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fxak68 pafc 181203 

Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
403 am akdt Tue Sep 18 2018

Analysis and upper levels...

The convoluted and rather unusual upper air pattern remains over
Alaska as general high pressure in the background "competes" with
slow moving and weak cutoff upper lows. The most prominent of
these lows is centered over the Bering Strait, and this low is
merging with another, albeit smaller and weaker shortwave trough,
over the Brooks range. There is another departing upper low over
the eastern Gulf of Alaska with broad northwest flow over the
eastern Bering Sea. A subtle shortwave has stalled over southwest
Alaska (more specifically the Kuskokwim delta) with an unusually
formed deformation zone over southcentral. All in all, dynamic
upper forcing is rather meagre this morning with more cloud layers
and patchy areas of fog than precipitation. Rounding everything
out, there is a strong gale force front clipping the far western
Aleutian chain, but this front will rapidly weaken as it moves
eastward into the Bering Sea today.


Model discussion...

The numerical models are in excellent agreement with the large
scale synoptic pattern through the remainder of the week. They
still remain in lesser agreement with a closed upper low over the
North Slope and how quickly it shifts south. The GFS remains an
outlier with the NAM, ECMWF, and Canadian in better agreement with
the southward track of this low into the weekend. The main impact
is how much precipitation/clouds can advect "under" the upper low
with the GFS favoring a wetter scenario for parts of southern
Alaska. For the forecast, the NAM and European model (ecmwf) were favored which
features a farther south track of this low by Friday, and less

Over the Bering Sea, the next low of significance is Thursday
evening through Friday, reaching close to gale force. While all
the models are showing slightly different tracks and strength,
they are all relatively clustered around one another so a blend
was used to capture this low.


panc...ceilings are expected to stay above 5000 feet through the taf
period with winds less than 10 knots. Dry conditions, with the
exception of a stray sprinkle, is expected.


Short term forecast southcentral Alaska (days 1 and 2)...
fairly benign weather conditions are expected for the next couple
of days, as the flow aloft and at the surface is light. For today
and tonight, some moisture over the area will keep conditions
somewhat cloudy. There will also be some showers around, but most
areas will stay dry. There will be a subtle shift in the pattern
on Wednesday, as the flow aloft becomes westerly and increases a
little. This will bring a decrease in clouds along with a small
increase in low level offshore flow. Clouds will increase on
Wednesday night as an upper level low in northern Alaska sends
some moisture southeastward into the area.


Short term forecast southwest Alaska and Bering Sea/aleutians(days
1 through 3)...

A weak boundary draped across the southwest combined with upper
level short waves propagating from the Bering Strait initiate
scattered rain showers across the region today...mainly across
the Kuskokwim valley. Since low level moisture remains high,
patchy fog will develop during the morning hours as temperatures
cool to near dew point. Across the expansive ridge
slides slowly east keeping low clouds/fog along with light north
to west flow. A strong front brings unsettled weather across the
western waters through tonight as it pushes into the western
perimeter of the ridge.

Relatively quiet conditions are expected on Wednesday as the front
diminishes and the surface ridge becomes the dominant synoptic
pattern across the eastern half of the Bering and southwest.
Patchy fog and clouds persist in this pattern as abundant moisture
advects around the ridge. However, rain chances increase across
the southwest Wednesday into Thursday as another short wave moves
inland. The next front to moves up from the North Pacific to the
western Aleutians late Wednesday spreading north to the central
waters by Thursday afternoon.


Long term forecast (thursday night through Monday night)...
to begin the long term forecast Thursday night, the models are
beginning to come into consensus with a further southerly track of
an Arctic trough approaching from the interior. This will allow
for cooler temperatures and the possibility of mountain snowfall
by Friday morning, especially from Talkeetna Mountains northward
where there is the best agreement for precipitation. Further south
over the chugach confidence is much less that precipitation will
spread that far to the south.

Behind this trough, weak ridging moving over the south Mainland
should provide for a break in the active weather, however this may
be very brief for the southwest coast as a frontal system from
the Bering quickly moves into the replace it. With the weak nature
of this ridging and poor agreement in how far east the remnants
of the Bering front will reach, forecast confidence for the
upcoming weekend is very low, especially in the rainfall forecast
around the south Mainland. The general trend then looking into
early next week will be for continued deteriorating conditions for
much of the south Mainland as the ridge axis shifts east of the
area and allowing for a stormy pattern over the Bering and North
Pacific to track into the area.


Afc watches/warnings/advisories...
Marine...gales 178.



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