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FXAK68 PAFC 201242

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
442 AM AKDT Wed Sep 20 2017


The polar jet remains positioned over the area in a northwest to
southeast orientation from the northern Bering across Southwest
Alaska and into the western Gulf. A wave of energy moving along 
the north side of the jet is currently spreading light rain across
the Kuskokwim Delta up through the Lower Kuskokwim Valley.
Proximity to the jet appears to be hampering much southward
progress to the rainfall, however.

Another low centered south of the central Aleutians consists of a
strong warm front transporting a subtropical air mass northward
across the eastern Aleutians. The moisture-laden air mass is
producing rainfall that is heavy at times across much of the chain
this morning. This very warm air mass aloft is helping push the
jet north and east with time, which will play into the forecast
for mainland Alaska over the next few days. More on that below.

Mainly clear skies and light winds across Southcentral are 
allowing temperatures to plummet across the region this morning, 
with numerous locations dropping down into the 30s and threatening
a first frost for many inland locations.



The models all are in very good agreement for much of the short-
term. One significant difference appears by Thursday evening where
the GFS depicts a much stronger secondary low developing over the
northern Gulf in advance of the subtropical air mass lifting
northward as compared with all of the other models. The other
models keep a trough over the north Gulf Coast, but it never
really develops. The result is the GFS shows much stronger winds
across the northern Gulf than the other models, and being the
outlier, was eliminated as part of the forecast. The differences
between the other models were not significant with regards to that
feature. During the day Friday, the models diverge on
precipitation coverage, with the GFS and to a lesser extent the
NAM ending the rain earlier in the day, with the other models
keeping more persistent rainfall. Forecast confidence remains 
high with excellent overall model agreement, though confidence is
lower regarding those other differences between the models.


PANC...VFR conditions and light winds will persist. A brief shower
or sprinkle will be possible during the day, but should not cause
any long-term changes to the VFR conditions.


A fast moving upper level short-wave trough over Southwest Alaska
this morning will cross Southcentral today, bringing a quick shot
of rain to most inland areas, including the Mat-Su and Copper 
River Basin. Areas to the south will remain dry with just some
passing mid to high clouds. 

All eyes will then shift to the storm south of the Aleutians,
which will send a warm front, with very warm and moist air toward
Southcentral. There will be a break tonight between the departing
short-wave and this warm front, but mid to high clouds will 
stream well out ahead of it. Thus, temperatures will not fall as
far as they did this morning for most of Southcentral. 

Rain will overspread Kodiak Island this evening then continue
north and east overnight through Thursday. It looks like a fairly
quick shot of steady rain for most places right out ahead of the
warm front. Development of a triple point low over the northern
Gulf on Thursday afternoon combined with progressive flow in the
upper levels will then help shunt the deepest moisture eastward 
toward Southeast Alaska. A persistent supply of upper waves
transiting Southcentral combined with the fairly warm and moist 
air mass in place will maintain a threat of rain Thursday night 
and beyond. The forecast challenge will be to try and lock in on 
specific features and associated rainfall.


There is a fast moving low and and associated frontal boundary
moving into western Alaska. More precipitation is expected for the 
Kuskokwim Delta. A break in activity is expected and then a 
second wave of moisture will enter the region from the south and 
move directly into the Bristol Bay coastline. Rain looks to 
continue into Thursday evening and beyond as the longwave trough 
extending from the Arctic phases with the upper low south of the 
Aleutian chain and helps to establish broad southwest flow over 
the region into the latter half of the week. 


The latest ASCAT pass has the low south of Adak. The associated
warm front is overtaking the Aleutians. Expect rain and gale 
force winds. These unsettled conditions will spread from the 
Central and Eastern Aleutians to the Alaska Peninsula and the 
Pribilof Islands as the forecast period unfolds. The eastern half
of the Bering will remain in a wet and windy pattern while 
western portions of the Bering including will remain on the drier
side on the back side of the longwave trough.


.LONG TERM FORECAST (Days 3 through 7)...
Beginning the extended period forecast Thursday night, there will
be an arctic trough merging with a North Pacific warm front over
the state. Broad southwest flow will bring rain to most of
Southern Alaska heading into Saturday as the upper trough remains
in place. Uncertainty develops on Saturday night with another warm
front moving north toward the Gulf of Alaska with the ECMWF
favoring a deep 978 hpa surface low tucked into the west side of
the Gulf. However, the 12z GFS and Canadian GDPS models are
notably weaker and much farther east with the surface low.
However, even with this uncertainty, there will still be plenty of
forcing for continued coastal rain and rain chances inland through
the weekend. Eventually, the upper low will move southward into 
the North Pacific and Gulf of Alaska, which, in theory, should
result in less precipitation chances heading into mid/late week.


MARINE...Gales: 170 173 175 177 178 179 411 412 413 414



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