Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 201718 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
918 AM AKDT Wed Jun 20 2018

.ANALYSIS AND UPPER LEVELS...
The pattern remains in a persistent synoptic regime featuring a
stationary upper level trough over the eastern Bering Sea with a
strong upper high over Western Canada and Yukon. This pattern
keeps Southern Alaska under the influence of continued onshore
flow, bringing in coastal showers and clouds, especially for
locations west of Prince William Sound. At the base of the trough,
there is a weak low tapping some subtropical moisture around 50N.
This low will usher in more clouds and rain into Southwest today,
with the trough axis lifting north into Southcentral Thursday.

&&

.MODEL DISCUSSION...
The models are in excellent agreement with the general synoptic
scale pattern through the weekend. In general, the trough over the
eastern Bering Sea will slowly weaken with a drying trend across
Southern Alaska heading into the weekend. The challenges will
revolve around more local scale patterns. For this forecast the
hi-res models were favored.

&&

.AVIATION...
PANC...VFR conditions will persist. Some southeast winds may move
over the terminal this evening into the overnight hours, but
should be much less than past days with winds only being in the
10 to 18 kt range. Showers this morning should end quickly with
dry conditions expected thereafter.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA...
An upper level disturbance over the west coast will continue to
bring areas of clouds and showers to most areas west of the Copper
River Basin today. While a majority of these showers should occur
from the Western Gulf into the Southern Kenai, isolated showers
should continue for most of the day today as far east as the
Matanuska Valley. Further to the east over the Copper River Basin
conditions should be much drier and warmer today as a ridge of
high pressure noses in from the east, bring temperatures this
afternoon well into the 70`s with gusty southerly winds along the
Copper River. Starting on Thursday a shortwave moving through the
Gulf will spread these showers into most of Southcentral,
especially during the afternoon-evening as daytime heating causes
showers forming over the Chugach to move northward in the
southerly flow aloft. Cyclonic southerly flow behind then
shortwave will then bring continued mostly cloudy skies and rain
showers into Friday, however these are expected to be much less
organized, allowing for more breaks in the sun and clouds with
warming temperatures.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Under the cyclonic flow of the low over the eastern Bering and
small troughs rotating around it, showery conditions and moist
southeasterly flow will remain over much of the southwest Mainland
through Thursday afternoon. This system will phase with a fast
approaching disturbance from the North Pacific, and redevelop
over the Northern Bering Thursday afternoon through Friday night
while slowly tracking westward. This system`s new position over
the Bering will shift the pattern to a more onshore/southwesterly
flow pattern, feeding moisture into the area and causing some
breaks in cloud coverage. This will cause some scattered
convective showers to develop over the area Friday afternoon.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The upper level low centered over the Eastern Bering, Alaska
Peninsula and Bristol Bay area will remain in that general
location through Thursday before beginning to drift north Thursday
night and Friday. At the surface, the Eastern Bering low will
phase with a weaker North Pacific system speeding through the
Southwest Mainland, and redevelop over the Northern Bering
Thursday afternoon through Friday night. Meanwhile the western
half of the Bering remains under stable high pressure through
Friday, with fog and low stratus being the main impacts over the
area. The ridge begins to push southward into the North Pacific
Friday afternoon, as a front approaches the Western Aleutians and
the above mentioned low tracks further westward into the
Northcentral Bering.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The long term forecast begins Friday night with the persistent
long-wave trough anchored over the Eastern Bering /Western
Alaska. Upstream, an elongated ridge extends across the Canadian
Rockies north into the Yukon and Eastern Alaska. Between the two,
an upper-level shortwave is moving northward through the Gulf of
Alaska. A more potent upper-level disturbance and surface low in
the Northeast Pacific is continuing toward the Alaska Peninsula.

Models are in fairly good agreement with the track of the low
moving toward the Alaska Panhandle for the weekend. As it does, a
surface ridge will lift northward across the Gulf and into
Southern and Southwest Alaska for Saturday, reducing any residual
onshore flow and producing a warming trend for the weekend.
However, the aforementioned shortwave will also move across the
region, providing enough instability to generate clouds and
showers. Scattered clouds and showers continue into Sunday as a
second, weaker shortwave moves through the interior of southern
Alaska.

The overall pattern becomes more progressive by the end of the
weekend with a zonal jet stream south of the state bringing two
lows across the Aleutians toward the northern Gulf. The first will
slide from the Western Aleutians Saturday to the Gulf by Monday
evening. The second will approach the Central Aleutians by
Tuesday afternoon. Although it is still a long way out and there
is significant model divergence by early next week, there is some
indication that the development toward a more progressive pattern
by the end of the weekend could shift the longwave pattern and
kick the persistent upper-level low over the Bering to the east.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 145.
MARINE...None.

&&

$$

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


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