Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 270026
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
426 PM AKDT FRI AUG 26 2016
.ANALYSIS AND UPPER LEVELS...
A somewhat messy upper level pattern presents itself this
afternoon as a broad, weak upper ridge stretches from the western
Bering Sea through most of the Great Land. However, several areas
of low pressure aloft are within the ridge. One upper center is
over the central Bering while the other is a more compact
disturbance over the Alaska Peninsula. Both features have weak
fronts associated with them bringing light rain to the Pribilof
Islands and southern Kodiak Island.
The main weather feature to speak of is low stratus/fog left over
from the plentiful low level moisture of the past week. A deck of
stratus stretches from Cook Inlet northward through the Susitna
Valley, but is beginning to erode from the edges as daytime
heating mixes the low levels up. The surface high pressure over
most of the state combined with lingering low pressure along the
Gulf Coast has us in an outflow pattern with strong winds in
favored areas such as Seward and Whittier.
The synoptic pattern is well agreed upon by guidance as no
developing systems are progged in our area for the short term.
Model consensus is that the eastern area will remain under
cyclonic flow with an upper low centered over the Northwest
Territories with no surface features or major embedded shortwaves.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A pleasant weekend is in store for most areas as dry offshore
flow is established over the region between a narrow upper level
ridge over the west coast of Alaska and a broad trough over
western Canada. For tonight, fog and low stratus is expected to
be much less extensive than last night as northerly flow helps to
mix out the remaining low level moisture. However, some patchy fog
and increased cloudiness could persist over portions of the Cook
Inlet region where these conditions have lingered well into the
afternoon. Otherwise, clearing skies and warmer temperatures will
prevail through the weekend, with the main forecast challenge
being gusty outflow winds through favored areas along the north
Gulf coast such as Seward, Whittier, and Valdez. Looking farther
east towards the Copper River Basin, broad cyclonic flow on the
western periphery of the Yukon trough coupled with increasing
instability will lead to the potential for showers and even a few
thunderstorms near the Wrangell Mountains on Sunday.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The main forecast challenge will be the development of fog after
midnight tonight for the Southwest Alaska region. The upper level
synoptics has a ridge over the Bristol Bay region, and extends to
the northwest through Nunivak Island. Therefore, look for dense
fog/stratus to develop from King Salmon southward along the
Alaska Peninsula. Onshore flow in the aforementioned region
will advect marine layer stratus and combine with radiational
cooling to kick-off the formation of dense fog. This fog will
dissipate by late Saturday morning before redeveloping by early
Sunday morning along the Alaska Peninsula. The rest of Southwest
Alaska will have northwesterly flow bringing cooler air from the
north, and drier conditions as we head into the weekend.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The weak low in the Central Bering continues to stall before
dissipating by early Saturday morning. The Eastern Bering has a
1020 mb high pressure parked over Nunivak Island gradually moving
northward through Saturday. This will bring a southeasterly to
southerly flow through the majority of the eastern/central Bering,
and the Aleutian chain. Therefore, marine layer stratus and dense
fog will develop under the subsidence inversion associated with
this high pressure after midnight tonight before dissipating by
late Saturday morning. Meanwhile, the Western Aleutians has an
upper level ridge positioned over the region with westerly flow.
This is allowing marine layer stratus to be advected into the
region accompanied by areas of fog over the Shemya region. Look
for dense fog to develop in the aforementioned region by early
Saturday morning before dissipating by midday.
.LONG TERM FORECAST (Days 3 through 7)...
At the beginning of next week an upper level ridge over the
Eastern Bering will be wedged between two low pressure systems
to the east and west. The eastern bordering low sends energy from
the Yukon toward the northeast Gulf. This provides enough lift to
initiate rain along the eastern areas of the Copper River Basin
Sunday night with a few embedded thunderstorms. Precipitation
chances spread westward across Southcentral through mid week as
the upper low center slowly tracks into the northern gulf.
Meanwhile, a strong disturbance dives out of the northern Bering
across the Southwest and moves to the Alaska Peninsula on Monday.
This fast moving wave is followed by drier conditions Tuesday into
Wednesday across the southwest.
The storm to watch, for the Bering and southern Alaska, pushes a
front into the southern Bering Monday night. This storm system
breaks down the ridge across the eastern waters and sends a front
to west coast at the end of next week. Models are above average
with their synoptic agreement through the middle of next week and
then differ in strength and timing of the Bering low into Friday.
A conservative approach was used to update the extended forecast
with a heavy weight toward the ECMWF solution which had the
slower propagating low.
PUBLIC...Flood Advisory 145.
SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD