Special Weather Statement, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 PM AKDT Sun Aug 6 2017

.ANALYSIS AND UPPER LEVELS...
Broad cyclonic flow around an upper low complex south of the
Alaska Peninsula and an amplified ridge stretching northwest
across the Gulf to over the interior continue as the dominating
upper features. A short wave over the Kenai Pen/Cook Inlet,
evident of afternoon satellite imagery, is moving rapidly West.
This feature will likely help to trigger convection across
Southwest Alaska this later this afternoon/evening. A weak frontal
boundary, associated with the low complex south of the Alaska
Peninsula is continuing to bring rain over the Aleutians, Alaska
Peninsula, and portions of Kodiak Island. Generally fair skies
are evident over inland areas, though some cumulus with developing
afternoon convection are starting.

&&

.MODEL DISCUSSION...
Models overall are in good agreement across Southcentral Alaska.
Out west, agreement is pretty good with handing of the low complex
south of the Alaska Peninsula. However, as the focus shifts to
the low over the western/central Bering Mon night/Tue, it appears
that the GFS/EC have a better solution.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist into Monday
afternoon. There is a possibility of some MVFR (or lower) stratus
at the airport, but not enough of a chance to warrant putting it
in the TAF yet - thus the "FEW020". Southerly winds will pick up
Monday afternoon as a weather system approaches the area.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
After a beautiful Sunday afternoon across much of Southcentral
Alaska, cloud cover will begin to build back across the region
late tonight. Before that happens, a series of weak upper level
impulses will combine with steepening low-level lapse rates to
help produce showers and even a couple of thunderstorms across the
Copper River Basin, Susitna Valley and possibly over parts of the
Chugach and Kenai Mountains. The limiting factor for
thunderstorms today is a weak pocket of subsidence behind a weak
shortwave that transited the area during the morning hours. With
cloud cover increasing on Monday, the amount of incoming solar
radiation will be much weaker which will keep the low-level lapse
rates from steepening too much. This will lead to nearly NIL
chances for thunderstorms across southcentral Alaska and will
begin to usher in a cooler and slightly wetter pattern.

By Tuesday, a front will begin to impact the northern gulf coast
and decent rainfall amounts will be seen along the coast as
moisture upslopes along the coastal mountains. Models are in good
agreement that decent cross barrier flow will develop which will
make it difficult for most inland locations to see much in terms
of rainfall. Some rainfall may be able to move over the mountains
on Wednesday as the column begins to saturate and downsloping
winds may not entirely dry the low-level column out. Regardless, a
more cloudy, cooler and wetter pattern is in store for the
upcoming week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Isolated to scattered thunderstorm activity this evening initiating
along a thermal trough that extends from the western Alaska Range
through the Kuskokwim Valley and Delta will be the primary
forecast challenge in the short term. This convection has the
potential to persist well into the overnight hours as storms
propagate westward along a corridor from roughly Aniak to Bethel
to Kipnuk following the progression of a compact but potent upper
level disturbance moving from east to west. Slightly cooler and
cloudier conditions are then expected for the beginning of the
work week as a cooler air mass associated with an inverted trough
moves into the region. This coupled with lingering cloud cover
from the overnight convection should help to limit thunderstorm
potential for Monday, although scattered showers will persist
into Tuesday given the broad cyclonic flow in place.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Numerous showers will continue to impact the Alaska Peninsula
through the day on Monday as the upper low remains just to the
south. Otherwise, extensive stratus and fog remain the dominant
weather features beneath a narrow and weakening ridge. A front
entering the western Bering will spread rain across the Western
Aleutians through Monday, eventually reaching the central part of
the chain by monday night.

&&

.LONG TERM FORECAST (Days 4 through 7)...
A progressive wet weather pattern remains through the long range
forecast. On Wednesday night, a front will be pushing across the
southern mainland with ongoing precipitation into Thursday as
additional upper level waves pinwheel around a low tracking near
the Alaska Peninsula to the southeastern gulf. For the end of the
week, the pattern is expected to stay active as energy dives south
from the the arctic and another wave moves toward the western
Bering. The models all shifted out of agreement on the long range
solutions today, so kept the inherited forecast intact as the
previous tracks of the synoptic systems looked more reasonable.
The most challenging forecast problem for both the ECMWF and GFS
is that there are multiple strong waves moving toward the mainland
into next weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...MC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...KH

- PAFC 000000
SPSAFC

Special Weather Statement
National Weather Service Anchorage AK
616 PM AKDT Sun Aug 6 2017

AKZ155-070300-
Kuskokwim Delta-
616 PM AKDT Sun Aug 6 2017

...AN AREA OF STRONG THUNDERSTORMS WILL AFFECT SOUTHERN COASTAL
AREAS OF THE KUSKOKWIM DELTA...

At 616 PM AKDT, strong thunderstorms were clustered 35 miles
southeast of Tuntutuliak, or 50 miles south of Bethel, moving west
at 15 mph.

Pea size hail is possible with this storm.

Locations impacted include...
Tuntutuliak, Eek and Apokak.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

LAT...LON 6048 16144 5991 16167 5988 16209 6018 16222
      6012 16231 6029 16245 6024 16260 6019 16245
      5999 16250 6000 16273 5981 16335 5980 16385
      6037 16352
TIME...MOT...LOC 0216Z 112DEG 18KT 6003 16177

Hoeppner