Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 121246
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
446 AM AKDT Fri Oct 12 2018

.ANALYSIS AND UPPER LEVELS...
The synoptic pattern has the main upper level low over the eastern
Bering and ridging over the Gulf that extends over the southern
mainland. The eastern Bering low is vertically stacked with its
associated front pushing through southwest Alaska bringing rain to
the area as it tracks northeast. West of the Bering low is weak
ridging that has built in over the western Aleutians.

&&

.MODEL DISCUSSION...
The models are on the same page concerning the overall synoptic
pattern but are struggling with some of the details. One of which
is a low approaching the Aleutians Friday. As it tracks east,
staying south of the islands into Sunday, models are having
difficulty locking in on its exact position. The other area of
concern is with a weak low moving through the Gulf toward Prince
William Sound Friday night. Some are pushing it into Prince
William Sound while others take it further to the west.

&&

.AVIATION...
PANC...VFR conditions will persist until a band of rain that will
setup west of town moves over west Anchorage this afternoon
dropping the ceiling. Conditions look to improve by the evening,
but there is some uncertainty with exact timing. Winds look to
stay out of the southeast with gusts around 20 kt for much of the
TAF period. This afternoon the Turnagain Arm wind will increase
overhead bringing windshear to the airport.


&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The north-to-south oriented front over the area will remain
roughly in place through this evening. A tight pressure gradient
is associated with this front, and fairly strong southeast winds
are expected through Turnagain Arm and other gaps in the terrain.
A surface low will move rapidly into the Gulf from the south
tonight, reaching the Gulf Coast Saturday morning. A barrier jet
will form along the Gulf Coast into Prince William Sound, with
Gale force winds expected. Storm force winds will also briefly be
seen east of Montague Island in the northern Gulf. This is a
fairly moist system and rain is expected over virtually all of
Southcentral. The low will then push inland Saturday afternoon,
with winds and rainfall tapering off through Saturday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Today through Sunday)

A weak front, currently near a Bethel to Kodiak Island line, will
continue to move northeast through the day with widespread
rainfall accompanying it. In its way, showers will continue to
develop as a few shortwaves rotate across the area in the cyclonic
flow aloft. Given the low freezing levels of ~3800 feet, any
heavier shower will probably mix down some small hail, with the
morning Doppler radar imagery also strongly suggesting this with
the heavier cores, as a lower specific differential phase and some
bright-banding has been observed. However, areal coverage seemed
low enough to preclude including this in the zones.

Heading into the late morning and afternoon, instability will
build some with the diurnal heating cycle, and given model progs
of steep mid-level lapse rates (70-500 mb lapse rates near 8 deg.
C/km) amidst 0 to -2 surface lifted indices and a few hundred
joules of CAPE, an isolated thunderstorm or two is certainly
possible and this was added to the forecast. Heading into Saturday
morning, a low moving through the Gulf will allow a steadier
rainfall to retrograde into the interior near the Alaska Range, as
deformation dynamics take hold. Otherwise, look for a showery
pattern to continue through the weekend as the stagnant cyclonic
flow pattern remains virtually unchanged.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Today through Sunday)

A broad cyclonic flow around a Saint Matthew Island low will
maintain an overall showery regime across the region through the
weekend. Disturbances rotating through the area will bring a burst
of ascent at times, helping to increase rainfall coverage while
also helping to mix down some stronger winds aloft. Overall small
craft conditions look to be about the worst of it for the Bering
Sea through the weekend.

Further south though, an area of low pressure will bypass the
Aleutian chain through the weekend, as it dives south of Shemya
tonight before turning eastward. However, the storm will be close
enough to bring widespread rainfall to the Shemya area by late
this afternoon, with sustain low-end gales developing by tonight.
This steady rainfall with sustained gales will then progress
eastward along the chain, affecting both the Bering and Pacific
coastal waters, as it treks eastward while reaching the AKPEN by
Sunday.

&&

.LONG TERM FORECAST (Days 4 through 7: Sunday Night through
Thursday)...

A sprawling low centered south of Dutch Harbor Sunday evening will
be the driver for the weather all across Southern Alaska through
the long-range forecast period. The center of the low will track
northward and move to about halfway in between the Pribilofs and
Cape Newenham by Tuesday morning. Beyond that the model
differences increase markedly. The GFS and Canadian turn the low
center back to the southeast and move it into the southwestern
Gulf by Thursday afternoon. Meanwhile the EC parks the low in the
same place for about 60 consecutive hours through Thursday
evening, gradually weakening as it sits and spins. Along this
track, most of Southern mainland Alaska will be east of the low
center, which means it will be subject to nearly continuous flow
off the Gulf and the Bering. This will support a cloudy and wet
weather pattern with scattered showers likely each day. An
associated front located east of the low is expected to continue
to progress into Southern mainland Alaska on Monday. That will
make it the wettest day of the week and also the windiest day
through the gaps.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...125 130 131 132 174 176 177 178.
 Storms...119.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...RA/JW


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