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Southcentral and southwest Alaska forecast discussion 
National Weather Service Anchorage Alaska
410 PM akst Friday Dec 26 2014

Analysis and upper levels...
a strong low has moved into the western Bering Sea this morning
and has likely reached peak intensity around 958 mb. The
associated front is moving through the eastern Aleutians...but is
encountering resistance in the form of a rapidly building ridge
currently amplifying into southwest Alaska from the North
Pacific. Most of the bite associated with this front is in the
form of gusty winds...with the area of precipitation quite narrow
from Dutch Harbor up to the pribilofs. East of the Alaska
Range...the aforementioned amplifying ridge is causing a series
of shortwave troughs to drop quickly across the area from the
west. The atmosphere remains unstable enough that these troughs
continue to kick off numerous showers around the Cook Inlet
area...but are most intense and persistent over westward facing
slopes of the chugach and Kenai Mountains. The first shortwave
will zip down into the Gulf this afternoon...providing a brief
break in shower activity. A second trough is currently moving
into the Kuskokwim valley and will bring another round of shower
activity generally from Anchorage northward this evening.


Model discussion...
as a ridge continues to amplify over southern Alaska over the next
few days the front currently in the Bering will slow or stall in
the vicinity of Kodiak Island and southwest Alaska. At the same
time...the large parent low in the western Bering will continue
to send shortwave troughs northward along this stationary
boundary...and it is the timing and placement of these waves that
represent the biggest forecast challenges during the short term
period. The reason this is important is because strong winds will
certainly develop through favored gaps in the Aleutian Range and
also through Turnagain Arm as this front moves into the area...but
each time one of this weak impulses moves through represents an
additional tightening of the pressure gradient and the potential
for even high winds. Predictably...the models are struggling a bit
with these and other small details that will have higher-than-
normal impact on observed weather. The global models (namely the
GFS and ecmwf) are in better agreement...which makes sense given
strong meridional flow. Thus when it comes to some of the smaller
local wind details the NAM will not be trusted too much...even
though it does have Superior resolution in the complex terrain.


Short term forecast southcentral Alaska (days 1 and 2)...
scattered showers will linger around parts of southcentral as a
series of upper level shortwaves track through the area from west
to east tonight. The initial wave will bring showery conditions
from parts of the Kenai Peninsula all the way north into the
Talkeetna Mountains....where the second wave right behind it will
focus the showers further north (from Anchorage north). Upsloping
will cause showers to be most intense over westward facing slopes
of the chugach and Kenai Mountains with the initial wave...then
over the chugach and Talkeetna Mountains with the next wave.
Showery conditions will then taper off by tomorrow morning as the
high amplitude ridge builds in over the area from the west. As the
ridge axis tracks further to the east tomorrow winds
will begin to increase as the gradient over parts of south central
tighten up. Winds specifically along Turnagain Arm will become
very gusty by Sunday afternoon.


Short term forecast southwest Alaska (days 1 and 2)...
a strong warm front moving north across the Bering this evening
will clip the southwest areas. Ahead of the front...snow will
develop this evening and bring easterly winds capable of blowing
the snow around severely enough where a Winter Weather Advisory
for blowing snow is warranted for the northern coast of the
Kuskokwim Delta. Snow will end from south to north...ending last
around Hooper Bay and Chevak around midnight or shortly

Much warmer temperatures will push north..and combined with much
drier air just above the surface widespread fog should develop
across the southwest. In fact...interior Bristol Bay areas have
already seen visibilities struggle to get above zero during the
afternoon today.

The main threat after this will be a strong Pacific storm system
forecasted to move south to north across the akpen into the
eastern Bering. A powerful jet streak...both aloft and in the
lower levels of the atmosphere...will become situated across the
Bristol Bay zone. This will bring increasing east to southeast
winds possibly approaching High Wind Warning criteria for a large
portion of Bristol Bay late Saturday night through Sunday
evening...with the possibility of strong winds continuing through
as late as Monday night.


Short term forecast Bering Sea/Aleutians (days 1 and 2)...
strong southerly winds and rain and snow showers will continue out
in the western half of the Bering and Aleutians...with a pair of
lows impacting the eastern Aleutians and akpen with more
persistent rain and winds through the weekend. After a period of
rain and winds tonight...the pribilofs may get somewhat lucky and
remain between these two unsettled weather regimes for a good
portion of the weekend.


Long term forecast (monday through friday)...
a highly amplified northeast Pacific Ridge will have situated
itself over southcentral Alaska by Monday morning. This will allow
southerly flow to advect warmer air and moisture from the
subtropics toward southcentral Alaska. This will lead to
increasing precipitation chances to the northern Gulf Coast...with
most if not all falling as rain. Due to the moisture flux
associated with the plume of moisture from the subtropics...rain
will likely be heavy at times. It would not be surprising to see 4
to 5 inches of rainfall with possibly locally higher amounts along
the immediate Gulf Coast area. Precipitation will initially be a
mix of rain and snow showers across the interior with Stout cross
barrier flow keeping a good chunk of the Kenai
Peninsula...Anchorage and the mat-su valley initially on the dry
side. As the ridge slides eastward late Tuesday into upper level trough will push across southcentral
Alaska and weaken the cross barrier flow and bring widespread
precipitation to the region. Rainfall will likely be mostly rain
from Anchorage southward...with a mix of rain and snow observed
over the mat-su valley.

In addition to the warming temperatures across southcentral
Alaska...the potential for a high wind event for the Anchorage
Hill Side and Turnagain Arm is becoming more favorable. A tight
pressure gradient from the amplifying northeast Pacific Ridge and
a low in the Bering Sea colliding with this ridge will act to set
up the stage for the potential wind event. There is however some
uncertainty in with the placement of a low late Monday into
Tuesday which could have an impact on winds hitting warning level
or ending up just shy. As with the previous forecast
packages...have continued to trend winds up with gusts to 60 miles per hour
now mentioned in the forecast. Should conditions continue to
remain favorable...a watch could be issued as early as tomorrow

The Bering Sea will continue with the active weather pattern. As
an area of low pressure moves across the eastern Aleutians early
Tuesday and continues to move north rapidly through the remainder
of the day...a second low pressure system originating from the
Kamchatka peninsula will begin impacting the western Aleutians. A
front will push across the central Aleutians Wednesday morning
before stalling and falling apart during the second half of the
week. Ridging will build across southwest Alaska beginning
Wednesday morning and persist through the work week with
diminishing precipitation chances.


Afc watches/warnings/advisories...
Marine...Storm Warning 185.
Gale Warning 130 131 132 138 150 155 160 165 170 171 172 173 174
177 178 179 180 181.
Heavy freezing spray 185.
Fire weather...none.



Synopsis and model
southcentral Alaska...pepe
southwest Alaska/Bering Sea/Aleutians...cirrocumulus
long term...cirrocumulus

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