Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 271352
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
452 AM AKST Mon Feb 27 2017

.ANALYSIS AND UPPER LEVELS...

A rather dramatic shift in the upper air pattern is underway this
morning as a low amplitude shortwave trough tracking over the
Northern Bering Sea moves toward the northwest coast of Alaska. A
rather impressive 580 dm 500 hpa high pressure zone over the
North-Central Pacific is dominating the upper air pattern to the
south of Alaska. In between is a weak atmospheric moisture river,
seen on satellite imagery as a ribbon of enhanced high clouds
streaming across Southern Alaska and the Bering Sea.

Morning polar-orbiting nighttime microphysics satellite imagery
depicts a region of rather widespread low stratus and fog over
most of lower elevations interior Southcentral. However, the
higher clouds mentioned earlier are beginning to have an impact on
local fog as visibility is slowly improving. Across southwest
Alaska, snow showers are upsloping along the Western Alaska Range
and Ahklun/Kilbuck Mountains.

&&

.MODEL DISCUSSION (through Tuesday afternoon)...
The numerical models are in excellent agreement. There is little
to no significant differences, and all the models are locked into
this impressive outflow cold air advection pattern heading deep
into next week. Forecast confidence is very high.

&&

.AVIATION...
PANC...LIFR conditions will improve this morning as higher clouds
move in and fog and low stratus slowly lift. By this afternoon
conditions should improve as light snow moves in. Drier air behind
a front will finally lift CIGS to VFR this afternoon and evening.

&&

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

Widespread fog over the northern Cook Inlet and Mat-Su Valleys
overnight is improving as the warm upper level ridge is pushed
southward by the trough that is beginning to dig west of the
Alaska Range and will move over the region this afternoon. There
is expected to be a shot of snowfall over the Susitna Valley
through the northern Kenai Peninsula late this morning through the
afternoon (including the Anchorage, Palmer, and Wasilla areas)
with this trough. It does not appear that there will be a lot of
snow with accumulations around an inch being the most likely
scenario for lower elevations with a chance for a little more at
higher elevations.
After this trough moves through temperatures will begin to drop
and winds will increase in strength from a north to west direction
over the region. The weather story will then become one of dry
air, cold temperatures, and gusty northerly winds the remainder of
the week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
While snow showers over much of the Southwest Mainland are
beginning to dissipate early this morning, they will then be
reinvigorated as a shortwave over the Northern Bering moves
overhead. These showers will however be much lighter than those
that moved through yesterday as much of the deep layer moisture
has moved east of the area. This shortwave will also bring the
first surge of cold air to the region, causing temperatures to
fall throughout the day. On Tuesday, cold temperatures will
continue to spread through the area as deep northerly flow drags
cold air in from the interior. This pattern will then persist into
mid week, bringing continued clear and cool weather.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Rain and snow showers along the quickly dissipating frontal band just
north of the Aleutians will diminish this afternoon and be
replaced by cold air and northerly flow descending from the
Bering. Strong cold air advection in this northerly flow will tap
into any remaining moisture along the front to bring snow showers
to the Bering Sea coast of the AK Pen, though significant
accumulations are not expected. Otherwise a nearly stationary and
strong ridge of high pressure will setup over the Central Bering
beginning this afternoon, which will keep any active weather
confined to the Western Aleutians/Bering into the middle of this
week.

&&

.LONG TERM FORECAST (Days 3 through 7)...

A shift in the upper air and sensible weather pattern will be
well underway beginning the extended forecast period on Thursday
as a stout Western Bering Sea ridge unleashes the cold arctic over
southern Alaska. 850 hpa temperatures will be -20C to -25C across
the southern mainland with cold offshore flow along the coast.
Heading into the weekend, all three major global deterministic
models, (the ECMWF, Canadian GDPS, and GFS) along with their
respective ensembles, are depicting a rather significant bora
polar air outbreak across the entire state of Alaska. This will
bring down 500 hpa heights in the 475-485 dm range (-2 to -3
sigmas) and 850C temperatures possibly as cold as -35C into the
weekend. This would represent the coldest temperatures of the
season at 3000-5000 feet. The March sun will moderate some of the
cold air at the surface, but the reality of this situation is it
will be quite cold and well below average for the next 7-9 days,
dominated by dry offshore flow and cold interior temperatures.
Details, as usual are uncertain, especially with respect to low
amplitude shortwave troughs diving south toward the Gulf of Alaska
coast. Some solutions are depicting rather impressive bora wind
events across parts of southern Alaska. But, it is currently too
far away to get caught in the details.

What do we know? Old man winter is going to make a forceful
return, reminding Alaskans that Spring is most definitely not here
yet.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 120 121 127 129 132 136-139 150 160 165 175-178 180.
         Storm 130 131.
Heavy Freezing Spray 121 129 130 136-139 141 150
160 165 180 185.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JRA
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DEK
LONG TERM...JRA



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