High Wind in Alaska, November 6, 2017

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

.ANALYSIS AND UPPER LEVELS...
The two main features currently impacting Mainland Alaska are a
low complex over the southern Gulf and an amplifying upper level
ridge centered over the Western Coast. Strong outflow conditions
along the North Gulf coast and Eastern Kenai Peninsula are
beginning driven by increasing pressure gradients along Gulf
coast, increasing thermal gradients with cold air building over
the Copper River Basin, and favorable (northerly) upper wind
support. Elsewhere, gusty local terrain generated winds are
evident inland across Southcentral.

Further west, subsidence under the strong upper ridge is
producing areas of low clouds and fog across the Delta but
generally fair skies inland. A long wave trough and associated
frontal zone is evident across the western and central Bering Sea.
Gusty south winds and rain were observed along the frontal
boundary.

&&

.MODEL DISCUSSION...
Models are in good agreement in the overall synoptic pattern
through the short term. Differences occur with local wind effects
across Southcentral and precipitation types over western Alaska.
Generally will be leaning toward the high resolution models for
details.

&&

.AVIATION...
PANC...Gusty north winds will diminish overnight. VFR conditions
should prevail through the TAF period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Monday night through
Wednesday)...

Strong "outflow" winds across southern Alaska are likely at their
peak right now. Beginning this evening, the low in the Gulf will
be moving away from the area and the cold air/high pressure over
the interior portions of the state will also be shifting eastward
and away from the area. Winds will diminish at all locations, but
will likely stay gusty a bit longer over Valdez/Thompson
Pass/Copper River Delta but will still be dropping significantly
after midnight tonight.

With warmer air at mountaintop level moving into the area from
the west, but continued clear skies allowing radiational cooling
across the area, we are still expecting the thermal profile of the
atmosphere to become increasingly favorable for fog formation,
especially around upper Cook Inlet. It often takes a day or so
after a wind event for the different layers of the lower
atmosphere to become stable enough for fog formation, so at this
point we're not thinking much more than patchy fog will have a
chance to form by Tuesday morning, but the Tuesday night into
Wednesday morning timeframe looks more favorable for not only more
widespread fog but potentially thicker/lower visibility fog as
well.

On Wednesday, a weak (and weakening) front will try to move into
the area from Southwest Alaska, but at this point it still looks
unlikely to produce anything more than an increase in clouds and
potentially a few flurries, especially north in to the Susitna
Valley.

&&

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

The main forecast challenge this afternoon lies with any fog
development tonight. This afternoon's visible satellite imagery
and surface observations show no fog currently present. A
shortwave ridge is shifting east across the region, ahead of a
weakening front pushing ashore along the western coast. However,
there's no real thermal ridge seen in the 850mb fields to suggest
a strong inversion will be in place. Overhead stratus/cirrus will
also mitigate the radiational cooling/fog formation process. On
the flip side, the approaching front will bring more moisture to
the region from west to east, so we maintained a patchy fog
mention while improving visibilities in the forecast zones.

As for precipitation, surface convergence along the front will
become less and less with time, but, there is just enough forcing
for some light precipitation to persist. Precipitation type will
be tricky near coastal locations as the marine layer modifies the
low-level environment. Initially we expect mainly rain south of
the Kuskokwim Delta before changing over to a rain/snow mix, then
all snow. Further north along the coast a quicker transition is
expected. Inland the precipitation will be all snow. Snowfall
accumulations will generally be less than one inch area-wide.

For the remainder of the week, a generally zonal pattern will be
in place, with some potential for episodic snow shower development
as weak impulses aloft cross the region.

&&

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

This afternoon, a weakening cold front stretched from west of
Saint Paul Island to the central Aleutians west of Dutch Harbor.
This front will continue its eastward journey through the next 24
hours while continuing to weaken. A corridor of rain showers will
precede frontal passage, with rainfall amounts mainly in the 1/10
to 1/4 of an inch expected. A surge of warmer air at 850mb will
also allow for some patchy fog development across this region.

Gale force winds along and immediately west of the front will
lift off to the north. Another 993mb low moving along the eastern
Kamchatka Peninsula will send an associated surface front into
the western Bering overnight tonight and into Tuesday, with Gale
Force winds a good bet. Similar to its predecessor, the upper
support will lift north of the region, with a weakening front/
diminishing wind field as it reaches a western coast of Southwest
Alaska to Cold Bay line by Tuesday evening. With a zonal pattern
in place through the end of the week, look for sub- Gale winds to
persist.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The numerical models are in excellent agreement for the long term
portion of the forecast, therefore confidence is higher than
normal. The basic pattern is anchored by the long wave ridge along
about 170 degrees west longitude, and a long wave trough 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 WPC guidance uses an equal
blend of the GFS and ECMWF, incorporating considerable amounts of
the deterministic solutions along with the ensemble means.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 131 until 6pm
MARINE...Storms: 352
Gales: 128 130 132 138 139 177 178 181 185 351 411 412 413

&&

SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...PS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...BL