Gale Warning 107, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
522 AM AKST Sun Feb 10 2019

.ANALYSIS AND UPPER LEVELS...
There is a surface low near Bristol Bay with a front bisecting
the Eastern Aleutians. This baroclinic zone extends southwestward
and into another surface low east of Japan. There is a dome of
high pressure over the Central Pacific that extends into the Gulf
of Alaska and Eastern Pacific. Aloft, there is an upper level low
over the Western Bering with multiple shortwaves embedded in the
cyclonic flow. The latest ASCAT/advanced scatterometer pass has
detected gale force winds in portions of the Bering and both gale
and storm force winds with the low east of Japan. The latest night
time microphysics pass on satellite imagery has an area of fog
and low stratus in the Copper River Basin and portions of Knik
Arm, a distinct cloud shield associated frontal boundary and the
Bering is mottled with tufts of cold air strato-cumulus clouds.
The polar jet arches over mainland and there is a second jet core
with an intensity of 150 knots just south of the Western and
Central Aleutians.

&&

.MODEL DISCUSSION...
The global models are virtually in lock step through 72 hrs with
the placement synoptic features, albeit with some differences in
low pressure and high pressure intensities. This pattern where the
longwave trough has dominated the western/central Bering and NW
Pacific has been going on for weeks now. The consistency of this
pattern may be playing a role in the performance of the models.
There are some minor differences with the timing of the onset and
duration precipitation and the p-type. The confidence level for
this forecast package is above average especially considering
that it is February in Alaska. The only potential fly in the
ointment will be the fog situation in Southcentral, specifically
the Anchorage Bowl.

&&

.AVIATION...
PANC...The possibility of fog continues to be the concern around
PANC. Fog has been observed at PAMR, at the NWS facility near PANC
and over Knik Arm on the latest microphysics satellite image. At
1255Z or 3:55 AM local the PANC observation recorded VV001 and
1/4 FZFG and the winds were calm. Low level southeasterly flow
will increase across the Kenai and Chugach Mountains from 12z-16z,
as well as along Turnagain Arm. Expect this should be just low
enough to erode the fog from the top down over the next few hours,
though some shallow surface fog could linger a bit longer. After
that VFR conditions will settle in.

A weak trough will approach Anchorage 06z-12z tonight which will
cause ceilings to lower. Don't expect more than flurries with
this, but if light snow did materialize would likely drop to MVFR
for a brief period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Monday night)...
Areas of dense fog which formed overnight from the Western Kenai
Peninsula to Anchorage and the Matanuska Valley will linger early
this morning. Strengthening low level southeast winds coming
across the Kenai and Chugach Mountains should be enough to erode
the fog from the top down over the next few hours. Meanwhile, a
strong short-wave tracking just south of Kodiak Island will
develop into a closed low this morning. It is in the favorable
left exit region of an approaching jet streak while low level cold
air advection is increasing baroclinicity. A building upstream
ridge will force this low eastward into the southern Gulf. The
cold advection on the back side will bring strong and gusty winds
to the western Gulf waters and Kodiak Island this afternoon and
evening. As far as precipitation, rain will fall across Kodiak
with rain and snow showers in the Prince William Sound region.

A trailing upper level short-wave over the eastern bering will
weaken as it pushes across Southcentral tonight. The most likely
place to see some light snow is the northern and eastern Susitna
Valley thanks to a little extra lift from low level southwest
flow. An upper ridge will rapidly build in once again behind this
trough, so may see some redevelopment of fog in the Cook Inlet
region. The upper level flow will remain progressive, with a weak
front arriving in Southcentral from the west Monday night. This
looks like another quick shot of light precipitation, with very
little in the way of snow accumulation.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Saturday
night through Tuesday)...

Two lows will impact the southwest coast over the next three
days. The first low is currently moving through the Aleutian
Islands and will impact the southwest coast tonight into
tomorrow. This low is the weaker of the two lows. It is
anticipated to bring rain and/or snow depending on temperatures
at your specific location in southwest Alaska, so be sure to
check out your local forecast. Also expected are gale force winds,
mainly in coastal areas around Bristol Bay.

The second low is a much stronger low that will move through the
central Bering. Due to its large size and strength, it will have
multiple impacts on southwest Alaska. The first and likely most
significant impact will be sustained gale force winds along
coastal areas of southwest Alaska. Before winds reach shore, a
front dropping fresh snow will (along with recently fallen snow
from the first low) will leave a loose snow pack to allow for
blowing snow to be an issue with these winds. As of now we are not
expecting visibility to reduce below a half mile due to warm
temperatures and southerly winds, which have a tendency to limit
the amount of blowing snow. However, this bears close watching.

The second issue also pertains to winds, which is coastal flooding.
A south to southwest fetch of strong winds has the potential to
create a coastal flood issue for the Kuskokwim Delta and Bristol
Bay. However, taking a look at today the current thinking (after
discussing with the ice forecaster) is that shorefast ice will
hold and protect the coast. This will mitigate impacts in the
Kuskokwim Delta, which will be the most exposed in this wind
direction. The Bristol Bay area will not see enough of a fetch
for long enough to be an issue. Therefore, not expecting coastal
flooding at this time. We will take another look though.

Lastly, freezing rain could be present as this system will be
sending warm air into southwest Alaska. Looking at forecast
soundings and temperatures today, it does not favor freezing rain.
Therefore, backed off on the freezing rain forecast to a chance
but did not discount it completely based on one model run.

All in all, a busy few days of forecasting for southwest Alaska.

&&

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

The first of two low pressure systems for the Being Sea is
currently pushing through the central and eastern Aleutians. It is
still forecast to bring gale force winds the Aleutians and Eastern
Bering as it moves east tonight into tomorrow.

The more significant low is still anticipated to enter the
western Aleutians tomorrow night. Forecasts are still going for
this to be a deep and strong low, with a tight pressure gradient.
Therefore, were are still expecting storm force sustained winds
with hurricane force gusts for the western Aleutians and Bering
tomorrow night into Monday. Seas will be correspondingly high with
this low...with wave heights over 40 feet.

The low holds together well as it tracks northeast through the
Bering Monday into Tuesday, maintaining storm force strength over
the central Bering while sending strong gales into the Pribilofs.
The storm will largely exit our forecast area midday Tuesday.

&&

.LONG TERM FORECAST (Days 3 through 7)...

The long-term forecast pattern, generally well agreed upon in the
models will feature a weakening front moving over southern
mainland Alaska Tuesday and Tuesday night causing mostly mountain
snow showers with no one particular area expected to see much in
the way of precipitation activity. Behind that, a seasonably cold
air mass will move into Southwest Alaska early on Tuesday and
spread across Southcentral by Tuesday night. This cold, dry air
will increase the strong westerly gap winds through the Barren
Islands, Passage Canal, and northerly offshore winds through the
Copper River Delta, Resurrection Bay and possibly Valdez marine
areas. Other than those windy conditions, a period of calm, quiet
weather is expected and the air mass should be dry enough that
areas of fog should be few and far between. A dying front will
approach the coast of Southwest Alaska Thursday evening, but
should dissipate entirely before making any more progress inland
than that. The cold air mass across inland areas of Alaska appears
likely to persist through Saturday, which will also keep the gap
winds going much of that time, especially in those areas with
openings to the Copper River Basin.

Elsewhere, the storm track for much of the long-term will remain
across the western and northern Bering, as strong lows move across
or near the Western Aleutians about once every 36-48 hours. The
first low moves well west of Shemya Wednesday morning, a second
low moves north over the Central Aleutians Thursday night, and a
third low impacts the western Aleutians Saturday night. Each of
these will have gales and potentially storms with them, along with
heavy rains. By Saturday, the models are having their usual
struggles by day 7, but all suggest another dying front will make
somewhat better progress than its predecessors at spreading
precipitation across Southwest Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 191.
 Winter Weather Advisory 155.
MARINE...Hurricane Force Wind Warning 177 178.
 	     Storm Warning 132 150 175 179 185.
	     Gale Warning 130 131 136 137 138 155 160 165
					  170 172 173 174 176 180 181.
Heavy Freezing Spray Warning 160 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PS
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...JW