Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 271243
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 AM AKDT SAT AUG 27 2016
.ANALYSIS AND UPPER LEVELS...
The general weather pattern has become much quieter than earlier
this week. A weak upper level wave moving over the Copper River
basin and surrounding areas is bringing mid-high level clouds, but
little in the way of precipitation due to limited moisture. Behind
this wave around Cook Inlet, patchy fog and stratus have developed
again overnight under clear skies and light winds. However what
little wind there is is generally offshore, so it has not been
near as widespread or dense than was observed yesterday. These
offshore winds are forming in response to the weak lee side trough
that is beginning to develop over the Northern Gulf as the upper
level wave moves offshore. Quiet conditions also continue further
west into the Bering as a broad but weak complex low is bringing
showers to the central Aleutians, but with sub-Small Craft Advisory
winds. To the west of this low, weak ridging has allowed for
fog/stratus to form over most of the western Aleutians/Bering.
The models once again remain in very good synoptic agreement
through the short term with the relatively weak synoptic pattern.
As a result the higher resolution NAM/WRF-ARW were mostly used for
forecast updates overnight, especially around Southcentral and the
Gulf Coast to handle the outflow and offshore winds. Out west the
GFS was primarily used as it was better handling the winds/rain
from the next weak low approaching the Central Aleutians, but the
models are all handling this low reasonably well.
PANC...There is an outside chance that fog/stratus over Cook Inlet
may approach the terminal early this morning, however with weak
offshore flow and cloudy skies this was left out of the TAF.
Otherwise N-NW winds will begin to increase today with VFR
conditions as the down inlet flow strengthens. The strongest
winds should hold off until after 00z, however gusty N winds will
likely start by 21z as the gradient increases.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Unseasonably nice weather is still on tap for this weekend. A few
patchy areas of fog are trying to develop again around the western
Kenai Peninsula and in parts of the Matanuska Valley, but this
should not become widespread or persistent the way it did
yesterday as we are now firmly entrenched in a dry offshore flow
pattern. A shortwave is crossing the area from the north
currently, and has brought some high cloudiness into the region.
This feature will be continuing south and into the ocean this
morning, leaving abundant sunshine in its wake. Subsidence/sinking
motion behind this feature should also help mix down to the
surface some of the gustier winds aloft on the coast and even over
the upper Cook Inlet area by early this afternoon.
Another shortwave trough will move across the Copper River Basin
on Sunday, and may be able to kick off a few thunderstorms
primarily over the Wrangell mountains (east of Glennallen) but
perhaps also toward McCarthy area. The shortwave is not terribly
strong or well-organized, and low-level moisture availability
might be a limiting factor as well, so will plan to keep minimal
(slight chance) wording in the morning forecast package.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An upper level ridge will continue to build over the Southwest
coast, bringing more dry and stable conditions to the region
today. With these stable conditions and winds starting to
transition to more of an offshore/northerly flow today, has made
it tricky to pin-point the potential for fog to develop early this
morning along the Southwest coast and over the Bristol Bay zone.
Any fog or lingering marine stratus over the area will dissipate
by the late morning, as offshore/northerly flow increases and
gives way to drier conditions. There remains some potential for
fog to redevelop after midnight tonight and into Sunday morning
over the Alaska Peninsula and Kuskokwim Delta coast, as clearing
skies overnight may aide in the redevelopment of radiational fog.
Confidence in this redevelopment remains low, as this scenario
will be highly dependent on how much moisture is available near
the surface and how much mixing occurs with regards to the winds
over the area.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The weak low in the Central Bering continues to stall before
dissipating this morning. A high continues to build over the
Eastern Bering, bringing 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
continue to develop under the subsidence inversion associated
with this high pressure, before dissipating by late this morning.
Meanwhile, the Western Aleutians has an upper level ridge
positioned over the region with westerly flow. This is allowing
marine layer stratus to continue advecting into the region
accompanied by areas of fog over the Shemya region. Look for dense
fog to develop in the aforementioned region by early this 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 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 in 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 ECMWF solutions which
had the slower propagating low.
PUBLIC...Flood Advisory 145.
SYNOPSIS, MODEL DISCUSSION, AND AVIATION...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP