Area Forecast Discussion
Issued by NWS Anchorage, AK

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539
FXAK68 PAFC 040406 AAA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
706 PM AKST Sat Dec 3 2016

Updated Long Term Forecast

.ANALYSIS AND UPPER LEVELS...

Radar imagery this afternoon shows a well defined low pressure
circulation that is centered to the west of Middleton Island and
slowly spinning southward. An upper level low located just east
of the Kenai Peninsula continues to dive eastward into the Gulf of
Alaska. Meanwhile an impressive upper level ridge continues to
slide eastward from the Bering into the Alaska mainland. The
pressure gradient between the high pressure and low is supporting
strong gusty winds across the Alaska Peninsula and offshore across
the northern Gulf of Alaska. Widespread cloud cover this morning,
and gusty winds resulted in moderate temperatures this morning.
However, in wind-sheltered areas of the Southwest mainland,
temperatures plummeted last night well below zero. These are some
of the coldest temperatures of the season, and cold air will
continue to pour into Southcentral as strong cold air advection
continues through local gaps.  &&

.MODEL DISCUSSION...

Models are in great synoptic agreement in the short term as the
upper level ridge over the eastern Bering slides into the
Southwest Mainland, and a steep trough follows quickly behind it.
To best capture local gap wind accelerations, the high resolution
WRF ARW was used.

&&

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

The remnant trough across the northern Gulf has spun up into a
more organized surface low centered southwest of Middleton
Island. Moisture associated with this low extending over the
northern Gulf coast and into the eastern portions of Prince
William Sound will gradually taper off and pull offshore tonight
as the low sinks south. Strong west to northwest winds will
persist over Kodiak Island, Shelikof Strait and the Barren
Islands...especially to the lee of gaps in the Aleutian Range as a
final shortwave trough rotates through this evening as the
broader, very cold upper level trough progress east. Strong
northerly and westerly outflow winds will also continue through
Resurrection Bay and Passage Canal tonight and then gradually
diminish Sunday and more rapidly Sunday night as the surface and
upper level low drop to the southeast and exit the Gulf.

The very strong north to northeasterly outflow winds will persist
longer through Thompson Pass and Valdez area. The pressure
gradient initially more supported by the surface low in the Gulf
pulling the offshore flow will be reinforced by the cold air pool
and subsequent high pressure building in the Copper River Basin
as the cold air deepens. Very cold temperatures throughout
Southcentral Alaska continue to be a concern through the weekend
with frigid wind chills up near the Alaska ranges where breezy
north winds persist through the passes. An upper level ridge
building in from the west will begin spreading warmer air aloft
and possibly some moisture back into the southwestern Gulf Sunday
night and Monday.

&&

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

Cold and dry conditions will persist through the weekend beneath
mostly clear skies as a high amplitude ridge builds over the
southwest mainland from the west. A tight pressure gradient will
continue to produce gusty winds along the Aleutian and Alaska
Ranges through tonight before diminishing a bit Sunday as the
strong Gulf low departs to the southeast. Dry northerly flow will
begin to give way to easterly winds and increasing mid and high
level clouds along the coast on Sunday as a Bering front makes
slow eastward progress towards the mainland. A triple-point low
forming along the front over the southeast Bering will push some
overrunning snow across the Bristol Bay region, with heaviest snow
accumulations likely toward the best isentropic lift from
Dillingham north along the eastern slopes of the Wood River
Mountains Monday into Monday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Northerly storm-force outflow winds with hurricane-force gusts
continue into tonight before gradually diminishing through Sunday.
Otherwise, the strong gale-force front currently moving through
the central Bering is the primary feature of interest through the
weekend as it makes slow eastward progress across the
Bering/Aleutians. Precipitation may begin as a rain/snow mix with
the arrival of the front, but should then quickly transition to
all rain as warm North Pacific air streams northward. A triple-
point low develops near the Eastern Aleutians Sunday night and
moves to between the Pribilofs and the Alaska Peninsula Monday.
This will help to focus stronger cold advection along the backside
of this low over the southeastern Bering Monday, with gale-force
northwesterly winds possible.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The upper levels on Monday depict an amplified ridge building
over the Seward Peninsula extending northward into the Beaufort
Sea, and extending southward through the central Bering/Aleutians
through Tuesday. Meanwhile, a trough from the Lower Kuskokwim
Valley through the Copper River Basin will swing into the gulf.
Therefore, look for a snow showery regime accompanied by cooler
temperatures filtering into the Southwest Alaska region and the
northern Alaska Range. Beginning Wednesday, this ridge of high
pressure further amplifies, and builds westward through the
northern Bering into the Beaufort Sea. This results in high
pressure dominating the Southwest/Southcentral region through
Thursday. The temperatures will fall below average through the
midweek for the aforementioned regions. Over the western Bering a
gale-storm force front moves through the Western Aleutians. This
system will move northward as the weather front moves into the
central Bering/Aleutians, but falls apart as it runs into the high
pressure ridge over the Bering.

Meanwhile over the Gulf, a weak low pressure system entering the
eastern Gulf by Tuesday will bring a showery regime from Kodiak
Island northward into the northern coastal communities before the
systems dissipating mid week. By Wednesday there is uncertainty
with the possibility of an upper level shortwave moving through
the Tanana Valley region southward into the Anchorage Bowl as we
head into Thursday time-frame.


&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...high wind warning 131. Wind chill advisory 141 145.
MARINE...Storm warning 130 132 138.  Gales 127 128 131 170 172
173 174 175 176 179 181 185 351 352 412 413 414.  Heavy freezing
Spray warning 126 127 130 131 136 137 138 139 140 141 160 181.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...EN
SOUTHCENTRAL ALASKA...JR
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CC
LONG TERM...PD/DEK



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