Area Forecast Discussion
Issued by NWS Anchorage, AK

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

617
FXAK68 PAFC 130140
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
440 PM AKST Thu Dec 12 2019

.ANALYSIS AND UPPER LEVELS...
A vertically stacked low over the southern Gulf is slowly
weakening as it lifts northward. An occluded front is wrapped
around the northern to western Gulf, bringing small craft to gale
force winds and rain. Low level easterly flow is bringing some
light rain to the eastern Kenai Peninsula. Otherwise, Southcentral
is dry with a mix of sun and clouds. A series of weak waves are
tracking westward across the Gulf and Southwest Alaska, leading to
small areas of light precipitation over Bristol Bay and the
Kuskokwim Delta. 12Z soundings from King Salmon and Bethel show a
significant above freezing warm nose. Thus, precipitation is
falling as rain or freezing rain. A fairly weak deformation band
extends from offshore of the Kuskokwim delta southward to the
Alaska Peninsula, with precipitation. Tight pressure gradients
combined with a bit of cold advection on the back side of the
deformation band is leading to gusty gap winds along the southern
Alaska Peninsula. Lastly, a deep low is passing south of the
western-most Aleutian Islands, with gale force winds and rain
approaching the islands.

&&

.MODEL DISCUSSION...
Numerical guidance is in overall agreement with evolution of
large scale features over the next few days. There are some minor
differences with location of a deep surface low which sits in the
vicinity of the eastern Aleutians Friday night through Sunday.
This will determine whether or not Dutch Harbor experiences some
of the strongest winds with this system.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Low pressure south of Seward is moving towards the northern Gulf
coast. Winds will remain out of the east through Friday night with
gusts between 30 to 40 knots as the low center moves into the
central Gulf. The Mat Valley wind looks to strengthen a bit with a
thermal gradient component that has been established alongside
the pressure gradient from yesterday. The Copper River Basin
continues to cool as cold air moves in from the east. Expect a
period of light snow with minor accumulations over the Copper
River Basin tonight. The snow then tapers off Friday morning as
drier conditions take hold.

Rain will continue for coastal locations from Kodiak Island across
to western Prince William Sound as temperatures remain above
freezing through Saturday. The downwind side of the mountains
will remain mostly dry into tomorrow morning as cross-barrier
flow squeezes out precipitation on the windward side. After that,
a wave of energy looks to provide enough support to increase the
possibility for precipitation to make its way over the mountains
and into Cook Inlet and the Anchorage Bowl at times on Friday.
Temperatures at or above the freezing mark look to keep this
precipitation as liquid, or possibly a rain/snow mix. A secondary
wave, beginning tomorrow night, will bring greater chances of
precipitation all across Southcentral. Expect a chance for a
wintry mix of precipitation as temperatures hover around freezing
across the area.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Tonight conditions will mostly stay dry across the Southwest as
the trough axis that brought showers today shifts across the
eastern Bering. Patchy fog will be possible along the coasts over
night as a weak boundary stretches from Nunivak Island to the
eastern Alaska Peninsula. Isolated to scattered rain showers will
develop across the region on Friday as several weak upper level
waves northwestward from the low in the Gulf.

The next storm system pushes a front to the Southwest coast Friday
night bringing Gusty easterly flow through the weekend. Strong
winds will down slope precipitation, so only minor accumulations
are expected for valleys and west facing slopes. Precipitation
types will fall mainly as snow across the Kuskokwim Valley and
Delta with a mix of rain and snow across Bristol Bay.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Gusty northerly flow across the easter Bering and Alaska Peninsula
tapers down for Friday as the pattern begins to shift with the
surface low near Kodiak beginning to fill on a northward track.
The next low is currently pushing a front toward Shemya which
moves to the southwestern Bering tonight bringing snow and
blowing snow to Shemya as it crosses over. The Central Aleutians
will see gusty southeasterly winds and mixed precipitation along
the boundary. Elsewhere across the Bering...a ridge brings a brief
pause to the weather through Friday afternoon.

A new surface low develops south of the Central Aleutians Friday
afternoon. This system will organize into a stacked system Friday
night, rapidly deepening as it crosses over the eastern Aleutians
into Saturday morning. Storm force marine winds can be expected
around the core of the low. There is still some uncertainty on the
strength and track of this system; however, all models are
showing it track to the southern Bering and stalling through
Saturday night. This system generally weakens on Sunday, but
another North Pacific low re-energizes this storm on Sunday.

&&

.MARINE (Days 3 through 5/Sunday through Tuesday)...
Overall confidence with position of lows and fronts is above
average. The primary forecast challenge is whether wind speeds
top out at gale force or storm force. For now, will largely max
winds out at gale force until have more confidence in the depth of
these storm systems. The areas which will experience the
strongest winds include the northern Gulf and eastern Aleutians
Sunday through Sunday night, then out along the Aleutian chain
Sunday night through Tuesday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The global models are in good agreement in the northern Pacific
through Wednesday December 18th. There are some minor differences
with the timing of two different cyclones that develop in the
north Pacific and move into the Aleutians but the forecast
intensity is only a few millibars off. And the forecast tracks are
very similar. With above normal confidence expect back to back
lows to impact the Aleutians. The respective precipitation shields
and pressure gradient force will move from West to East...in
typical mid-latitude fashion. There is a dome of high pressure in
the central Pacific with a ridge axis that moves into the chain
and serves as a lull between systems.

For southcentral/AKPEN and southwest Alaska, the forecast period
commences with a mature cyclone entering the region. Even though
the Canadian/GFS and ECMWF are not in sync with the placement of
the surface low there is widespread consensus for onshore flow
from the Kenai Peninsula to Kodiak and the AKPen. And this low
decays and fills rapidly becoming absorbed into the longwave
pattern. The first aforementioned Pacific low is moving straight
into the Gulf Of Alaska and will undergo the process of
frontolysis there...which means minimal impacts for Southcentral.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 170 172 174.
 Gale Warning 119 150 155 160 165 171 173 175>180.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...CK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
MARINE/LONG TERM...PJS



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.