Special Marine Warning 2, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
546 PM AKDT Mon Jun 5 2017

.ANALYSIS AND UPPER LEVELS...

An upper-level low is spinning over the Gulf of Alaska. The
Pacific jet stream is just south of the Aleutian Chain with a
speed max of 165 knots. As the jet rounds the south side of the
Gulf low, a southerly flow moves over the eastern Gulf around 65
knots. This pattern is moving another wave of moist air from the
Gulf toward Southern Alaska today while continuing the north and easterly
flow that is currently driving this morning's wet and cloudy
experience along with convection in southwest Alaska.

An upper-level high sits near Saint Lawrence Island. This ridge is
just strong enough to keep stable weather with stratus and fog
over parts of the Bering Sea. Parts of the Alaska Peninsula began
the day with a clear sky... quite a treat.

At the surface, a thermal trough lies north of the Alaska Range.
This feature includes a weak thermal Low slowly moving southwest
from the upper Kuskokwim Valley. Convection began early over this
region this morning. The combination of the thermal trough and low,
combined with the easterly flow, warm temperatures and decent
instability will help to inspire convection this afternoon and
evening west of the Alaska Range and especially over the
Kilbuck/Ahklun Mountains.

A look upstream shows that a new wave is developing over the
Western Alaska range midday while yet another wave is developing
over the Interior.

&&

.MODEL DISCUSSION...
The models are similar in timing and placement of the main
synoptic features through Tuesday afternoon. After this point,
solutions diverge on placement of the next Gulf low nearing
Kodiak Island and the strength of a ridge building over the
Aleutians.

For Southcentral and Prince William Sound, the NAM was the
preferred model as it seemed to handle a developing triple-point
more effectively.

In Southwest and the Aleutians, a general blend of models was
used.

&&

.AVIATION...
PANC...VFR conditions will prevail. With more of an easterly flow
developing from present northeasterly conditions, drier weather
should be the result. However, a few showers can't be ruled out
entirely.

.FIRE WEATHER...
In the Kuskokwim Delta... High temperatures in the the upper 70s
combined with northeasterly winds up to 20 miles-per-hour and
relative humidity in the 25 percent range led to the issuance of
the season's first Red Flag Warning.

For the Kuskokwim Valley in general...an increasing amount of
lightning mainly near the mountains has been seen in the region in
recent days. With today's convective indices looking more favorable
for thunderstorms and lightning, the risk for weather to continue
to enhance fire danger is certainly present. In addition to the
new starts due to the abundant lightning, outflow winds from the
thunderstorms are another concern.

Fire weather conditions will moderate somewhat tomorrow with
increased cloud cover and slightly cooler temperatures however,
thunderstorms will be a concern again over the Kuskokwim Delta on
Wednesday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

An area of low pressure over the southern Gulf of Alaska will
continue to push an occluded frontal boundary toward the northern
gulf coast which will help upslope rainfall to continue along the
coast. At the same time, a sharp upper level trough located just
east of the Cook Inlet region will continue to shift westward this
afternoon and evening which will allow the rainfall over Anchorage
and the Kenai Peninsula to shift westward. One last shot at
thunderstorms will be possible this afternoon in vicinity of the
Alaska Range over the Copper River Basin and Susitna Valley but
will lack the areal coverage over the past few days as cloud cover
has limited the amount of destabilization this afternoon.

A triple point low will develop over the eastern Gulf of Alaska
Tuesday afternoon and pivot over the northern gulf by Tuesday
evening. This will help bring another slug of moisture to the
northern gulf coast but the vast majority of the rainfall will
upslope along the coastal mountains and struggle to move further
inland. Winds across Prince William Sound and the northern gulf
will be quite strong on tuesday with 20 to 35 kt winds sustained
with gusts upward to 45 kt through Hinchinbrook entrance but will
subside significantly on Wednesday. Convective potential looks
rather slim as increased cloud cover will prevent any significant
diurnal destabilization but a few showers in the afternoon will
remain possible across interior locations.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Rather impressive instability across Southwest Alaska associated
with the thermal trough stretching across the Kuskokwim Valley,
Kuskokwim Delta and western Greater Bristol Bay area is beginning
to trigger isolated to scattered thunderstorms as warm, northerly
flow pushes across the area this afternoon. Currently, notably
stronger thunderstorms likely containing hail are forming along
the Kilbuck Mountains where there appears to shear induced by the
leading edge of northerly flow hits the mountains. Thunderstorms
will continue to fire off through the afternoon and evening with
the most abundant lightning likely to be centered over the Kilbuck
and Ahklun Mountains as well as along and near the western Alaska
Range.

Conditions will cool and stabilize somewhat tomorrow as the
remnants of a front from the Gulf spreads a few more clouds
across the area. Precipitation will take the sparser form of
isolated to scattered showers mainly along the mountains. Another
trough rotating from the western Gulf across the Aleutian Range
Wednesday will tug the end of the interior thermal trough back
over the Kuskokwim Delta bringing back the chance for some
afternoon and evening thunderstorms.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A surface low over the western Aleutians with an associated front
extending across the southern Bering into the central Aleutians
will remain in that general location tonight through Tuesday then
gradually weaken Wednesday and Thursday. A frontal system from
the north Pacific will approach the central Aleutians Thursday.

&&

.LONG TERM FORECAST (Days 4 through 7)...
The pattern moving into the long term has a blocking high over
northern Alaska with a low south of the Gulf of Alaska. Another
upper level low will be over the far western Bering that will
track east staying just south of the Aleutians through the week.
Meanwhile, the high and low near the mainland will mostly stay in
place into late next week bringing moist, southeast flow to the
southern mainland. Most of the rain will stay along the coast with
afternoon showers for inland locations. On Thursday this pattern
will break down making way for the western low to move near the
Alaska Peninsula on Friday. However, there is disagreement with
where this low will end up with some models bringing it north
towards the Bristol Bay area while others stay south of the Alaska
Peninsula.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale warning 119 120 131 132 136 138 150 351 352.
FIRE WEATHER...Red Flag Warning 155.

&&

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...RD/MC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...EN

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
434 PM AKDT Mon Jun 5 2017

AKZ152-155-161-060445-
Lower Kuskokwim Valley-Kuskokwim Delta-Bristol Bay-
Including the cities of Aniak, Sleetmute, Bethel, Hooper Bay,
Nunivak Island, King Salmon, Dillingham, Naknek, and Pilot Point
434 PM AKDT Mon Jun 5 2017

ORGANIZED THUNDERSTORMS HAVE DEVELOPED SOUTH OF THE KUSKOKWIM
RIVER NEAR THE KILBUCK MOUNTAINS. NUMEROUS LIGHTNING STRIKES HAVE
BEEN DETECTED. STRONG OUTFLOW WINDS ARE EXPECTED. THE POTENTIAL
FOR DIME TO QUARTER SIZED HAIL IS POSSIBLE. THUNDERSTORM ACTIVITY
WILL LIKELY CONTINUE FOR THE NEXT SEVERAL HOURS NEAR THE KILBUCK
MOUNTAINS.

SZATANEK

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