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Southcentral and southwest Alaska forecast discussion 
National Weather Service Anchorage Alaska
133 PM akdt Friday Aug 1 2014

Analysis and upper levels...
a semi-classic Omega block pattern remains in place across the
northern Pacific and alaskan domain...featuring a huge upper level
high over the central Bering with deep lows over the northeast
Pacific and south of the Kamchatka peninsula in Russia. A
weak front has moved over the top of the Bering high and dropped
into the Mainland and has brought steady rain to the southwest
Mainland and interior portions of the state...but is now being
largely hung-up along the Alaska Range. The supporting upper
level shortwave is quickly stretching out and weakening...and as a
result the whole thing will slowly fall apart more-or-less in
place over the next 12 hours. Some middle-level clouds have pushed
into southcentral...but this will be the largest impact from this
front on the major population centers in the area. Out
proceeds as normal in the Bering Sea under high pressure...with
lights winds and widespread fog/stratus remaining the main
forecast challenge.


Model discussion...
the overall atmospheric evolution over the coming days has broad
support among the models. This will consist of the strong high in
the Bering weakening and gradually retreating to the northwest.
This will allow the large upper low south of the Gulf to drift
northward toward the Mainland. As this happens...the southern
Mainland and southwest Alaska will increasingly come under the
influence of easterly flow and embedded shortwave troughs. For the
most part the models are in decent agreement on where the actual
low center will move...but there is less agreement on the
strength...timing...and positioning of shortwave troughs
pinwheeling around the circulation. In general the European model (ecmwf) and
gemglb have shown the most agreement...and are the preferred
models today...but even these models leave some run-to-run
consistency to be desired. To start out...these shortwaves will
impact mainly the Gulf coastal areas. However...there are
starting to be some indications that a more significant
shortwave/rain shield may move through the entire area sometime
on Tuesday. Stay tuned.


Short term forecast...

Southcentral Alaska...
clouds and drizzle/light rain have spread into the northern
Susitna Valley and northern Copper River basin. As mentioned
above...this rain is going to have a very hard time spreading much
further south before the upper level support completely falls
apart during the day Saturday. Most of the area will only see middle
and high clouds from this feature. Meanwhile...Kodiak Island is
already under the influence of the big northeast Pacific low...and
the broad easterly flow from this low will be what spreads north
to the rest of the area by the end of the weekend. This will
commence a series of upper level shortwaves moving around the low
and impacting the forecast areas. The first shortwave will spin
around the low and may even stay south of Kodiak Island tonight.
The second in the parade of shortwaves will form a weak surface
front and move further north and impact the north Gulf Coast by
Sunday morning. Winds will increase through the usual gaps...but
most of the rain will stay along the coast. By late Monday...with
increasing model disagreement...a third and possibly more
significant wave will move through the area...with the potential
to bring rainfall even to inland areas.

Southwest Alaska...
the worst is over in terms of rainfall amounts across the region
as the same weather front mentioned above is falling apart over
southwest Alaska as well. Rainfall amounts range generally from a
tenth of an inch to a quarter of an inch...with the heaviest
amounts over the Delta and lower Kuskokwim valley. Showers have
now spread into Bristol Bay...but are struggling to hold together
and will diminish entirely once diurnal heating ends late this
evening. As the weekend unfolds...the fairly weak but persistent
thermal trough will team up with breaks of sunshine and cooling
temperatures aloft (associated with the upper low moving into the gulf)
to steepen lapse rates and likely increase diurnal shower
activity. This destabilization will be very gradual...and by
early next week a few thunderstorms in unstable northeast flow
cannot be ruled out at this time.

Bering Sea/Aleutians...
the weakening remnants of the Mainland front will bring a touch of
rain to the eastern Aleutians and Alaska Peninsula tonight and
tomorrow before flow becomes drier and increasingly northerly for
the remainder of the weekend. The rest of the area will continue
to see the effects of strong high pressure...meaning fog and
clouds and light winds. As the upper level pattern slowly changes
back to a more typical pattern the anomalous Tranquility (and in
some cases record warmth) will come to an end. By Monday night...a
front with gustier southerly winds will enter the western


Long term forecast (tue Aug 5th - Friday Aug 8th)...

Beginning Tuesday...a high latitude block will remain a dominant
feature...centered over the Bering Strait and Chukchi Sea...with
continued ridging over the Bering Sea. A large and sprawling low
will also remain in the Gulf of Alaska...with this feature driving
sensible weather across southern Alaska at the beginning of the extended
period. As alluded to in the short term forecast...there is
uncertainty with the strength of an easterly wave rotating into
southcentral along the periphery of the Gulf low. This will
dramatically impact temperatures and rainfall potential Tuesday
across portions of southern Alaska...but much of this uncertainty
is erased by Wednesday and Thursday as the Gulf low weakens and
splits. This will leave a typical Summer type pattern over most of
southern Alaska...with afternoon showers and near normal
temperatures. Across the Bering...the ridge will finally break
down...with trough and associated Pacific jet crossing the
Bering Tuesday through Friday...bringing rain to much of the
Bering Sea and Aleutians.


Afc watches/warnings/advisories...
Fire weather...none.


Amend/ahsenmacher Aug 14

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