Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 201401
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
501 AM AKST Mon Feb 20 2017

.ANALYSIS AND UPPER LEVELS...
The progressive pattern change across AK and the north Pacific is
underway this morning. An upper level trough over the western half
of the AK mainland is bounded by a weakening ridge to the east and
a strong ridge covering much of the Bering. A cut-off upper low is
over the western Bering. At the surface, a deep 950 to 955 mb low
is recurving over the western Aleutians with the associated storm
to hurricane force front curving back across the central Bering
and Aleutians. A ridge is over the eastern Bering and southwest
AK, and the remnants of a low continue to weaken over Prince
William Sound. Weakening snow shower activity persists across the
Kenai Peninsula, the Anchorage Bowl, and the Matanuska and Susitna
valleys.

&&

.MODEL DISCUSSION...
Models are in agreement that the Prince William Sound low will
continue to weaken through tonight, allowing the Bering/Southwest
AK ridge to migrate east. This will allow the Bering front to
reach the southwest coast of AK on Tue, producing a triple point
low that moves into the Gulf of AK on Wed.

While all of this is happening, a new low from Japan races to the
western Aleutians later on Tue, merges with the original low and
then spins through the western Bering into Thu. The leading gale
and storm force front from this merged system crosses the Bering
and Aleutians Tue, and reaches southwest AK on Wed. The trailing
gale force front moves across the Aleutians through Thu.

&&

.AVIATION...
PANC...A threat of snow showers will continue through the
morning as lines of showers rotate up from the Western Kenai. If
one of these lines tracks over the airport expect a period of MVFR
conditions. Otherwise, VFR conditions are expected to continue
through tonight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The negatively tilted arctic trough will slowly lift northeastward
across Southcentral today through Tuesday with a ridge building
in behind it. Snow shower activity in the Cook Inlet/Susitna
Valley corridor will gradually diminish today as the trough
passes overhead, lingering longest along the mountains. Showers
will spread out ahead of the trough from eastern Prince William
Sound northward into the western Copper River Basin. Meanwhile,
arctic air will continue to pour across the Alaska and Aleutian
ranges into Southcentral and across the Gulf. Temperatures will
not rise much today, especially over western areas of Southcentral
where the coldest low level air mass will lead to steady or
slowly falling temperatures. Gap winds will be in full force along
the western Gulf and western Prince William Sound.

Temperatures will drop off quickly tonight as clouds decrease
behind the upper trough axis. The cold temperatures will be
short-lived however, as the frontal system currently along the
Aleutians reaches Southcentral Tuesday night. Have upped the
probability of precipitation significantly area-wide Tuesday
night into Wednesday, with high confidence that a fast but stout
upper trough will move through. With arctic air in place expect
snow for all but Kodiak Island.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A pattern change will be coming for the mainland, but first
another day of the status quo. The stationary upper trough and
arctic air mass will continue to sit over the area for another
day, keeping temperatures firmly in the negatives. There are
pockets of low level clouds forming under the inversion but these
should not be widespread today. Late tonight, an approaching front
from the Bering Sea will begin to bring some offshore flow and
slightly mixed air near the surface which will stop temperatures
from falling as drastically as the last couple nights.

The front will impact the mainland on Tuesday, spreading light
snow from southwest to northeast through the day. Snowfall
accumulation looks to be in the 3 to 6 inch range, with isolated
areas getting more than 6 inches. The front exits from west to
east late Tuesday night, but an active, progressive pattern
persists.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The big story currently is a hurricane force front over the
western Bering Sea. It looks to have peaked in intensity and
should dissipate today with the strongest of winds remaining over
the far western Bering. The front continues northeastward over the
Bering and will bring blizzard conditions to the Pribilof Islands
this evening and overnight into Tuesday. As the front moves
eastward Unalaska will get a quick shot of snow and blowing snow,
but temperatures will warm enough from downslope to mitigate the
threat by the afternoon hours. The actual warm air will change
precipitation to rain by the late evening. The Alaska Peninsula
is in a similar situation as the front traverses the area Tuesday
morning. As the front exits onto the mainland, another strong low
and front already enter the western Bering Tuesday afternoon. This
one will have more of a south to north trajectory with the warm
front followed by a west to east moving cold front.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Midweek through the weekend forecast will be dominated by a ridge
that will nose in across Southcentral and a trough that will dig
south across the Bering. Guidance remains in good agreement with
bringing a shortwave across the ridge and sliding southeast across
the Gulf of Alaska. There are select differences with the models
with how strong of a wiggle in the flow it will be and more
importantly how far south it will develop. The forecast today
sided closer with the surface placement and features from the EC
and GFS as the NAM seems to be the outlier in how fast it
transitions the low to SE AK. Mid week that will likely bring a
shot of snow back to the area. Ridgetop and higher flow is
looking to remain southwesterly, keeping downsloping chances down.
Beyond that time for southcentral, the trough over the bering will
slide east and allow a front to stall out from the AKPEN and along
the Alaska Range. This will increase overall surface temperatures
as well as bring back the chances for mixed phase winter weather
come late in the week and again over the weekend.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 195.
MARINE...Heavy Freezing Spray Warning 121 129 130 131 138 139 141
                                      150 165 185.
 Storm Warning 177 178.
     Gale Warning 130 131 138 155 160 165 170 171 172 173
  174 175 176 179 180 185.

&&

$$

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



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