Area Forecast Discussion
Issued by NWS Anchorage, AK

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910
FXAK68 PAFC 020259
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
559 PM AKST Thu Dec 1 2016

.ANALYSIS AND UPPER LEVELS...
The upper level low center remains in the Arctic northwest of
Barrow. The main trough extending southward out of this low is
over the Bering Strait and has dug southward into the Pacific to
around 40N. The 250 mb jet stream is tracking over Japan at over
150 KT and then moves around the southern periphery of the trough
and then heads northeastward toward the Alaska panhandle with
speeds upwards of 180 KT. This jet is also in position to feed the
strong surface low south of Kodiak Island.

&&

.MODEL DISCUSSION...
Models are in good agreement and all seem to recognize the very
cold air moving into the southern part of the state. There are
some small differences in the timing of winds increasing in the
tightening pressure gradients but other than that models seem to
be performing well.

&&

.AVIATION...
PANC...Just like last night and this morning it appears a band of
heavier snow will set up over right over the Anchorage Airport
between 06 and 09z. Have trended ceilings/vis down to near IFR
conditions, though there is potential for a brief period of LIFR
conditions. The other challenge is determining how fast ceilings
will lift Fri morning as dry cold air moves in. Lower ceilings
will definitely persist along the mountains, but expect some
improvement near the airport.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The large low south of the Gulf will track northeast to the
eastern Gulf tonight and then swing inland on Friday. A
deformation band associated with a trough extending from the
surface low up across the Kenai Peninsula and into Southcentral
Alaska will strengthen overnight as a weak shortwave rotates in
from the east. This band has the potential to bring an additional
2 to 4 inches of snow to the Anchorage area as well as the
Matanuska and Susitna Valleys. Snow showers supported by the
incoming cold advection will continue off and on through Friday
but diminish in intensity and likely be more confined to the
westward facing mountains as the day progresses.

A stronger secondary frontal wrap associated with the big low is
moving into Kodiak Island this evening. The combination of this
band converging with the increasing northwesterly flow and cold
advection pushing over the area will bring a period of heavy snow
to Kodiak Island with between 4 and 9 inches possible overnight
and increasing winds picking up that snow and reducing
visibilities to less than 1/2 mile at times.

Offshore west to northwesterly flow will build rapidly through the
day Friday as the surface low tracks east and the cold upper
trough over western Alaska begins to push east. Storm force winds
will develop over a number of the western marine areas with
hurricane force gusts out of bays, passes and through terrain
gaps in the Aleutian Range. Kodiak Island will likely see rather
strong wind gusts through this event as well as the more typical
offshore gap wind locations such as Whittier/Passage Canal,
Thompson Pass/Valdez and Seward/Resurrection Bay.

Another concern as this pattern progress will be the very cold air
mass pushing into southcentral Alaska Friday Night and Saturday.
In calm, wind sheltered locations temperatures could drop to
colder levels than they have been in several winters and elsewhere
where the winds are brisker wind chills could become a broad issue.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A low in the Arctic swings a trough axis eastward tonight across
the Southwest. An associated weak surface low lags behind the
upper level system and drops south across the eastern Bering
waters. This surface low will be the focus for snow development
tonight along the Kuskokwim Delta as a fast moving disturbance
moves in from the Central Bering. There is some uncertainty how
far inland the air mass saturates, but there is enough confidence
to increase snow chances across much of the Delta. Snow will
continue along and near the Alaska and Aleutian Ranges through
Saturday as the air mass remains saturated and several short waves
move across from the Bering. The most dynamic system sets up late
Friday with an Arctic wave diving south, combining with the
shortwave energy along another sharpening trough axis moving from
the parent low in the Arctic.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The jet stream keeps an active weather pattern across the Bering
through Saturday. The main challenge tonight will be winds across
the Alaska Peninsula with gusty conditions peaking through
midnight behind a cold front that passed through late day. A high
wind warning was issued just before the afternoon package as winds
gusted up to 85 mph for King Cove.

The next system to monitor is a stronger disturbance which develops
this evening across the northwest Bering. This system digs
southeast to the Pribilof Islands Friday morning then continues
toward the Alaska Peninsula during the afternoon keeping strong
gusty winds for the area and increased chances for snow. An upper
level ridge begins to amplify behind this storm system across the
western Bering, shifting east as another rapidly developing storm
system moves into the western Bering Friday afternoon. This storm
force system will be accompanied by a rain and snow mix as it pulls
a warmer air mass from the Pacific and wraps into a powerful
system with cold polar air advecting around the core of the low
as it closes off a center over the Kamchatka Peninsula into Saturday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The coldest weekend of the season is anticipated for this weekend
across much of mainland Alaska. High pressure will continue to
build in across the mainland, with low pressure remaining in the
Gulf of Alaska. This will help to support strong gusty offshore
flow across much of the Gulf of Alaska, with particularly strong
winds occurring across Kodiak Island, Shelikof Straight, and the
Barren Islands. By Saturday afternoon wind gusts will continue to
build through Thompson Pass and the Valdez area, as pressure
gradients increase, and very cold interior air builds.

By early next week, a pattern change will begin to bring slightly
warmer temperatures back to the Alaska mainland as a trough begins
to push across the southwest mainland. This will help to bring
more moderate temperatures to the region with widespread clouds
and southerly flow across the Southwest Mainland. It will also
help to dramatically decrease the pressure gradient between the
mainland and Gulf of Alaska and greatly diminish the offshore
flow and strong cold air advection.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind warning 181. Winter Weather Advisory 171.
MARINE...Heavy Freezing Spray Warning 129 130 140 160 180 181.
         Storm Warning 150 130 131 132 136 138 177 178.
         Gale Warning 119 120 121 127 137 155 160 165 170 171
                      172 175 176 180.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...JR
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...EN



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