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** Important notice for upcoming changes to NWS Anchorage web site ** Read more...

FXAK68 PAFC 271241 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
437 AM AKDT MON JUN 27 2016

.ANALYSIS AND UPPER LEVELS...

High pressure remains well entrenched across the eastern two-
thirds of the Bering Sea this morning although the axis of the
ridge continues to slowly drift to the east/southeast. Both
visible and shortwave IR satellite imagery shows an expansive
area of marine stratus and fog that remains trapped underneath
this high pressure. 00Z RAOBS from PACB and PASN show strong
subsidence inversions that have changed little over the past 24
hours which brings high confidence that this fog and low stratus
shouldn't be departing any time soon.

An area of low pressure to the west/southwest of Shemya has
continued to advance northward this morning while steadily
strengthening. A frontal boundary associated with this low has
made steady progress eastward across the western Bering Sea this
morning but the overall speed is limited due to high pressure over
the Bering. However, with upper level support coming in the form
of a weak jet streak and an amplifying upper level trough, the
ridge over the Bering will continue to be displaced eastward
during the next 24 to 36 hours.

Looking back at the Alaska mainland, a broad trough continues to
linger across the region with the bulk of the upper level energy
confined to a broad low in the southern Gulf of Alaska. Several
weak disturbances have been rotating along the northern periphery
of this upper low which has brought rain showers across portions
of Southcentral and Southwest Alaska. Some of these showers still
linger this morning but much of the rainfall is confined to the
northern gulf coast where synoptic forcing is greater.

&&

.MODEL DISCUSSION...

No real change in guidance this morning as models remain in good
agreement through much of the short-term. The GFS is a bit quicker
and stronger with the front moving across the Bering Sea and was
therefore not heavily used for forecast updates. With the ridge
making slow progress eastward it makes more sense for the slower
solution observed by the NAM/ECMWF/GEM to unfold. The NAM still
continues to be a bit hot with the convective parameters but seems
to be nailing the areas of thunderstorms a bit better than the GFS
so a blend toward the NAM (while weakening it a touch) was used for
convection across Southwest Alaska. Models are struggling the most
with the marine stratus and fog currently situated over the eastern
Bering Sea under the upper level ridge. Best bet for forecasting
this fog and stratus will likely be persistence as models want to
raise visibilities and ceilings a lot during portions of the
afternoon and evening which has yet to be observed on any given day.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A large vertically stacked low over the far southern Gulf will
remain nearly stationary over the next couple days. A series of
short-waves rotating around the northern periphery will help focus
showers and/or rain along the Gulf Coast and Kodiak Island. Weak
ridging inland combined with weak instability will produce
isolated to scattered convection...mainly during the afternoon and
evening hours and mainly near the mountains. Although there will
be quite a bit of mid to high clouds moving up from the Gulf,
there should be just enough instability along with some localized
downslope winds to produce occasional breaks of sun for inland
valleys. A quasi-stationary surface ridge along the northern Gulf
coast will continue to produce gap winds through Turnagain Arm
and Knik Arm along with a bit of wind along the Copper River
Valley.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Mon and Tue)...

Departing upper disturbance over Southwest Alaska moves south and
away from the area this morning, with upper ridging building
overhead through the day. A northerly mid-to upper-level
disturbance drops south by early evening that will help initiate
convection once again mainly over the Kuskokwim Mountains the next
couple of days. The lightning coverage should be greater than
Sunday mainly because of the proximity of much drier mid-level air
over the Y-K Delta region moving toward the area of maximum low-
level moisture along the thermal trough. Tuesday will be much the
same in terms of convection with increasing easterly flow and a
mid-level disturbance approaching near peak heating, along with
very dry air just to the north over the Interior allowing for
increased instability along the thermal trough.

The other major weather feature of note will be fog. With the
thermal trough remaining over the interior of Southwest Alaska,
onshore flow particularly at night will allow for the marine layer
to push well inland, especially in the Kuskokwim Delta. By late
Tuesday night, the advection fog may transition to more of a
radiational fog scenario especially away from the coast as flow
becomes fairly weak and from the east, at least during the
overnight hours underneath the diurnal near-surface inversion.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Mon and
Tue)...

A weather front over the southwestern Bering and Western Aleutians
will push east to the northern and eastern Bering through Tuesday
night. Winds will increase to as high as low-end advisory level in
the strong warm advection just ahead of the front. Outside of the
low pressure system/front and its rain and wind impacts, the main
story will be the widespread fog over much of the region, with
particularly low visibilities anywhere there is any kind of
moisture/wind convergence or terrain sheltering beneath the near-
surface inversion.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A mountain rain shower pattern with breaks of sun will begin the
forecast on Tuesday afternoon and continue through Wednesday
afternoon. Conditions look to begin to go downhill on Thursday.
Several disturbances in the flow will affect the forecast there
after. Thursday's mountain showers/thunderstorms will be enhanced
by one of these disturbances, but a stronger one comes through
Southcentral overnight Thursday into Friday. The disturbance should
bring widespread wetting rain from east to west Friday. The
details get a little fuzzy over the weekend, but the gulf system
responsible for the active weather will still be in the area
promoting cloudiness and at least mountain showers. Expect mostly
cloudy conditions with generally light winds and seasonable
temperatures. Cloud cover will increase and temperatures will
moderate through the remainder of the week.

Technically...the upper level low pressure spinning in the Gulf
will control the weather through the remainder of the week. Early
on, Southcentral Alaska is on the periphery of this system leaving
any diurnal convective activity over the mountains on
Tuesday/Wednesday, enhanced by any embedded easterly shortwaves.
A stronger shortwave (not embedded, actually a limb of the upper
low), looks to amplify on the baroclinic zone between the Yukon
ridge and Gulf trough late week, bringing widespread moderate
rain. However, model guidance is notoriously bad at resolving
these type features, and some spread among guidance still exists
as far as where to track this wave. 

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...MMC 
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CC
LONG TERM...MTL

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