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Southcentral and southwest Alaska forecast discussion 
National Weather Service Anchorage Alaska
518 am akdt Friday Oct 9 2015

Analysis and upper levels...

A broad longwave trough sits over the entire state...encompassing
an area from the western Bering Sea through to the coast of the Pacific
northwest. A weakening low at the center of the trough is slowly
drifting northwestward just south of the Alaska Peninsula. Broad
onshore cyclonic flow around the low with embedded upper-level
disturbances has brought rain to portions of southcentral Thursday
evening. The heavier rain over the Prince William Sound has
dissipated overnight as an upper-level disturbance and most of the
moisture exit to the northwest. Downsloping has kept Anchorage and
the western Kenai Peninsula mainly dry. Rain over the western Cook
Inlet and western Susitna Valley is starting to taper off
southeast to northwest as the upper disturbance moves
northwestward. A jet streak wrapping around the base of the
longwave trough is providing large scale ascent in the left exit
region over the eastern Gulf of Alaska. This is supporting
intensification of a surface low moving northward through the
eastern Gulf. The deepening low...which is associated with the
tropical remnants of former hurricane transporting a
moisture rich subtropical airmass into southeastern Alaska that is
quickly moving towards the southeastern fringes of the aor.

Further west...a weakening occluded front associated with the
North Pacific low is tracking westward through the Bering
Sea. Northerly gale force winds associated with the front are
sweeping through the eastern and central Bering and Aleutians.


Model discussion...

Models are in good synoptic agreement in the short term with the
North Pacific low weakening today south of the Alaska
Peninsula. Slight differences arise in the handling of the
tropical remnants of oho...with the main impacts to our area being
the extent to which the subtropical moisture impacts southcentral
tonight. The latest models runs have come into better agreement
with the energy associated with the system moving westward through
southcentral overnight...bringing rain through the area. The
higher resolution models were the models of choice for this system
to account for the effects of terrain in the easterly flow.
However there was a notable difference between the 00z NAM run and
the 06z NAM run in the amount and placement of precipitation
moving through southcentral...with the 06z run appearing to have a
better handle on the local downsloping effects with the easterly
flow by keeping the Lee of the mountains considerably drier than
the 00z run.

Larger forecast uncertainties arise with another low moving into
the eastern Gulf Sunday. All models are tracking a low into the
northeastern Gulf by Sun night...however there is model
disagreement on the exact track of the low and the intensity of
the system. This impacts the early week forecast for
southcentral...with the furthest west ec solution supporting a
Rainier and windier beginning of the work week while the American
models keep the brunt of the impacts confined further east over
Southeast Alaska. Forecast confidence is slightly below average in
the short term due to the uncertainties with this system.

Short term forecast southcentral Alaska (days 1 and 2)...
winds and rain will gradually diminish across much of the area
through this afternoon as the low slowly weakens near the Alaska
Peninsula...with a few showers lingering across Kodiak Island into
the evening in closer proximity to the low center. Attention then
quickly turns to the intense extratropical low tracking into the
eastern Gulf this afternoon. Although the most significant
impacts from this system will remain confined to the Panhandle and
eastern Gulf...modest gap winds will develop along the north Gulf
Coast this evening in response to the tightening pressure
gradient. Rain will then spread back into Prince William Sound and
the Copper River basin tonight as the low moves inland near
Yakutat and interacts with an upper level wave traversing the
Gulf. There is still some question with respect to the westward
extent of this precipitation...but it appears for now that the
western Gulf and Cook Inlet region will remain dry through tonight
before rain spreads back in from the east Saturday into Saturday
night...with strong cross barrier flow helping to limit rain
potential from the Anchorage bowl northward into the mat-su


Short term forecast southwest Alaska (days 1 and 2...Friday and
sat)...most of the sensible weather continues to be controlled by
the slow moving upper level low parked just east of the Alaska
Peninsula. This feature continue to push light rain from east to
west...mainly through Bristol Bay. This low is also making for a
moderately tight gradient which is producing some gusty east to
northeast winds blowing. The low will continue to meander in that
area through the day...but it will weaken and wrap some drier air
in from the east. This should allow for a break from the more
widespread steady precipitation. This dominant low will not move
much by Saturday when the remnants of hurricane oho pushes north
through the eastern Gulf. Models are still in the process of
figuring out how these two features will interact. While general
thinking is that not much of the precipitation associated with oho
will make it over the Alaska Range...there will still be enough
moisture and instability overhead to make for chance to likely
probability of precipitation by Saturday evening.


Short term forecast Bering Sea/Aleutians (days 1 and 2...Friday and
sat)...northerly flow pretty much sums up this forecast. The low
near the Alaska Peninsula and high pressure over northeast Russia will work
in tandem to keep gale-force northerly winds over much of the
central Bering. Some colder air will be able to sneak south with
these winds. With sea surface temperatures near their
climatological maximum for the year...this will yield enough
instability to keep showers across the area. The best chance for
more persistent rain and heavier accumulations will be on the
Bering side of the Aleutians on north facing slopes.


Long term forecast (days 3 through 7)...
the remnants of the occluded low near Chirikof Island will begin
to phase in with another North Pacific low that will be tracking
into the lower southeastern Gulf by Sunday. This will continue to
produce a showery weather regime in the coastal marine communities
and drier conditions in the inland areas as we head into the
weekend. The models are having difficulty with the storm path with
this storm force low as it continues to move farther north in the
Pacific. By Sunday evening the European model (ecmwf) model depicts this storm
farther west in the northern Gulf...while the NAM/GFS models
positions the low closer to Sitka. The forecast confidence is
average due to uncertainty as this system is prognosticated to track
into the northern Gulf by Monday before moving inland near Prince
William Sound by late Monday evening. This system will bring
widespread rain and strong gusty southeasterly winds to the
eastern Gulf before transitioning to the northern Gulf through the
early part of next week. By Tuesday a low pressure system will
cross the eastern Aleutians before tracking just east of Kodiak
Island by Wednesday. This will bring southwesterly flow before
shifting to southeasterly winds through the Gulf by midweek. Look
for warmer temperatures accompanied by moisture spreading into
the southwestern Gulf and into the northern Gulf coastal marine

While...the Bering Sea remains under northerly flow with a
scattered showery weather regime as an occlude low remains stationary
near the Shumagin Islands through the weekend. By Monday...a low
drops down from the Kamchatka peninsula into the western Bering
and the Aleutian chain region before crossing the eastern
Aleutians by Wednesday. This system will bring warmer temperatures
with the southwesterly winds accompanied by rain/stratus before
spreading eastward along the chain through the midweek. By late
Wednesday evening another low pressure system with an associated
weather front will enter the western Aleutians. The models are
having difficulty with the placement/strength of these systems as
they track into the western Aleutians. Therefore...the forecast
confidence is low as we head into the midweek.


Afc watches/warnings/advisories...
Marine...172 173 174 175 176 179 185.



Synopsis and model discussion...rsd
southcentral Alaska...cumulonimbus
southwest Alaska/Bering Sea/Aleutians...mso
long term...period

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