Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 291246

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
446 AM AKDT WED JUN 29 2016


The most impressive feature on the surface map this morning is a
stalled frontal boundary draped across the central Bering Sea.
Latest observations including scatterometer data puts the frontal
boundary ever so slightly west of the Pribilof Islands. Gusty
winds have developed from the Pribilof Islands south to the
souther tip of the Alaska Peninsula where southeasterly winds is
being channeled through the terrain. Weakening high pressure
remains across the west coast of Alaska and Bristol Bay but latest
satellite imagery shows a much less expansive marine layer than in
previous nights. A stacked low remains over the western half of
the Bering Sea this morning with a pretty decent jet streak
digging down the westward flank of the upper low. This is helping
to amplify the longwave pattern which now encompasses much of
Alaska and the southern Gulf of Alaska. Light rain shower activity
continues this morning but has been confined to the northern gulf
coast with the low pressure in the Gulf of Alaska so far south.



Models remain in good agreement through Thursday before a pattern
shift begins to take hold and models become quite inconclusive on
timing of precipitation. To start off, models are in excellent
agreement on the timing and placement of a decaying frontal
boundary draped across the central Bering Sea near the Pribilof
Islands. This front should weaken over the next 36 to 48 hours
before it shears apart and a broad sfc trough moves into the
western and southern Gulf of Alaska. The persistent upper level
low in the Gulf of Alaska will be shifted northward which will
allow interior portions of southern Alaska to transition into a
wetter paradigm by Thursday. This is due in large part to
numerous easterly waves propagating along the northern periphery
of the upper level low. Do the low amplitude of these waves,
models struggle quite a bit with the timing of placement of these
features past 6 to 12 hours. Needless to say, forecast confidence
Thursday night onward is a good bit below average but it is safe
to say rain showers should envelop much of Southcentral and Southwest
Alaska heading into late this week.


The large low over the southern Gulf will send another short-wave
westward across the western Gulf and Kenai Peninsula today
bringing another round of showers. Stability indices over the
western Kenai Peninsula look quite favorable for thunderstorms
today, with sfc LIs of zero to -2 degrees C and CAPE of 200 j/kg
or more. Even the 12z anchorage sounding shows dry adiabatic lapse
rates through about the first 7000 feet and this is likely
representative of the western Kenai as well. The short-wave will
cross the Kenai early to mid afternoon, so if there are
thunderstorms expect they would form during this time...with the
likelihood of some weak subsidence moving overhead behind the
short-wave later in the afternoon. A separate short-wave will
cross the Copper River Basin this morning and the Susitna Valley
this afternoon helping initiate scattered convection. The best
chance of thunderstorms will be along the Talkeetna Mountains with
upslope flow in an already unstable environment. Decent easterly
flow aloft will carry showers (and any thunderstorms which do
form) westward across the Susitna Valley.

Much wetter and cooler weather is on tap for Southcentral as we
head to Thursday and beyond. The upper level low will swing back
northward across the central and eastern Gulf sending a parade of
short-waves from British Columbia and the Yukon westward to
Southcentral. Model guidance continues to struggle with the
details of these "easterly waves", but there is agreement that a
strong wave will move across Southcentral late Wednesday night
through Thursday Night. Thus, have upped PoPs to likely to
categorical for most areas and introduced heavy rain. There is
potential for widespread rainfall amounts of one half inch to one
inch during a 9 to 12 hour period. This would cause rapid rises in
small creeks as well as minor urban street flooding. The kenai
river is already running near bankfull, so this could produce
minor flooding. Will wait one more model cycle to try and nail
down details of this rain event and consider whether any
statements or advisories are appropriate for this upcoming heavy
rain event.



The synoptic pattern that will last into Friday features an upper
level low over the Gulf, that will slowly track east, and high
pressure to the north. This will leave Southwest Alaska in an
unstable easterly flow pattern. The combination of afternoon
heating and easterly waves rotating around the low will kick off
showers over the area as the waves pass during times of peak
heating. Even with the weaker waves moving through there should be
enough instability for thunderstorm development through Thursday.

The Thursday into Friday storm looks to be the more impressive
feature, with phasing between a well-defined upper level
disturbance with strengthening lower and mid-level northeasterly jet
streaks providing for the potential for locally heavy rainfall,
with the best potential for heavy rainfall amounts from the
Lower Kuskokwim Valley south through Bristol Bay.



The front that has pushed into the Central Bering is now stalling
and will stay in approximately the same place through Thursday
night before dissipating. Rain and winds will be associated with
this front, but will also gradually diminish as the front fades.
Throughout much of the Bering and Aleutians abundant low level
moisture continues to allow for low stratus and fog to predominate.


.LONG TERM FORECAST (Days 3 through 7)...

The extended forecast beginning late week will two large lows, one
over the western Bering and another over the Northern Gulf of
Alaska. Easterly shortwave troughs rotating around the Gulf low
will be supporting rain across all of southern Alaska, some
possibly heavy at times. The first of these will be rotating into
southwest Alaska by Thursday night, with the possibility of
another moving into Southcentral Friday and Friday night.
However, this is where confidence drops significantly as the model
solutions diverge greatly with secondary waves rotating around
this parent low. Either way, by the weekend, expect more showery
conditions inland as the Gulf low begins to move east. The
warmest temperatures will likely be west of the Alaska Range with
moderate temps across Southcentral. By Monday, weak ridging will
build in behind the departing Gulf low with continued inland
showers driven by diurnal heating. The Bering low will move east
into Southern Alaska by Tuesday, with more clouds and rain chances
coming with it.





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