will clear overnight as the interior ridge
shifts south. Northerly flow is expected to be light overnight
with the settling ridge which will allow for patchy fog to develop
toward morning as steep low level temperature inversions set up
just off the surface. The one limiting factor to morning fog is
the dryness of the air column inland. The sounding at Bethel this
afternoon sampled a much drier column in comparison to this
morning's data. Subsidence on the southern edge of the advancing
ridge may dry out the air mass significantly across much of the
Southwest preventing fog development.

The general flow remains relatively light through Monday across
inland areas. Along the coast, winds increase out of the east late
Sunday with gusty conditions along the Kuskokwim Delta as a warm front
approaches from the Eastern Bering. The front washes out as it
makes landfall on Monday morning...however, a few snow showers are
expected to develop along the coast as the warm subtropical air
mass overruns the cold dome over the Southwest.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Southerly flow returns across the Bering as a warm front along
the Aleutians advances north and east into Sunday. Winds are
expected to increase tonight along the Aleutians and Alaska
Peninsula as the surface low driving the front advances north
across the West Bering. Re-enforcing energy from the parent low
moving toward the western Aleutians keeps a gusty pattern across
the Bering through Monday. Snow and mixed precipitation with this
front changes over to all rain with the frontal passage which
should make it through the eastern and northern Bering by Sunday.

&&

.LONG TERM FORECAST (Days 3 through 7...Fri through Tue)...
Monday night will begin a transition period to a stormy regime
over the Bering Sea as the cold Siberian high reestablishes
itself. A blocking high over Southcentral will weaken Tuesday as
an undercutting Bering shortwave trough moves across the area
bringing some rain and snow to Southwest Alaska...and clouds to
Southcentral. After Thursday, uncertainty grows with respect to
another shortwave trough moves through the ridge with the GFS much
more bullish than the ECMWF/Canadian GDPS. That uncertainty is
already considered in this forecast which calls for increasing
chances for precipitation late week. Thereafter, the possibility
for increased storminess and multiple precipitation chances
increases as the ridge over Southcentral shifts east and weakens,
allowing for a series of shortwave troughs in southwest flow to
impact the southern mainland through the weekend. This should
usher in a return to above normal temperatures along with an
active weather regime across the forecast area.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 127.
 Heavy Frz Spray 126 127 181.

&&

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