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fxak68 pafc 291306 

Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
506 am akdt Wed Mar 29 2017

Analysis and upper levels...
an upper level closed low remains over the Yukon Delta region
with an associated negatively tilted trough extending through the
Bristol Bay interior into the northern Gulf this morning. This
synoptic feature is reflected at the surface with a 998 mb low
located over the western Prince William Sound region with an
inverted trough extending northward towards the northern Alaska
Range. The radar imagery is showing a large swath of echoes
across the northern Gulf spreading inland into the Anchorage bowl,
and the mat-su valleys. The surface temperatures are ranging from
the lower 30s along the northern Gulf coastal communities, and in
the upper 20s for inland areas.

The eastern Aleutians has an amplified ridge extending into the
central Bering with a weak closed upper level low located over the
western Aleutians. While south of the central Aleutians a 968 mb
storm force low is positioned at 44n 177w with an associated
occluded front approaching the chain. The satellite imagery is
showing clear skies over the southwest Alaska, and the eastern
Bering. The western Aleutians/Bering has marine layer stratus.
South of the central Aleutians a storm force North Pacific low is
intensifying this morning with an associated occluded front
approaching the central Aleutians.


Model discussion...
the models are coming into better agreement as the low pressure
system in the northern Gulf tracks into the Prince William Sound
region. The biggest forecast challenge remains the location of
the heaviest snowfall for our aor this morning. As the upper level
trough swings through this evening, look for the snowfall to
taper-off to a showery regime. Meanwhile, the models have a storm
force low just south of the central Aleutians with an eastward
storm-track following the chain. The preferred models of choice
were the GFS/NAM, and the forecast confidence remains average.


panc...look for steady snowfall with IFR to possible LIFR conditions
through midday today before transitioning to prevailing MVFR
conditions by this afternoon time-frame.


Short term forecast southcentral Alaska (days 1 and 2)...
the negatively tilted upper trough will slowly push northeast
today. This will force the Prince William Sound surface low inland
as well, with snowfall moving inland and diminishing. There will
then be a brief break tonight before the next front works its way
into the Gulf and south central Alaska from the southwest on
Thursday. This next system is much stronger than the current one.
It also has a lot more moisture, but most of the precipitation
will fall in coastal areas as the strong flow down-slopes off the
coastal mountains. The strong southerly fetch will also usher in
warmer air, with temperatures at many locations approaching or
even exceeding their normals for the first time this month.


Short term forecast southwest Alaska (days 1 and 2)...
expect increasing clouds today with northerly flow across the
southwest and easterly flow developing along the Alaska Peninsula.
A North Pacific low moves a front to the peninsula late tonight
bringing snow and strong gusty winds through Friday. Added areas
of blowing snow to the forecast Thursday into Friday morning as
the front elongates and stretches north to Bristol Bay. The low
center approaches Sand Point Thursday afternoon, spreading snow
to the Kuskokwim by Friday morning. Gusty easterly flow increases
along the frontal boundary Thursday night which will bring a warm
air surge across the Aleutian range; therefore, precipitation
will be mixed along and near the range. Temperatures warm across
the southwest on Friday as the front weakens and fills.


Short term forecast Bering Sea/Aleutians (days 1 and 2)...
ridging along the northern Bering shifts south to the western
Aleutians by the end of the week as the strong North Pacific low
moves over the eastern Bering waters and southwest. The main
impacts will be around the surface low as it moves near the
eastern Aleutians late tonight with gale to storm force winds
accompanied by rain and snow. Winds taper down late Friday as the
surface low fills and lifts north. Cold air advects from the
Arctic to the Bering and the Aleutian chain into the weekend.


Long term forecast (days 3 through 7)...

(from yesterdays forecast discussion)

Thursday will be the day that the upcoming pattern change
will begin to be felt across much of southern Alaska. An upper
level trough is still expected to amplify in the coming days as
strong jet support traverses with western periphery of the trough.
This will help to advect and warmer and wetter air mass up
towards Alaska. On Thursday, a surface low pressure south of Dutch
Harbor will begin to weaken. However, the frontal boundary
associated with this low and a surplus of upper level energy will
help a triple point low to develop south of the Alaska Peninsula.
This low is expected to move inland sometime late Friday morning
into Friday afternoon but a a bit of model divergence has made it
quite difficult to pinpoint all of the big details at this time.

With certainty, southern Alaska will see a pattern shift that
will entail temperatures warming a good bit above normal and
periods of rain and snow that will largely be confined to the
northern Gulf Coast. As it appears now, Anchorage and other areas
on the Lee side of the Chugach Mountains will be downsloped from
any precipitation with impressive cross barrier flow developing.
Confidence is increasing that a Chinook type pattern will set up
over the Anchorage bowl on Thursday and/or Friday which could
allow temperatures to warm into the lower to mid 40s. The
temperature forecast is heavily dependent on the amount of wind
that makes it into town but a warm up is nevertheless inevitable.
A mix of snow and rain will likely be seen along the coast
Thursday afternoon ahead of the frontal boundary but precipitation
across most of southcentral Alaska will quickly change to all rain as
850 temperatures warm to near zero degrees celsius and sfc to 850 mb
thicknesses rise quickly. Any precipitation from Friday morning
Onward across southcentral Alaska will likely be rain except for
portions of the Talkeetna Mountains, the Alaska Range and the
Aleutian Range where some minor snow accumulations could be seen.

Heading into the weekend, chances for rainfall and possibly a mix
of snow at times cannot be ruled out thanks to an active synoptic
pattern. A series of lows will pass through the Gulf of Alaska
heading into next week but trying to come up with any low position
is near impossible this far out as the models are quite volatile
past 4 or 5 days. The weekend looks warm as temperatures will
likely remain above normal and no reprieve is seen through the
first half of next week.


Afc watches/warnings/advisories...
public...Winter Weather Advisory 101.
Marine...Storm Warning 131 155 172 174.
Gale Warning 119 120 130 132 136>139 150 160 165 170 171 173 175
176 180.
Heavy frz spray 179 185.



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