FXAK68 PAFC 110112
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
412 PM AKST Sun Dec 10 2017
.ANALYSIS AND UPPER LEVELS...
A stable long-wave pattern continues to dominate the Alaska
region, with a trough centered over the Aleutians/North Pacific
and a high amplitude ridge over the western U.S. extending
northwestward to the eastern Alaska mainland. Thus, the warm
and windy weather for the Gulf and Southcentral will continue.
Satellite imagery shows a deep low over the northeast Pacific
continuing to strengthen as it tracks northwestward toward Kodiak
Island. Conditions are rapidly deteriorating out ahead of the low
across the Gulf and Kodiak Island.
An upper trough tracking westward across Southwest Alaska is
weakening as building heights out ahead of the low overtake it.
Thus, expect precipitation to also follow suit and fall apart.
Out west, a stationary front is strung out along the Aleutians
and North Pacific between two low centers. The Bering Sea is
dominated by northerly flow on the east side of a blocking high
centered over the northwest Bering and eastern Russia.
Models are all in excellent agreement with handling of the
existing low, but quickly diverge with development of another low
over the north central Pacific and the exact track northward to
Southwest Alaska on Tuesday. Solutions are grouped in the same
general area, so forecast confidence is generally high. The main
forecast challenge will be details of the wind forecast as the
deep low affects Southcentral tonight through Monday. A secondary
challenge is precipitation type along the periphery of the low,
namely portions of interior Southcentral as well as Bristol Bay.
PANC...Based on the track of the deep low to Kodiak Island, down
Inlet flow will dominate late this afternoon and evening, which
should keep strong easterly flow above mountain-top. Thus, expect
a period of rain and lower ceilings to move into the terminal by
early this evening. Have stuck with low end VFR ceilings, but
there may some lower ceilings in the vicinity. As the low passes
west of PANC's longitude, pressure gradients will quickly increase
leading to opening of a downslope hole. Expect ceilings to rise
and rain to end. With a much stronger storm system passing to the
west on Monday, southeasterly winds will once again come into
Anchorage but will be even stronger than this morning.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
As the low south of the Gulf tracks toward Kodiak it will swing
its front over the north Gulf coast. As the front passes late this
evening it looks to bring warning level wind gusts to Cordova.
However, these winds should be short lived and diminish quickly
early Monday morning after the front passes.
Some of the stronger winds with the front will push through the
Prince William Sound area and through channeled terrain to
Portage Valley and Turnagain Arm. Winds along Portage Valley and
Turnagain Arm will also see warning level wind gusts late tonight.
Eastern areas around Turnagain Arm will first see strong winds
from the winds being funneled through the gaps. Then this will be
aided later in the night by the movement of the main low pressure
center as it tracks over southwest Alaska. This new position will
help create a strong pressure gradient and bring winds further
west along Turnagain Arm. Monday morning winds will begin to drop
below warning level.
The overall pattern will be warm southerly flow over Southcentral
Alaska bringing mostly rain to the area through Tuesday. In this
setup, with multiple waves progressing through the pattern over
Southcentral, the majority of the precip will fall along the
north Gulf coast.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A strong low pressure system will move northward just to the west
of Kodiak Island tonight and bring a trough through Southwest
Alaska during the short term. There is not as much warm air aloft
with this system as the last one, so precipitation is expected to
be a mix across Bristol Bay, with snow falling mainly northwest of
Naknek. Southeasterly winds on the leeward side of the Aleutian
Range should warm temperatures enough that freezing rain will not
be a concern for this system. Showers will linger in the area
through Tuesday night, when another low approaches from the North
Pacific and enters Bristol Bay. This is a weaker system and few
impacts are expected across the area.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The low in the Gulf of Alaska will tighten the gradient across the
Bering Sea and push the wind direction to be more northerly. This
will keep gale force winds across the northern Bering, along with
snow showers in the north and some mixed precipitation along the
Aleutians. Seas are relatively high across the western Aleutians
as a remnant from the last low pressure system to move through.
Although they'll decrease as we move through the short term, seas
will increase in the Bering with the northerly flow and the long
fetch. It's not until Tuesday night that we begin to see winds
and seas settle across the Bering and Aleutians.
.LONG TERM FORECAST (Days 3 through 7)...
An active pattern remains through the long range forecast as
multiple low pressure systems track to the southern mainland from
the Gulf and Bering. Tuesday night the persistent pattern
continues with amplified ridging over the Southeast, however a
pattern shift begins to take place across the Bering and
Aleutians. A zonal subtropical jet begins to nose closer to the
Aleutian Chain sending a storm system northward which pushes a
front across the Central Aleutians and Western Bering on
Wednesday. A jet streak puts this storm into eastward motion along
the Chain which works to break down the high amplitude ridge by
the end of the work week. There is high confidence for the
anticipated pattern change as all the models are showing these
trends. Only minor changes were made for the extended range
forecast Wednesday through Saturday night with an equally
weighted trend of the operational models and ensembles.
PUBLIC...High Wind Warning 101 125 135.
MARINE...Gale 121 127-129 136 140 141 150 173-175 178 179 181 185.
Storm 119 120 125 130-132 137-139.
Heavy Freezing Spray 185.
SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...LF