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Southcentral and southwest Alaska forecast discussion 
National Weather Service Anchorage Alaska
431 PM akdt Thursday Oct 20 2016

Analysis and upper levels..

Water vapor satellite imagery this morning shows two main upper
level features driving much of the weather across the state this
afternoon. An impressive upper level trough associated with
copious amounts of cold air covers much of the western Mainland.
It is currently draped just to the west of the alaskan and
Aleutian mountains and will push towards Anchorage in the
overnight hours. A second wave is associated with the upper level
low centered in the Gulf of Alaska, and is helping to usher
moisture westward across Prince William Sound and even has
supported a few sprinkles in the Anchorage bowl this morning. The
interaction between these two features will help to fuel the
development of widespread showers by this evening.


Model discussion...

Models remain in great synoptic agreement and all indicate the
development of widespread showers across the Kenai Peninsula,
Anchorage bowl and Matanuska Valley by this evening. However,
small differences between the models are contributing to
noticeable differences in the timing and amount of precipitation
across Anchorage. The NAM is much faster, and develops
precipitation by mid-afternoon, with snow showers by this evening.
The GFS model indicates a slower development of showers and later
transition to rain. Currently went with The Middle Ground in terms
of snowfall amounts, but amounts could be an inch or so higher.


Ceilings are expected to drop below 5000 ft by late
afternoon/early evening as a rain and snow mix develops.
Precipitation is expected to transition to all snow by late this
evening with IFR conditions and a few inches of accumulation. VFR
conditions are expected by Friday morning as the weather system
pushes to the east, and gusty northerly winds will develop by
Friday afternoon.


Short term forecast southcentral Alaska (days 1 and 2)...

An Arctic trough is digging into southern Alaska this evening
which is merging with a remnant low from the northern Gulf of
Alaska. A rather impressive stretching deformation zone can be
seen on satellite imagery from Cook Inlet to the Brooks range.
Clouds and precipitation are forming along this region of
convergence with cold Arctic air beginning to move across the
Alaska Range. Snow has already developed in the Susitna Valley
with rain showers from Anchorage to the Matanuska Valley. The 00z
sounding shows a freezing level of 3100 feet and a wet bulb zero
level of 2500 feet. It will take time to erode the low level warm
layer, so snow is expected to hold off until after dark. Snow
accumulations will generally be light with 1-2 inches in lower
elevation areas with higher amounts in the upslope faces of the
chugach and Talkeetna Mountains. Temperatures drop below freezing
by morning, so this may create some slick driving conditions for
the morning commute.

The low moves into the Gulf Friday, with the associated
precipitation band also moving east into the Copper Basin.
Increasing offshore flow will allow for impressive outflow winds
near the coast, with some gusts pushing into the 60 miles per hour range
near Thompson Pass Saturday. With accumulating snow Friday, there
will be some blowing snow and reduced visibility in Thompson Pass
Friday evening into Saturday. Saturday will see drying conditions
and a maximum in the pressure gradient along the coast with slowly
diminishing gap winds Saturday evening.


Short term forecast southwest Alaska (days 1 and 2)...
the southwest Alaska region remains under offshore northerly winds
resulting in sunny skies during the day, and clear/brisk nights
heading into this weekend. Look for cooler temperatures resulting
from an upper level trough as it drops down from the north before
tracking eastward into the southcentral region. Therefore, cold
air advection is moving into the region combined with a high
pressure ridge building in from the eastern siberian region. As
this synoptic feature moves eastward it will result in favorable
conditions for fog developing in the Kuskokwim Delta Region by
Saturday morning.


Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the storm force low dropping off the Kamchatka peninsula tracks
toward the western Aleutians with the associated weather front
moves into the western Aleutians/Bering by Friday morning. This
results in gale force southeasterly winds before shifting
southwesterly with the frontal passage by Friday afternoon. Look
for widespread rain/stratus as this synoptic feature moves into
the western Bering. Meanwhile, a storm force North Pacific low
tracks toward the western Aleutians by late Friday evening
bringing another round of gale force southwesterly winds to the
aforementioned region. Otherwise, the rest of the Bering will be
under the influence of high pressure moving in from the eastern
siberian region, except for some gusty northerly gap winds along
the eastern Aleutians as we head into the weekend.


Long term forecast (days 3 through 7)...
the mid term for the southern Mainland is characterized by cold
air advection and a ridge building in from the west that will
bring outflow winds through the weekend. As the ridge moves off to
the east, winds will die down, but it is unclear how long the
fair conditions will last. That is because of the uncertainty that
remains with the system moving into the Bering Tuesday. There
still remains a fair amount of variation in the models handling of
this feature. However, the deterministic solutions are starting to
lean towards a stronger low moving through the central Bering,
while the ensembles keep the main low further west. Overall there
is better agreement than yesterday but still enough uncertainty to
keep confidence low for specific impacts.


Afc watches/warnings/advisories...
Marine...gales 127, 128 130 131 132 138 177 178 185 411 412 413.



Synopsis and model discussion...en
southcentral Alaska...ja
southwest Alaska/Bering Sea/Aleutians...period

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