Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 180042
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
442 PM AKDT Thu May 17 2018

.ANALYSIS AND UPPER LEVELS...
There is an upper level negatively tilted low over the Central
Aleutians that is deepening while a closed high in the western
Bering Sea is beginning to deteriorate. A front from a Kamchatka
low is advancing eastward, grazing the Western Aleutians which is
helping to activate a second part of the jet south of the Western
Aleutians. The southern portion of the jet remains strong,
sitting just south of the Central Aleutians and shifting into a
north-south pattern in the western Gulf.

Moving eastward, a Pacific ridge that has dominated the northern
Gulf is moving inland and dissipating. What has been southerly
flow with several small fronts, has starting to shift to a slight
southeasterly flow with continued light rain in the Cook Inlet
region and Prince William Sound.

&&

.MODEL DISCUSSION...
The numerical models are only in fair agreement today. There are
some differences in frontal and surface low locations, as well as
moisture and precipitation fields. The NAM has not been performing
very well the last few runs with precipitation, and was steered
away from in the forecast. The NAM is also an outlier with a low
in the southwest Gulf Friday night. Forecast confidence is a bit
below normal today due to the differences in moisture/precip,
which are subtle but lead to differences in the sensible weather.

&&

.AVIATION...
PANC...Periods of light rain with VFR conditions and S-SE winds up
to 10 kt will persist into Friday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A weak deformation zone continues across Southcentral today. It
will likely get a little juicier with the intrusion of moisture
from a weakening frontal trough heading north into the Gulf
today. The brunt of the rain will continue along the coasts, but
will likely spread inland this afternoon and early evening and
with it with the frontal trough. Rain will diminish significantly
inland this evening with trough passage as weak upper ridge moves
in. Though, broad southeasterly flow in the lower elevations will
likely keep chances of light rain along the coasts through the
overnight hours. Some brisk winds will be likely across Turnagain
arm and through the Knik River Valley this afternoon and evening.

The next, in what seems like an endless series of systems, heads
northwest across the Gulf toward Kodiak Friday afternoon through Friday
night. It will likely bring renewed rains to the the western and
northern Gulf. Inland areas will be largely downsloped, so
precipitation potential will be fairly slight. Winds are expected
to increase across much of the Gulf. Should see a significant
increase in gap flow across Turnagain Arm and through the Knik
River Valley Friday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...

A warm front aloft has moved over SOuthwest Alaska this evening.
This is supporting a light rain/showery regime over the area along
with increasing easterly winds. A brief drying out will ensue on
Friday morning before a stronger shortwave trough moves inland
Friday night, bringing another round of rain into Saturday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...

A warm front is moving northwest through the eastern Bering Sea
toward the Pribilof islands which is enhancing northeast winds
ahead of it. The front will reach the Pribilof Islands late this
evening then shift west and fall apart, leaving behind it showery
conditions over the eastern half of the Bering. A gale force front
will clip the Western Aleutians through Friday night then shift
east into the North Pacific, leaving behind it broad cyclonic
flow.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The overall pattern will transition to a more progressive one over
the weekend, as the upper level ridge over southeast Alaska pushes
further eastward with a strengthening upper low tracking into the
southwest Gulf by Sunday evening. This will allow several systems
to rotate into the southern mainland over the weekend and into
next week. Strong southeasterly onshore flow associated with these
systems will favor coastal areas for precipitation, while the
typical lee-side locations will downslope out and remain mostly
dry, though seeing continued cloudy and cooler conditions.

Further west, an upper level ridge will amplify and track over the
western Aleutians to start the week, and start weakening as it
progresses eastward through mid-week. During this time, another
large upper level low will begin to track over the western
Aleutians Tuesday night. Models are in fair agreement now that
this low will closely follow the decaying ridge eastward, and
into the north Pacific by Thursday. it is looking more likely that
the persistent pattern of cooler, cloudy, and unsettled weather
that has plagued the southern mainland for nearly the past month
will remain in place through next week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 178.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...RJA/BL
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...TP



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