Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 211421
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
521 AM AKST Thu Feb 21 2019

.ANALYSIS AND UPPER LEVELS...

The main feature is a long wave trough sitting over the western
Gulf of Alaska. At the base of the trough, a negatively tilted
vertically stacked low spins near Middleton Island. The jet
stream is fairly zonal, but satellite imagery depicts a
developing jet streak northeasterly aligned over the central
Gulf. In the mid-levels, cold air is wrapping around the low,
while warmer air is pushing into the Gulf on the leading edge of
the trough. This cold air is also advecting over the Bering and
western mainland. Within the flow around the low, westerly
flow dominates the western mainland, while general southeasterly
flow is present along the northern Gulf coast. A second upper-
level low is advancing from Kamchatka towards the western
Aleutians. Between the two lows, a ridge is building over the
central Aleutians and Bering.

In the western Gulf, the surface low is moving eastward along the
coast line. Precipitation is occurring throughout Southcentral,he
concentrated in the northern Gulf coast. The snow that fell
overnight in the greater Anchorage are and Kenai Peninsula has
tapered off this morning. In the early stages, southeasterly flow
through Southcentral created Turnagain Arm winds overnight.
However, those winds have since ceased and become more northerly.
The Kamchatka surface low, that is just off-shore of Shemya, is
pushing a strong storm force front over the western Aleutians.

&&

.MODEL DISCUSSION...

Models are in good synoptic agreement through the period. Models
have locked onto a track with the low in the northern Gulf, and
the track/strength of the Kamchatka low. The main model
differences arise specifically with wind speeds in the Bering
associated with the Kamchatka low. The American models are
holding onto stronger winds, while the EC/Canadian are weaker
through Friday. The next feature, a North Pacific low moving over
the Aleutians on Friday morning, is well handled until it moves
into the central Bering Friday night.

&&

.AVIATION...

PANC...Northerly winds with a few higher gusts will continue into
the morning, along with VFR conditions as the snow has ended.
High pressure moving in later this morning will cause skies to
gradually clear.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Today and
Friday)...

As the last of this underwhelming snowfall event ends from west to
east across Southcentral this morning, the forecast focus will
turn from snow to winds as a modestly colder air mass moves into
Southcentral. The upper level support for the snow this morning
ended up tracking a bit further south than expected. This resulted
in most of the moisture and heavier snow staying further south, so
snowfall amounts were lighter than anticipated in most areas.

As the aforementioned colder air mass settles in to Southcentral
later on this morning, the cold air will be squeezed through the
mountain passes and gaps. This will result in stronger westerly
and northerly winds through the passes. The strongest winds are
expected through Thompson Pass and Valdez today. Winds could gust
up to 55 mph in the town of Valdez later on this afternoon. Gusts
through Thompson Pass will be in the 60 to 70 mph range into
Friday morning. Peak wind gusts of 40 to 50 mph are forecast
through Seward and Whittier.

Elsewhere across Southcentral, the strong offshore winds will
help clear out the cloud cover by this afternoon for most areas
making for sunny skies to end the day. The clear skies will
persist into Friday, though some additional clouds are possible
Friday afternoon as a dry front moves through. Fog will become
increasingly common each night starting Friday night as
temperatures aloft warm with the approach of a strong upper level
ridge.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Thursday
through Friday night)...

Lingering snow showers across the Southwest coast will eventually
taper today as a weak ridge over the eastern Bering advances
inland. The weather pattern will change by the evening as an
advancing front, associated with a Bering low, pushes eastward,
bringing another chance for precipitation. By Friday morning,
coastal areas along the northern Kuskokwim Delta may see gusty
southerly winds resulting in brief periods of blowing snow.
Initially, temperatures will be cold enough for most precip across
Southwest to fall as snow. However, a second system moves into
the Bering Friday night, bringing strong southerly flow. With
warmer temperatures and ample moisture, this will allow any
remaining precipitation, especially along the AKPEN and Southwest
coast, to change over to rain. Gusty southerly winds along the
Kuskokwim Delta coast, including Nunivak Island, are expected
through Friday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Thursday
through Friday night)...

A storm force low centered just west of Attu is bringing strong
southerly flow across the western and central Aleutians.
While some areas across the Bering will see a rain/snow mix with
this system, most precipitation will fall as rain as warm air is
advected northward. This trend will continue through the
afternoon as the associated front advances eastward across the
Bering. A wide swath of south to southwest gales will persist
until Friday morning.

Meanwhile, a second system in the North Pacific advances
northward, bringing another round of gales to the Bering Friday
afternoon. As the low lifts northward into the central Bering,
northerly gales will persist over the western Bering while the
leading front will bring southerly gales to the east side of the
Bering through Friday night. Due to the warm southerly flow,
precipitation with this system will fall mostly as rain across the
Aleutians and Pribilofs.

&&

.MARINE (Days 3 through 5)...

Thursday through Saturday continue to look windy across the
Bering Sea as a strong low moves up the Kamchatka Peninsula. Winds
ahead of the low in the central Bering are still expected to be
around gale force, but the forecast winds around the low center
have increased in strength and storm force winds are expected for
the Bering Sea and just north of the western Aleutians through
Friday. Saturday appears to be a bit of a break as most of the
energy will go into a low moving up into the central Aleutians,
but gales can be expected to persist across the central Bering.

In the Gulf of Alaska, a low moving into the Prince William Sound
and sliding off to the east on Thursday will increase
northwesterly winds through the gaps. These are expected to be
gale force through the Barren Islands and into the Gulf of Alaska
but higher gusts are possible especially along the coast in the
gap areas.

&&

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

The long term forecast continues to be on track from yesterday,
with a ridge of high pressure building in over mainland Alaska,
pushing the main storm track westward over the Bering Sea. Models
are trending towards a stronger high pressure system, which means
that it looks less and less likely for precip to make its way
inland from any lingering fronts over the weekend. This high is an
unusually warm high pressure system for this time of year,
looking more like a spring time high pressure system than the cold
winter highs we see in late January. Solar heating is still
marginal, so spring is not here yet...but temperature forecasts
for the weekend and early next week across the mainland look to be
a bit of a challenge.

As for the Bering Sea, the usual active storm track of the winter
will continue. The storms moving through aren`t expected to be
unusually strong, but the amount of warm air aloft will bring rain
much further north than we normally expect at this time of year.
Warm and wet conditions are expected to continue for the eastern
half of the Bering through early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 131.
MARINE...Gales: 119 120 121 125 127 128 132 137 170 173-177 179 185
                351 352 411-414.
         Storms: 130 131 178.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...RJA
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO/DK
MARINE/LONG TERM...SA


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