Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 240039
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
439 PM AKDT Fri Jun 23 2017

.ANALYSIS AND UPPER LEVELS...
The ridge of high pressure that stretched from Southcentral into
the Yukon Territory is beginning to move northward in advance of
an upper level low moving into the Gulf. This ridge along with
southwest flow up Cook Inlet created the widespread stratus layer
that developed over Southcentral, but this is beginning to break
up into showers as a shortwave along the northern edge of the Gulf
low approaches from the south. Just to the north over the Upper
Yukon Valley the thermal trough remains under the upper level
ridge, which is beginning to fire showers and thunderstorms that
may stretch as far south as the Eastern Alaska Range and Talkeetna
Mountains this afternoon.

Over the southwest Mainland the remnants of an old frontal band
brought widespread clouds and rain showers to the Kuskokwim Valley
today. Just behind this front, clearing has allowed for a few
thunderstorms to begin firing along the spine of the Kuskokwim
Mountains, which are expected to continue spreading eastward this
afternoon. The environment then changes dramatically to a much
cooler maritime airmass under a ridge of high pressure over the
eastern Bering and southwest coast that has brought widespread
stratus to those areas. This ridge is beginning to slide eastward
as a frontal system over the central Bering approaches, spreading
rain and small craft advisory level winds into the eastern
Aleutians.

&&

.MODEL DISCUSSION...
The models remain in decent synoptic agreement into early next
week, though there are some difference in the smaller scale
details that are important to the short term forecast. The first
is how much will the shortwave approaching from the Gulf erode
away the stratus that has kept the area much cooler and cloudier
this week. The models are in generally good agreement that this
stratus layer will continue to break up into showers with the
shortwave, however they aren`t clear on whether this will be
enough to prevent it from redeveloping overnight. The current
forecast indicates that it will not redevelop, which will need to
be monitored this evening. There had also been differences, mainly
from the 12z GFS in the speed that the upper trough over the
northern Panhandle will move inland tomorrow. This has since
improved with the 18z runs and the models are now in excellent
agreement in the timing of that feature.

&&

.AVIATION...
PANC...The low marine layer will continue to gradually break up,
with ceilings gradually lifting. Expect VFR conditions to persist
at the airport, though MVFR conditions may linger in the vicinity.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
There is no shortage of stratus today. According to the models in
some areas the stratus layer is thousands of feet deep. With all
of this stratus in place the sun has not been able to warm the
surface in many locations and generally speaking temperatures
across the region are well below normal for the third week in
June. With a dome of high pressure in place over the interior
where the clouds break up...the temperatures will rapidly warm but
the stratus has had a tendency to linger longer than expected. All
the while...the models have pinged into a few weak vort lobes
embedded in easterly flow which could result in thunderstorm
activity. This forecast package increased the area of isolated
thunderstorms for both Saturday and Sunday to include much of the
Copper River Basin and surrounding mountains.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Satellite imagery this evening shows a remnant of a stretching
deformation zone aloft as an upper trough across northwest Alaska
moves eastward. The instability axis is rather narrow and weak,
but it is enough, coupled with continued long summer days, for
very isolated thunderstorms along the higher terrain of the
interior mountains and the Alaska Range. Otherwise, weak sea
breezes are keeping coastal locations stable and dry.

The main story, heading into the weekend, will be increasing upper
level ridging. This will help to stabilize most of the region
and remove the chance of thunderstorms. The one exception will
once again be along the higher terrain around the Kuskokwim
Valley, but any threats will be rather marginal. Expect warming
conditions through Sunday as the ridge builds out ahead of a
Bering warm front which will arrive Sunday evening.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A stacked low circulation just north of Shemya is now in the
process of merging with a North Pacific warm front. The low will
reinvigorate and strengthen then move into the Western Bering Sea
Saturday night while the warm front moves east through the Bering.
This will bring widespread rain and increasing south/southeast
winds which will reach low end gale force. The front will reach
the southwest Alaska coast by Sunday evening.

&&

.LONG TERM FORECAST (Days 3 through 7)...
An amplifying longwave pattern looks to bring an end to the period
of nice weather over much of southern Alaska early next week. By
Tuesday, the broad upper level trough over the Bering Sea will dig
southward and begin tapping into some moist subtropical air,
while advecting that air northward into the Gulf of Alaska. This
moist southwest flow regime looks to set up a rather wet period
for much of southern Alaska late Monday through Friday of next
week, as a series of systems move into the Gulf of Alaska and
north toward the Gulf coast. There remains pretty remarkable
model agreement with this change in weather pattern so confidence
continues to rise in the change itself and the likelihood of it
persisting through the entire week. However, it is to be expected
that there could be some breaks in the rain and clouds but
overall, wet and cloudy will be the theme for much of next week
and into next weekend.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 170 172.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...PZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...TP



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