Area Forecast Discussion
Issued by NWS Anchorage, AK

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223
FXAK68 PAFC 220042
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
342 PM AKST Tue Feb 21 2017

.ANALYSIS AND UPPER LEVELS...
A strong frontal system is currently pushing onshore into
Southwest Alaska and spreading snow inland. Blizzard conditions
have developed along the Western Capes and are in the process of
spreading north to the Kuskokwim Delta Coast. A second frontal
system over the western Aleutians is lifting north and east into
the central Aleutians and Bering. Over Southcentral Alaska, an
upper level ridge out ahead of the Bering low is building east
into the area and pushing the arctic trough north and east as
well. Clouds from the leading edge of the frontal system in
southwest Alaska have reached Kodiak Island and the southwestern
Gulf.

&&

.MODEL DISCUSSION...
The leading edge of the snow associated with the frontal system
pushing into Southwest Alaska has arrived several hours ahead of
earlier model run timing. Of the 12Z runs, the GFS had the fastest
solution, which appears to be within an hour of the actual start
time and intensification of the snow, though the 18Z NAM is coming
in line with this faster timing as well. The 18Z GEM regional
also is picking up on the faster front and has the leading edge
of the overrunning precipitation to the southern Kenai by 03Z and
Anchorage as early as 06Z. All models continue to develop the
leeside low in the western Gulf tonight/Wednesday with the NAM
continuing as a bit of an outlier, tracking it the furthest north.

&&

.AVIATION...
PANC...VFR conditions will give way to MVFR with light snow
spreading north up Cook Inlet and reaching PANC around 06Z this
evening. A period of IFR in moderate to heavy snow is expected
Wednesday morning. A return to VFR conditions is anticipated for
Wednesday afternoon.

&&

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

The mid to late winter onslaught continues...

A slightly negatively tilted upper level shortwave trough will
move into Southcentral tonight with the associated surface low
crossing west to east across the Northern Gulf of Alaska
Wednesday. Snow will impact all of Southcentral with the best
accumulations near the coast. The surface low will induce some low
level southeast winds near the Chugach which will cut into
precipitation values, but with southwest flow aloft through the
favorable dendritic growth zone, snow-liquid ratios should be
rather high in the 20-1 to possibly 25-1 zone. With low density
snow and the heaviest rates expected during the morning rush,
visibility will likely be reduced due to commuting cars and
trucks, so even if snow totals don`t reach 6" or more required for
winter weather advisories, the expected impact to the morning
commute will still be significant. The low will move out by
Wednesday evening with snow quickly ending as ridging aloft builds
in. Quiet weather will ensue Wednesday night and Thursday before
the next storm moves in Friday (see extended discussion).

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The well-below normal temperatures throughout the Southwest
mainland today have started to warm this evening, as the warm
front begins to work its way further inland. This very moist warm
front will continue to bring moderate to heavy snowfall rates to
areas along the coast of the Kuskokwim Delta, areas North and West
of King Salmon, and over parts of the Lower Kuskokwim valley as
well today and into tonight. With the moderate to heavy snowfall
coinciding with the gusty winds over the above mentioned coastal
areas, blizzard conditions are expected to persist through later
this evening. Sub-warning level snow and winds are still expected
further inland around the Bethel area, including Aniak, and
interior portions of Bristol Bay. Falling and blowing snow will
reduce visibilities to one-quarter mile or less at times, mainly
along the coast, with inland areas experiencing slightly less
restricted visibilities.

The front will continue inland tonight, bringing the same
intensity snow to the Kuskokwim Valley. The cold antecedent
conditions will lead to light and fluffy snow that will build up
quickly. The front exits the southwest by the early morning hours
Wednesday, while another weak front brings another shot of very
light snow to the area by Wednesday afternoon.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Just as one front over the eastern Bering tracks inland over the
Southwest mainland, another one has started to work its way
through the western and central Aleutians and into the western and
central Bering this evening. This front will push eastward tonight
over the eastern aleutians and bering, including the Pribilof
islands, which will likely see another round of blowing snow and
reduced visibilities overnight tonight and into tomorrow morning.
The one caveat with this system is the warm air it is advecting
from the Northern Pacific, moderating temperatures and changing
snow to rain along the Aleutians. This warmer air will eventually
work its way northward into the Pribilof Islands by Wednesday
morning, and slowly put an end to the blowing snow conditions.
Further west on Wednesday morning, colder temperatures and gusty
winds will accompany the cold front tracking through the western
Aleutians/Bering and into the Central Aleutians/Bering by
Wednesday afternoon.

&&

.LONG TERM FORECAST (Days 3 through 7)...
There remains good confidence that a shortwave moving over the
Mainland on Friday will force the highly amplified ridge well
south of the area, allowing for widespread precipitation to move
into the south mainland. The one change in the models since
yesterday is they have weakened significantly with the associated
warm air advection, so the risk for widespread rain or freezing
rain is much less, especially around Southcentral. It still
appears that a precipitation will move into Southcentral sometime
Friday afternoon or evening, and most likely as a rain/snow mix.
Afterwards an active pattern, especially for the Bering Sea and
Southwest Mainland, will continue into this upcoming weekend as a
gale-to-storm force low moves through the Northern Bering. This
active pattern then continues into early next week and we
continue to have good confidence that an arctic trough will
descend through the Mainland. There still remains some uncertainty
in the exact track and how much cold air will be associated with
this trough, but the general trend has been towards stronger
solutions through the Mainland. This would allow for cooler
weather, chances of snow, and a generally active pattern for the
Mainland into the middle of next week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 155 161.
         Winter Weather Advisory 101 111 121 145 152.
MARINE...Storm Warning 185.
         Gale Warning 119 120 131 132 165 170 172>181.
         Heavy Frz Spray 129 139 185.
&&

$$

SYNOPSIS AND MODEL DISCUSSION...JR
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PEPE
LONG TERM...DEK



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