Area Forecast Discussion
Issued by NWS Anchorage, AK

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750
FXAK68 PAFC 251247
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
447 AM AKDT THU AUG 25 2016

.ANALYSIS AND UPPER LEVELS...
The very amplified pattern continues this morning as two distinct
synoptic scale waves are moving through the forecast area. The
first of which continues to stretch from the Eastern Bering into
the North Pacific and is beginning to slowly weaken as it moves
eastward and interacts with a massive ridge stretching from the
Southern Gulf into much of the Eastern Pacific. Between these two
features, an atmospheric river is surging tropical moisture from
as far south as 25N into the Gulf and much of Southcentral. This
is bringing widespread rain, heavy at times, to the North Gulf
coast and is beginning to spread into interior Southcentral. Over
the southwest mainland, showers are pushing through the Kuskokwim
River Valley and Delta as the cold air associated with the upper
level trough over the Bering approaches. Further to the west a
gale force front associated with a low just off the Siberian
coast is pushing through the Central Bering as it weakens. This
front will continue moving towards the Pribilof`s today before
dissipating Friday.

&&

.MODEL DISCUSSION...
The models remain in very good synoptic agreement into late this
week with the highly amplified but slowly moving pattern. The main
concern continues to be with the timing and intensity of rainfall
from the atmospheric river around Southcentral. Rainfall amounts
overnight, while widespread, where generally lighter than
expected, especially when looking at the NAM. This was especially
apparent over the Anchorage bowl which has received much less than
the models have indicated. So far rainfall amounts from the GFS
appeared to have verified the best, especially over Southcentral
where it has verified very well. As a result the amounts in the
morning forecast were dropped slightly, using mostly the GFS.

&&

.AVIATION...
PANC...Ceilings should remain above 5000 ft today, with the
exception of a few brief periods falling below during heavier
showers until 14z. By late this afternoon there is a slight chance
of thunderstorms in the area, however these should be confined
mostly to along the Chugach Mountains and were kept out of the
TAF. Clearing will however need to be monitored today as any
significant daytime heating will increase the chance of
thunderstorm developing further west towards PANC.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Widespread rain will move out of the Cook Inlet area this
morning, but continue across Prince William Sound and the Copper
River Basin through the afternoon. The main remaining forecast
challenge from this storm is the ongoing potential for
thunderstorms over the Mat-Su valleys, Talkeetna Mountains, and
maybe even the Chugach Mountains this afternoon. While the cold
thermal anomaly aloft will be moving over the area around the
typical diurnal peak heating time, we will need a decent amount of
sunshine to "realize" this instability and initiate deep
convection. This remains the limiting factor, as morning satellite
imagery shows a weak additional shortwave riding northward along
the jet toward Southcentral Alaska. While this wave will not be
strong enough to bring much precipitation back across the
mountains and into Cook Inlet, it may be strong enough to keep
cloud-cover around a few hours longer which would definitely make
thunderstorms development more difficult. It still seems likely
that at least the most unstable areas in the northern Susitna
Valley will see enough sunshine to initiate a few storms, so have
kept the general area of thunderstorms around the region similar
to previous forecasts, but have backed down thunderstorm coverage
just slightly for the morning forecast.

Meanwhile, rivers along the gulf coast and southeast Copper River
Basin will stay high for much of the day, but with current
rainfall totals coming in a bit lower than expected no significant
flooding concerns are expected. Up in the Susitna Valley, on the
Yentna river, overnight rainfall will keep water levels hovering
around flood stage, but should begin falling in earnest by this
afternoon as the steady rain continues to slide off to the east.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
As the upper level wave over southwest Alaska pushes off to the
east today ridging will move in behind it. Before the ridging
moves fully over the area there will be enough instability over
the Alaska Range for isolated thunderstorms to develop this
afternoon. Then tonight the ridging will help to clear out the
showers that have lingered over the area today. The ridging will
last through the weekend keeping the region void of rain, but it
will provide an environment conducive for fog. Fog looks to
develop overnight tonight and Friday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
The low over the western Bering is slowly tracking east with its
front draping across the central Bering and south over the Central
and Western Aleutians. The low will make it to the central Bering
tonight where it will then stall, being held up by ridging to the
east. Its associated front will make it over the Pribilof Islands
tonight as well where it will stall and dissipate by Saturday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The pattern change will be well underway by Saturday with high
pressure building across the interior of Alaska. Models are still
struggling with just how dry the new air mass is going to be with
a very broad and expansive area of low pressure expending from the
Alaska Panhandle up through Prince William Sound. Nevertheless, it
appears as if much of Southwest and Southcentral Alaska will see
clearing skies and warming temperatures with this new pattern
through the weekend. The stacked low extending from the Alaska
Panhandle to Prince William Sound will be an issue through the
forecast period as it looks like several easterly waves will
rotate around the feature. Depending on the strength of the ridge
over western Alaska, a few showers could move through the Copper
River Basin and possibly the Anchorage area by midweek.

The Bering Sea will be quite quiet through much of the time
period with weak high pressure over the eastern half of the sea
while the rest of the area will remain under cyclonic flow. Given
the nature at how moist the air mass has been recently, it looks
like fog will remain an issue over the eastern Bering and the
southwest coastline through the weekend. Just like the rest of
Alaska, a pattern shift looks likely for the Bering Sea as well.
However, this will come in the form of a potent frontal boundary
that will enter the western and central Bering Sea Tuesday
morning. Increasing winds and rainfall will be likely with the
frontal boundary but strength and timing is a little rough to
forecast at this time.

&&


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

&&

$$

SYNOPSIS AND MODEL DISCUSSION...DEK
AVIATION...DEK
SOUTHCENTRAL ALASKA...AD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...MC



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