Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 200053
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 PM AKDT Tue Jun 19 2018

.ANALYSIS AND UPPER LEVELS...
The upper level pattern has changed very little from yesterday
with the large upper level low in the eastern Bering Sea. The 250
mb jet stream remains well south of the Aleutians and then turns
due south-to- north through the Gulf of Alaska and then to the
Alcan border. What has changed is the surface low is now nearly
vertically stacked under the upper level low and has weakened
considerably. What remains of the front with this low stretches to
near the Bering Strait and then toward the Brooks Range. This has
allowed the pressure gradient over southern Alaska to ease and
reduced the winds. The atmosphere in southwest Alaska is a little
more unstable today being close to the cold core of the upper
level low center. This is allowing some showers to pop-up this
afternoon as well.

&&

.MODEL DISCUSSION...

Models remain overall good agreement the next few days. The wave
that will develop in the Gulf of Alaska and move onshore Wednesday
evening through Thursday is farther west on the NAM and GFS than
Canadian and EC models. The more open-wave structure of the
NAM/GFS seems more reasonable and was used for guidance in the
Gulf and Southcentral areas. Both GFS and NAM are indicating an
uptick in instability tomorrow for the Kuskokwim Delta and far
north Copper River Basin as compared to previous runs.

&&

.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 being only being in
the 10 to 15 kt range.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA...
Moist southerly flow aloft will continue through Thursday as the
synoptic pattern of an upper level low over the eastern Bering and
ridging east of Southcentral Alaska remains in place. This pattern
will bring showery conditions to Southcentral Alaska that will be
amplified as various shortwaves move through the upper level flow.
One shortwave is pushing through the Kodiak Island area bringing
rain to the island that will last into Wednesday. Another will
push in over the Kenai Peninsula late Wednesday bringing more rain
to the area. It will then track north spreading rain to areas
from the Susitna Valley through Prince William Sound Wednesday
night into Thursday. Showers will then diminish moving into
Friday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
With the upper low persistently centered offshore of Southwest
Alaska, southeasterly flow at the surface and southerly flow
aloft will continue to stream moisture up into Southwest Alaska,
with small troughs rotating around the main low further enhancing
precipitation. The next of these shortwaves will lift north
across the western Capes and Kuskokwim Delta this afternoon and
evening followed another trough further inland tracking the
Greater Bristol Bay and Kuskokwim Valley late tonight and
Wednesday morning. By Wednesday afternoon afternoon and evening a
broader trough rotating through and increasing general instability
will support showers across all of Southwest Alaska. Additional
troughs will continue spinning across the area through the end of
the week before the entire pattern starts to shift north.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
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. Troughs rotating south around the backside of
the low will move through the Pribilof Islands and eastern
Aleutians while further west the general flow remains diffuse and
out of the west northwest through Thursday. On Friday, a frontal
system will approach the western Aleutians.

&&

.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...EZ
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...TM


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