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Southcentral and southwest Alaska forecast discussion 
National Weather Service Anchorage Alaska
455 am akdt Tuesday Aug 4 2015

Analysis and upper levels...

The Rex block continues over the Mainland and is bringing clear
skies and high pressure from the southern Mainland into the
western Bering Sea. This ridge associated with this helped to
bring record high temperatures to many locations around
southcentral and southwest Alaska yesterday...and will bring
continued warmth to the same areas today. Over the Bering Sea
ridging has allowed for extensive fog/stratus to develop over much
of the region...which is beginning to spread into the Kuskokwim
Delta with onshore flow. Over the Gulf of Alaska the fog and
stratus is much less widespread...but can still be clearly seen in
nighttime microphysics imagery from the Cordova area into the
eastern Gulf...with a much smaller area near the Barren Islands.
To the south of the Rex block a weak and nearly stationary low
spins just to the south of the akpenn with an associated front
stretching into the western Gulf. Meanwhile over the northwest
Pacific another low is approaching the western Aleutians...but is
failing to make much progress to the east as ridging builds over
the central Bering.


Model discussion...

The models continue to handle the weakening Rex block over the
Mainland as it shifts westward and becomes an Omega block over the
Bering Sea. As this occurs..the models begin to break down as the
low currently south of akpenn shifts east and slowly weakens over
the Gulf on Wednesday. The ec has been the most consistent and
further south with this low and was preferred for the morning
package. These differences are small however and should not have
a significant impact on forecast confidence. Larger differences in
the models arise with an Arctic trough that will enter the
northwest coast and move into the central interior on Wednesday.
The NAM has been consistently the strongest and furthest south
with this feature...while the GFS and ec have been consistently
weaker and slightly further to the north by keeping the majority
of the trough north of the aor. For now the GFS and ec were
preferred for this feature...though there still remains
uncertainty in the timing of rainfall around southcentral late
Wednesday into Thursday.


Fire weather...

High pressure over the southern Mainland will bring one more day
of warm and sunny weather to the area. The driest conditions are
expected in the Copper River basin and Kuskokwim valley this
afternoon/evening...where relative humidity values are expected to drop to
around 30 percent. Red flag conditions however are not expected as
light winds will continue in the driest areas. There is a slight
chance of wet thunderstorms as well over the Talkeetna Mountains
and western slopes of the Alaska Range this afternoon as a weak
thermal trough diminishes. Any thunderstorms that do develop
however are only expected to be short lived and only produce a few
isolated lightning strikes.


Short term forecast southcentral Alaska (days 1 and 2)...
the upper level ridge over southern Alaska will slowly weaken and
begin to move east today. Most of the area will remain dry and
very warm...with record high temperatures forecast at some
locations again today. Kodiak is the main exception to this as a
low moves toward the island from the southwest. Also...a few
showers and thunderstorms could develop later today over the
mountains around the Susitna Valley as the subsistence inversion
weakens a bit. On Wednesday the ridge will slide into Canada. Much
of south central Alaska will then be in a "col" between the low
centered south of Kodiak and an approaching trough over central
Alaska. The areal coverage of scattered showers will become more
widespread on Wednesday into Wednesday night as the Kodiak low
and central Alaska trough begin to interact.


Short term forecast southwest Alaska (days 1 and 2)...
high pressure aloft will bring another day of warm and dry
conditions to the southwest Mainland today...which will begin to
cool slightly over the next few days with the approach of a upper
level trough digging through the Mainland from the north. There
are still some minor model discrepancies regarding positioning and
strength this trough dropping down from the north...which has
slightly lowered forecast confidence on exactly when and where
showers will develop and how far temperatures will drop. Rain will
continue to develop along the Alaska Peninsula this morning and
persist through Wednesday the associated North
Pacific low tracks into the western Gulf. Enough moisture and
lifting from the low will support the development of isolated to
scattered rain showers this afternoon/evening along the Alaska
Range...with the possibility of isolated thunderstorms developing
near the Alaska Range. Continued pushes of marine stratus and fog
will impact the Kuskokwim Delta during the overnight and early
morning periods...with patchy fog across valleys in the lower
Kuskokwim valley.


Short term forecast Bering Sea/Aleutians (days 1 and 2)...
high pressure will continue to dominate over much of the
area through Wednesday...keeping low stratus and patchy fog over
much of the Bering Sea under the stable high pressure. As the low
south of the Alaska Peninsula tracks further eastward...showers
will linger over the eastern Aleutians under the influence of
continued northerly flow through Wednesday night. A front
associated with a North Pacific low south of the western Aleutians
will approach the western and central Aleutians Thursday
morning...bringing another round of rain and gusty winds to the


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

Wednesday night through Friday will be a transitional pattern as
two low pressure systems break down the amplified ridge over the
Bering. Models were in better agreement with the Middle Range
forecast...however the ec solution was heavily utilized to make
changes as it had the most reasonable solution on the track of
the low pressure system moving up from the North Pacific.

On Wednesday night a strong shortwave digs south across the
interior with some of the energy diving around the Gulf low. Rain
is expected to develop along this wave Wednesday night with
activity starting along the Alaska Range to the Aleutian Range.
Rain then spreads south and east into Thursday. On Friday and
Saturday...weak ridging sets up behind the front keeping the
southern Mainland dry...except for a few afternoon showers along
the ranges.

To the west on Wednesday night...a North Pacific low pressure
system moves a front into the western and central Aleutians. This
front makes slow progress with a better developed rain band
on Thursday. The front occludes late Thursday and pushes its
leading edge into the central Bering with the boundary stretched
out to the Alaska Peninsula by Friday.

The Bering low stalls on Saturday and organizes into a stronger
and faster moving system Saturday night through Monday. This
system is expected to bring gusty winds and rain across the
southern Mainland and Gulf waters through the beginning of next


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



Synopsis and model discussion...dek
southwest Alaska/Bering Sea/

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