Small Craft Advisory 923, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
525 AM AKST Fri Nov 23 2018

.ANALYSIS AND UPPER LEVELS...
A vertically stacked, gale force low is spinning over Adak with a
long wave trough extending over the western, central and eastern
Aleutians. This low is part of a double barrel low system with
the second low sitting well to the west of the Aleutians. This
complex low system is preventing any cold air advection into our
forecast area and keeping most of the cold air over northern part
of the state. The front associated with this low is spreading
over much of the Bering Sea and Aleutians bringing stratus and
precipitation with it. The jet stream is quite active with a large
jet streak blowing south of the western Aleutians reaching speeds
of 180 kt. A second jet streak sits on the leeward side of a
trough south of the AKPEN, with winds approaching 140 kt at 300
mb. A ridge is building over most of the Gulf of Alaska ahead of
the dominant trough creating clear and dry conditions. The
combination of these features leads to mostly northwesterly flow
over the eastern part of the state and south-southwesterly flow
over the AKPEN, and eastern Aleutians. A short wave is moving
through the flow of the ridge which is aiding in fog development
through the MAT-SU, Cook Inlet and Copper River Basin.

&&

.MODEL DISCUSSION...
Models have come into better agreement with the low mentioned
Saturday in the previous discussion. They have less of a spread in
the low center, with a more westerly track now favored for the
incoming low. The model of choice we are favoring with this low
is the NAM. Differences do still exist with placement of this
system, so we will still have to hone the forecast. As with
yesterday, models diverge quite a bit again on Sunday, with
general agreement of a large area of low pressure, but no
agreement on where the next low on Sunday will go. Still favoring
NAM for Sunday at this time, but that may change with the large
spread between models.

&&

.AVIATION...
PANC...Foggy conditions and light winds are expected to persist.
Fog has moved out of and then back over the airfield tonight, with
satellite showing and extensive area of fog around the Cook Inlet.
Therefore, anticipating fog to be an issue at PANC at least until
tomorrow afternoon. Lifted fog for now tomorrow afternoon. However,
not entirely sure inversion will break enough to clear fog, so wouldn't
be surprised if fog lingers through the rest of the TAF period.

&&

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

Outflow winds will diminish through today over the eastern Kenai
Peninsula and the North Gulf Coast as the surface trough over the
Gulf retreats south. Areas of fog across the western Kenai
Peninsula, Anchorage Bowl, lower Matanuska Valley, and Copper
River Basin should see some improvement throughout the day. The
cold stable air over the Copper River Basin will likely keep low
clouds and some fog in through tonight and Saturday. Should get
enough gradient driven winds tonight over the western Kenai to
prevent fog formation. The big question is whether there will be
enough low level mixing to weaken the surface inversion and
thereby eliminating fog across the Anchorage Bowl and Knik Arm
tonight. Current models suggest not, so for now will at least put
some patchy fog along Knik Arm and north Anchorage tonight.

A developing storm system over the north Pacific will bring about
a pattern change toward windy, warmer, and wetter conditions this
weekend. The low center pushes rapidly north and intensifies
Saturday and ends up just south of Seward Saturday evening before
making landfall and heading into the Interior Sunday. The
occluded front will precede the storm center and produce
widespread gales to storm force winds across the Gulf and
surrounding marine areas and immediate coastal locations as it
heads to the north Gulf Coast Saturday. Expect Gap flows to
increase in Saturday especially in favored areas (Turnagain Arm,
Portage Valley, Matanuska Valley) Saturday. Another strong weather
front heads north into the Gulf Sunday so any slacking of winds
will be short lived. Warmer Temps will accompany these systems. So
look for a nod toward rain along the coasts and increasing
changes of light mixed precipitation types inland through the
weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

Snow is expected to impact the Southwest this afternoon through
the weekend as two fronts move up from the North Pacific. There
will be several challenges with these systems as the first front
brings gusty conditions along with snow and the second forces a
warmer air mass across Bristol Bay.

The first front, currently across the Alaska Peninsula, makes
slow northward progress today spreading clouds back across
Bristol Bay this morning with light snow developing during the afternoon.
An upper level disturbance pushes the front inland overnight. The
surface low driving the front holds over the Central Aleutians
before diving southward and dissipating Saturday afternoon. With
the surface low this far west, the general flow across the
Southwest remains out of the north and east direction which keeps
a shallow cold pool across the region. Snow spreads north toward
the Kuskokwim Valley during the evening as a warmer subtropical
air mass glides over the cold pool. The winter weather advisory remains
in effect for this evening through noon Saturday for Bristol Bay.

The next system is a fast paced low which is expected to move up
to the western Gulf early Saturday. This system is expected to
push a warmer air mass across the Aleutian Range Saturday morning.
Pockets of freezing rain may occur along an near these mountains
with precipitation changing over to rain Saturday afternoon.
Only trace amounts are expected as low level easterly flow down
slopes precipitation on the lee side of the mountains.

The front washes out as it moves across the Kuskokwim Delta and
Valley Saturday morning bringing light snow as it slowly
dissipates.

The next system on Sunday is another North Pacific low which is
another warm front which keeps precipitation type challenges in
the forecast into the beginning of the next week.

&&

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

Low pressure remains into control through the weekend as and upper
level trough moves several surface lows up from the North Pacific.
Expect frequent directional wind changes with these actively
moving systems. Cold air advects back across the western half of
the Bering during the latter part of the weekend as the parent low
over the central Aleutians moves south and the next low dives
across the Bering Strait.

&&

.LONG TERM FORECAST (Days 4 through 7: Monday Nov 26th through
Thursday Nov 29th)...

The latest runs of the GFS and ECMWF continue to be in relative
good agreement depicting the major synoptic features in the early
portion of this forecast period. This consensus is apparent at
the surface and the echoed in the 500 mb with the baroclinic
features aloft. A strong low south of Unalaska will bring another
round of enhanced winds into the Central Aleutians, gale force
winds will be possible. Both models show a second low south of the
chain that will rapidly intensify and is headed towards Kodiak.
The GFS has this low bottoming out at 951 MB while the ECMWF only
has it bottoming out at 964 mb. Also noteworthy is the emergence
of the "Atmospheric River" in the Gulf Of Alaska. The current
thinking is that the bulk of the "Atmospheric River" will move
into Southeast Alaska, and Southcentral will only get a sliver of
the "Atmospheric River". The pops were bumped up this forecast
package for the Prince William Sound Region. In addition to the
moisture, this system has a robust pressure gradient associated
with it, gusty winds expected in some inland locations as well as
over the water.

All the while, the Bering and the Sea Of Okhotsk is brewing up a
series a lows.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Fog Advisory 101 111.
         Winter Weather Advisory 161.
MARINE...Gale Warning 125 132 136 137 138 174 175 177 178 179 411
         412 413 414.
         Storm Warning 119 120 130 131 139 351 352.

&&

SYNOPSIS AND MODEL DISCUSSION...RA/BJB
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...PS