Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 271744 CCA

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 AM AKDT Mon Mar 27 2017

The large upper level high over the Bering persists with an upper
level low centered around 50N to the south of the eastern Aleutians.
An associated frontal system is lifting towards, but remains south
of the eastern Aleutians and Alaska Peninsula. Over mainland Alaska,
a broad, cold upper trough extends from the Arctic Coast south to an
upper low centered over the Kenai Peninsula and Kodiak Island.
Precipitation associated with a surface low in the northern Gulf
extends across the northern Gulf coast and is pushing into Prince
William Sound.


Models struggle with the finer scale details of the pattern shift
beginning during the first half of this week. Specifically there
are large differences in the handling of the various weak surface
lows moving into and through the Gulf. The model uncertainty
likely stems from resolution issues dealing with all of the
small, weak lows and the question of how the low in the
Southeastern Gulf will interact with the trough swinging into the
western Gulf.

By Wednesday models begin converging on a solution of the broader
low complex lifting north into Southcentral Alaska with model
confidence improving into Thursday regarding the front approaching
the Alaska Peninsula and western Gulf.


PANC...VFR conditions and light winds will persist.


Today marks the gradual change in the pattern across Southcentral.
The upper long wave trough which has been parked across the
Southcentral will slowly retrograde westward. This will bring a
shift to a more southerly flow aloft and allow moisture from the
Gulf to spread west of the Chugach Mountains. The first wave
moving west across the Gulf today will bring snow to much of the
Gulf coast(mixed with rain in a few places along the immediate
coast) and the eastern Kenai. Snow showers will likely spread
across the Chugach Mountains into the Anchorage Bowl by afternoon,
though western Anchorage and the Matanuska Valley are not
expected to see measurable snowfall at this point. Snow showers
will diminish across southcentral overnight as the low weakens.

Another developing low over the North Pacific heads toward the
northern Gulf late Tuesday and brings another shot at snow and
rain the Gulf and possibly a better change of measurable snowfall
across the Anchorage Bowl and Matanuska Valley. However, the
models are differing on the speed and position of this low. So
confidence is low on the timing of precipitation and location.

Cold and quiet weather continue to be the name of the game across
Southwest Alaska as northerly offshore flow continues to pump dry
polar air southward across the area. An upper level low and
associated trough currently over the western Gulf will gradually
push northwestward into Southwest Alaska. The center of the upper
level low represents the core of the coldest air. Thus, an
approaching upper level low spells out cooling temperatures.
However, during the day, a good amount of that cooling will be
offset by the strong late March sun, which will remain out in full
force for most areas. The lone exception will be late Monday
night into Tuesday for areas along the Alaska Range, as mid-level
moisture from a low over Southcentral spills over the mountains,
increasing cloud cover. At night, winds in most areas should
average in the 5-10 mph range, keeping full radiational cooling
from taking effect. Falling low temperatures over the next few
nights should be expected, with any sheltered locations having no
trouble dropping below 0.



A gale-force front parked south of the Eastern Aleutians will
rapidly weaken during the day today as the parent low retreats
south into the North Pacific. The upper-level rex block over the
Bering will break down over the next two days and the upper level
high and associated ridging will push south and weaken. The
resulting change in the sensible weather will be fairly benign, as
the pattern of stratus over the central Bering becomes more
convective with cold advection. Polar high pressure will persist
over the northern and eastern Bering, keeping any threats of
precipitation at nil. The only precipitation to be found once the
front weakens will be in the western Bering and Aleutians, as weak
upper level lows keep widely scattered showers passing through
that area.


.LONG TERM FORECAST (Days 3 through 7)...
The only significant change to the long range was to increase the
POPS along the North Gulf Coast and Prince William Sound into the
Copper River Basin Tuesday night and Wednesday. A surface low is
moving from the gulf into Prince William Sound beginning Tuesday
supported by an upper level low. The models continue to hold the
precipitation along and east of the Kenai and Chugach Mountains.

Over the Bering Sea, Aleutians and western Alaska we will see a
big pattern change by the middle to end of this week as a large
potent low will move toward the Alaska Peninsula Thursday and
Friday. The models are in good overall agreement with this
solution through Friday. The latest model run does begin to
converge on this low moving over the southwest mainland Saturday,
although the position and intensity of the low vary a bit. Models
are struggling to converge on a solution for the location and
intensity of a low south of the Aleutians. At this point we stuck
with nudging the current forecast toward the ensemble solution
making minor changes to the forecast.

Overall the pattern is shifting and will bring a more active
weather pattern and likely warmer temperatures to the southern
Mainland and Alaska Peninsula.


MARINE...Gales...155 172 174.
         Heavy Freezing Spray Warning 179 185 414.



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