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Southcentral and southwest Alaska forecast discussion 
National Weather Service Anchorage Alaska
455 PM akdt Thursday Aug 27 2015

Analysis and upper levels...

The weather pattern across the alaskan domain is in a state of
transition today. West/southwest flow across the entire area is
becoming more north/northwesterly as the upper level ridge in the
Bering and an upper trough are both beginning a rapid
amplification. A pair of shortwave troughs are moving
southeastward across southern Alaska representing the beginning
of this pattern shift. The first shortwave came through Cook Inlet
this morning mostly dry...with rain falling largely over the
Copper River basin and eastern Prince William Sound. The second
shortwave is crossing the Alaska Range as of midday...but is even
more moisture-starved than the earlier minimal
additional shower activity is expected. Southerly and easterly
winds across southcentral Alaska have largely ceased and the
widely anticipated northwesterly "cold and dry" winds are
beginning in earnest...with the predictable area in terrain gaps
such as Seward and Whittier already showing abrupt increases in
wind speed.

Out west...sprawling high pressure remains in place across the
central Aleutians...with freezing levels at Cold Bay on the 12z
morning upper air sounding an impressive 17000 feet. Fog and stratus
remain widespread over much of the area...including low clouds
that have been pushed onshore the Kuskokwim Delta and parts of
Bristol Bay.


Model discussion...

The models have "locked into" the evolution of major features over
the next day or so. There has been a small increase in forward
speed of some of the weaker shortwaves moving through the
amplified northwest flow on Friday and Saturday...but this is not
surprising given the impressive strong jet stream already in place
over the area (160+ knots at McGrath this morning on the 12z raob).
The higher resolution models (nam/wrf) will provide the basis for
forecast updates...due to obvious advantages as strong winds
accelerate through the complex terrain gaps around southern


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

The big forecast challenge for the day is pinpointing the
intensity of the winds expected overnight tonight and through
tomorrow morning across the area. The coastal communities will
still bear the brunt of the strong winds. Atmospherically
speaking...all the major pieces are in place for an impressive
"outflow" wind event from the north and west. This includes
impressive jet support (from the northwest)...a low in the Gulf to
setup an offshore pressure gradient...and relatively cold and
dense air moving into the area from interior Alaska. What is more
unusual in this case is the time of year. This is a fairly typical
late fall/winter pattern...but not a common late Summer/early
fall pattern. As such...high wind warnings have been issued for
the Valdez area (including in town) as well as Whittier and
Seward. The worst conditions will be overnight tonight...but winds
will remain generally gusty through Friday night in most areas.

The other side of the Coin is the unseasonably cold nature of this
airmass. Once winds start to diminish late Friday...the typical
cold spots of east Anchorage...The Butte...and Meadow Lakes area
in the Matanuska Valley will make a strong run at freezing
temperatures early Saturday morning. The main uncertainty at this
point is if any middle-level clouds will loiter around the area to
help keep overnight temperatures a few degrees warmer. This will
be monitored closely over the next 24 hours...but even if freezing
temperatures are not reached in these areas it will still be
close...with temperatures the coldest of the season so far across
the region.


Short term forecast southwest Alaska (days 1 and 2)...
precipitation over much of the southwest Mainland will continue
to diminish this the second of these waves departs
into the Gulf of Alaska and Ushers in brisk northwest flow on the
backside of the digging trough. This will produce gusty northwest
winds and also contribute to colder temperatures
overnight lows dip into the 30s across much of the area beneath
decreasing cloud cover. The period of dry weather will be short
lived as another wave pushes into the southwest Mainland Friday
afternoon. This will bring another round of rainfall to the area
Friday night into Saturday morning...before switching over to
showers late Saturday morning as the wave tracks into the western
Gulf. With winds shifting to a stronger onshore flow overnight
tonight...patchy fog off the Bering Sea is expected to push into
the Kuskokwim Delta and persist through Friday evening.


Short term forecast Bering Sea/Aleutians (days 1 and 2)...
aside from a few showers in northwest flow across the Alaska
Peninsula and eastern Aleutians...the bulk of the rainfall will
remain confined to the far western parts of the Aleutian chain
through tonight as the North Pacific ridge is re-established over
the central Bering Sea. Rain will then spread to the northern and
eastern Bering Friday as an upper wave rounds the top of the
ridge...with rain returning to the rest of the Aleutian chain by
the weekend as the ridge breaks down. Widespread stratus and fog
is expected to accompany the ridge as is persists over the Bering
through Saturday.


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

Beginning the extended period on Saturday evening...the state of
Alaska will be under the influence of a large Arctic trough
centered over western Alaska...with a weakly phased shortwave
trough in northwest flow tracking into the Gulf of Alaska. An
associated 140 knots upper level jet streak will be tracking over
the Gulf as the Arctic trough makes southward progress through
Monday. This will reinvigorate the offshore flow pattern as a
surface low forms in the Gulf of Alaska and tracks toward the
Panhandle. Rain and snow showers will remain largely over the
mountains through Monday...although lower elevation showers will
be more prevalent across the lower Kuskokwim valley and Bristol
Bay regions. Otherwise...rapid drying will commence on Monday as
the trough continues to dig through the southeast Gulf...and
offshore and downslope flow dries the atmospheric column. Weak
ridging aloft will move over the state from west to east Tuesday
and Wednesday...with a slow warmup but continued dry conditions.
Precipitation chances redevelop toward late week as a series of
weak lows track from the Bering into southwest Alaska. With that
said...the upper level flow will remain rather weak...with no
significant storms or wet periods anticipated with the change in
pattern. Due to slight uncertainty in the extended period toward
the end of next week...a blended wpc ensemble approach was favored
with some blend of European model (ecmwf) and GFS Saturday evening through Monday
to capture outflow winds along the coast.


Afc watches/warnings/advisories...
public...High Wind Warning 125 131.
Marine...gale 119-125 127-138 150 165.
Fire weather...none.



Synopsis and model discussion...amend
southcentral Alaska...amend
southwest Alaska/Bering Sea/
long term...ja

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