Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 221434
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
534 AM AKST Wed Feb 22 2017

.ANALYSIS AND UPPER LEVELS...
One front is moving from Southwest to Southcentral AK this
morning, while another system is pushing through the Central
Aleutians. The front across southern AK is bringing widespread
snow to the area. The heaviest snow overnight was over parts of
the Kenai Peninsula and Southwest AK. But radar clearly shows that
more is in store for Anchorage and the Matanuska and Susitna
Valleys. This snow is being aided by a fairly sharp trough that is
sweeping quickly to the east this morning. In between the two
systems, a 110 kt jet is moving over top of a short wave ridge
and keeping the pattern very progressive.

The second system is located over the Western Bering with a cold
front between Dutch and Adak and a warm front stretching towards
the Alaska Peninsula. It is being supported by a 150 kt piece of
the subtropical jet. This jet is carrying a healthy amount of warm
air northward with 850 hPa temperatures surging near 5C. This warm
air will cause precipitation type issues (snow changing to rain
and possibly freezing rain) over the Aleutians first, then into
the Pribilof Islands, and then into Southwest AK. It will also
allow the ridge to build over the southern mainland over the next
24 hours.

&&

.MODEL DISCUSSION/Uncertainty...
The NAM relented on its slower timing and further west solutions,
so all models are in solid synoptic agreement this morning. This
is leading to average forecast confidence with regards to synoptic
features. There is still uncertainty with snow amounts from this
morning`s system. Higher resolution models were used to tweak
those values. Out west, the GFS was preferred due to its
consistency, especially with winds. The area of uncertainty in the
mid-range (Fri-Sat) will be with precipitation types. How much
warm air will there be and how far inland will it go? At this
point, think at least a mix of rain and snow is in store for the
Cook Inlet by on Fri.

&&

.AVIATION...
PANC...Snow reducing visibility to IFR conditions will
continue through mid-morning, then should quickly end by late
morning. After that, VFR conditions are expected with light
winds.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Strong south to southeast flow across the Kenai and Chugach
Mountains has thus far limited snow intensity in much of Anchorage
and the Mat Valley. However, winds will rapidly diminish over the
next couple hours as the surface low tracks south of Seward. This
will allow for a quick shot of heavier snow before the upper
trough moves through between 10 am and Noon. The short-wave will
go from a negative tilt to neutral tilt as it continues eastward
over Southcentral and a lot of the moisture will be pulled south
over the Gulf with the surface low. Thus, expect generally light
snow amounts for the Copper River Basin on south to Eastern Prince
William Sound.

A warm front will then ride a building upper level ridge northward
across Southwest Alaska tonight. The overrunning precip associated
with this will largely stay out of Southcentral, though there
could be some very light precip along the Alaska Range. Southcentral
will warm as the ridge passes overhead on Thursday and southerly
flow behind the ridge increases. There is some uncertainty as to
exactly how much the airmass will warm. This will be key to
precipitation type as a weak front and upper trough approaches
from the west Thursday night into Friday. The best chance of
precipitation Thursday night will be across the Cook Inlet-Susitna
Valley corridor. There is potential for freezing rain so will have
to closely monitor the trends in temperatures with this front.

&&

.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. It still appears that 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...Winter Weather Advisory 101 111 121 145
MARINE...Storm 178 185
Gale 119 120 131 155 165 170 172 173 174 175 176 177 179 351 411
412 413 414
Heavy Freezing Spray 185

&&

$$

SYNOPSIS AND MODEL DISCUSSION...MO
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MTL
LONG TERM...DEK


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