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Southcentral and southwest Alaska forecast discussion 
National Weather Service Anchorage Alaska
530 am akdt Wednesday Aug 24 2016

Analysis and upper levels...

The dominant feature on satellite continues to be centered around
a broad and nearly stationary trough stretching from the central
Bering Sea well into the North Pacific. On the eastern edge of
this trough, signs of an atmospheric river are beginning to appear
in the forecast area as moisture from as far south as 35n is
beginning to stretch into the Gulf Coast. A warm front associated
with this system is beginning to make landfall over the Gulf,
bringing widespread rain from the northern Gulf to southern
Bristol Bay. Ahead of this front, partial clearing along with
saturated low levels from recent rains has allowed for patchy
fog/stratus to develop, especially over low lying areas of the
southwest Mainland. Out west, a front associated with a low
centered over the Kamchatka peninsula is beginning to move through
the western Aleutians. This system will slowly move into the
Bering through mid week.

&&

Model discussion...

The models remain in good synoptic agreement through the short
term, with the main issue being placing rainfall throughout the
south Mainland through Thursday. In particular the models are
struggling with how much downsloping will occur over the Anchorage
bowl today. The higher resolution NAM is showing its usual dry
bias in keeping Anchorage dry for much of the day today, however
this has not verified well overnight so the rainfall forecast
continues to be painted with a broad brush. By late tonight
however all of the models are in very good agreement that
southwesterly flow will move in, leading to high confidence that
most of southcentral will see widespread rainfall into Thursday.
Over the Bering the models are having some minor differences with
the strength of the front moving through the area into Thursday.
The GFS in particular has been the quickest to weaken it and was
mostly ignored for the morning package.

&&

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

Rain has once again spread into the area as the upper level
trough over the North Pacific and eastern Bering continues to
amplify. A strengthening jet stream is beginning to push straight
up Cook Inlet, and is ushering in blossoming radar echos across
the whole area. A weak warm front passing across upper Cook Inlet
is also helping to focus rainfall this morning, and will be
moving out of the area by late this morning. The heaviest
rainfall today will be along the north Gulf Coast...especially
from Cordova over to Valdez. By tonight, a more coherent shortwave
will pass right over the area and take advantage of copious
atmospheric moisture transport to produce one final big slug of
rain in most areas. Many areas, including the big population
centers, will likely see a multi-hour period of heavy rain
overnight tonight. Rainfall totals in these interior areas will
most likely fall between one-half and one inch with higher amounts
in the Talkeetna Mountains and northern parts of the Susitna
Valley. Meanwhile, the Prince William Sound and north Gulf Coast
area will again bear the brunt of the rain, with impressive 36
hour rainfall totals upwards of 4 inches possible around Cordova
where orographic enhancement in southwesterly flow will be
maximized.

Rivers are already high in much of the Susitna Valley, but
thankfully this area will Dodge the heaviest rainfall from the
storm today and tonight. Nevertheless, enough rain will fall to
likely keep some rivers around flood stage for another 24-36
hours...thus the Flood Advisory remains in effect. Meanwhile, the
slightly different wind flow from this storm will produce
favorable orographic enhancement along the Talkeetna Mountains
and along the north Gulf Coast. Both of these areas will need to
be watched closely as the day unfolds as water is bound to rise on
area rivers and streams and could approach bankfull.

Finally, thunderstorm potential remains elevated Thursday
afternoon across much of the area as the upper disturbance tracks
overhead with enhanced instability and likely breaks of sunshine.

&&

Short term forecast southwest Alaska...

Troughing extending from the West Coast across the Aleutians is
bringing shortwaves from the south over southwest Alaska. This is
bringing rain to the area that will continue until Friday when
weak ridging moves in from the west. The ridge will help to clear
out the rain through Saturday.

&&

Short term forecast Bering Sea/Aleutians...

The ridging over the western Bering will push east today and then
south tonight as the trough over the eastern Bering weakens. The
ridging is also helping with fog formation for much of the Bering
and Aleutians. Behind the ridge a new system will push into the
western Bering tonight from the west and then to the central
Bering Thursday. It will then stall and diminish through the end
of the week.

&&

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

The advertised pattern change is still on schedule but just a
touch quicker than expected yesterday. By Thursday evening high
pressure will begin to build across the Alaska Mainland which will
help begin the pattern change as offshore flow develops. This flow
will help dry out the lower levels of the atmosphere and decrease
rain chances Friday through the beginning of next week. The upper
level pattern is also changing. The upper level ridge will remain
anchored over southwest Alaska with a broad upper level low over
the Alaska/Canada border. This complicates the forecast a bit as
it will likely keep a few showers near higher terrain in the
Copper River basin and possibly Susitna Valley. However, much of
southwest and southcentral Alaska will easily see some of the
best weather this August has yet to offer. Confidence in the nice
weather decreases on Monday as models are struggling on how to
resolve an upper low near the Bering Strait. This feature,
although well outside of our forecast area, will impact the
amplitude of the building ridge over the western half of Alaska.
On the one hand, if this low ends up splitting its energy between
a low in the Arctic and another low near the aleutains, then the
Alaska Mainland could be in for an extended period of nice weather.
On the other hand, if this feature gets cut off, then it will
limit the extent of the upper level ridge and likely bring parts
of Alaska back into a wetter period by the middle of next week.

&&



Afc watches/warnings/advisories...
public...Flood Advisory 145.
Marine...gales 185 411 412.

&&

$$

Synopsis and model discussion...dek
southcentral Alaska...ad
southwest Alaska/Bering Sea/Aleutians...dk
long term...mc

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