Small Craft Advisory 456, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
452 PM AKDT Thu Jun 8 2017

.ANALYSIS AND UPPER LEVELS...
The upper level ridge continues to build into the eastern Interior
of the state from the Yukon with a weak trough extending over the
southern Gulf of Alaska to the northern Bering Sea. A
southeasterly jet streak between these features is enhancing the
precipitation in the northern Gulf. This jet streak will weaken a
little overnight and become more oriented toward the south
tomorrow which should help to bring some precipitation into
Southcentral Alaska...except for the Copper River Basin which is
far enough east to have the ridge be the dominating feature.

A large upper level low is just west of Attu and driving to the
east-southeast over the next few days.

The atmosphere is relatively stable overall for most of the
forecast area with the notable exception being the Kuskokwim Delta
into the Kuskokwim mountains. The big issue that is keeping
convection at bay (at least for the time being) is the large
amount of cloud cover limiting the surface heating.

&&

.MODEL DISCUSSION...
The most meaningful model issue to discuss is the way the
precipitation heading toward Southcentral Alaska is being handled
and, specifically, how far west the main precipitation will fall.
The NAM started the trend of increased downsloping limiting the
precipitation over the lee side of the mountains and the Canadian
model, and the GFS to a lesser extent, are starting to indicate
this as well.

Models are converging on a solution for the low that will be
moving into the Alaska Peninsula/Western Gulf of Alaska region on
Saturday with the farther east solution bringing the weakening low
over Kodiak Island the tune they are all now singing. The weak
surface low over Southwest mainland Alaska will slowly drift into
the eastern Bering Sea and rotate around the aforementioned low
before becoming a broad and weak area of low pressure in the
eastern Bering for the early part of next week. Models will
probably continue to struggle with the placement of the weak low
centers but that will have very little impact on the forecasts.

&&

.AVIATION...
PANC...Overall, VFR conditions and light winds will persist
through tomorrow. However, there is a chance for a brief MVFR
ceiling with a shower.

&&

.FIRE WEATHER...
The Copper River Basin is shaping up for a warm, dry, and
eventually breezy day tomorrow necessitating a Red Flag Warning.
There is still a little question as to when the winds will come in
and that will be a factor, but the temperatures will be quite
warm with low humidity even before the winds make it in. With
southerly winds there should be a few hours of critical fire
conditions until the moisture starts to raise dewpoints and
therefore RH values which will end the Red Flag threat tomorrow
evening. It looks to be a one-and-done situation with much cooler
temperatures and higher RH values on Saturday so a prolonged
period of critical fire conditions in not expected.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Cloudy conditions for most of the area will prevail with rain
developing for the Kenai Peninsula. An upper level ridge will
continue to move into Southcentral Alaska, bringing hot and dry
conditions to the Copper River Basin tomorrow. However, a stalled
front running from Cook Inlet into the Gulf will bring rain to
mainly the Kenai Peninsula, beginning Friday morning.

The upper ridging will retreat a little bit late tomorrow as an
upper level low pressure approaching the Gulf brings a cooler air
mass and several disturbances toward the area. Turnagain Arm wind
will increase Friday afternoon and last through Saturday as
ridging builds along the coast. Rain will stay confine to western
Prince William Sound and western Cook Inlet, while the lee-side of
the Chugach will remain downsloped and dry.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Fri and Sat)...
A fairly convoluted weather pattern will transpire over the next
few days. To start things off tonight, even with abundant clouds
and fog, we still do expect some isolated thunderstorms this
afternoon along the Kuskokwim Mountains and possibly drifting into
Bethel. Overnight, fog should redevelop once again as the thermal
trough acts to "pull" the Bering Sea marine moisture inland. Fog
could be dense again Friday morning in places likes King Salmon
and Bethel. An upper level warm front will establish itself right
over the Alaska Range. This will serve to usher in more moist,
southerly flow across the area making for another mostly cloudy
day. It should also bring more steady showers to the Lower
Kuskokwim Valley. Along the warm front, a weak surface low will
develop over the Delta. This is making the convective forecast
rather difficult, but still looks to provide enough of a trigger
to warrant some isolated thunderstorms along the Kuskokwim
Mountains Fri afternoon and evening.

By Saturday, a cooler, more stable airmass will override the
area. Widespread showers will remain in the forecast, but have
opted to not introduce any thunderstorms at this time. A new low
will move towards Kodiak through the day as a ridge builds along
the Gulf Coast. This will lead to a slight uptick in southeast
winds across the area, especially through the favored gaps.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Fri and
Sat)...
Ridging over the Central Bering will keep marine stratus going
while some weak North Pacific lows clip the southern part of the
chain. The marine stratus currently stretches from Dutch Harbor to
Nelson Lagoon and then up to St Matthew Island. This stratus
should spread to the Eastern Bering once again tonight and likely
compress to the surface resulting in fog for many of the same
areas. A weak low over the Western Aleutians will fill as it bring
some light rain through Fri. Another low will strengthen tonight
over the North Pacific as it tracks toward Cold Bay. It will sling
some light rain into Atka and Adak through Fri as it wraps up
south of Sand Point. It will also bring some sustained small-craft
easterly winds with gale-force gusts to some of the coastal marine
zones late Fri. By Sat these two lows start to join forces to make
for broad cyclonic circulation across the whole Bering. This will
make for good rain chances from Adak over to Bristol Bay.

&&

.LONG TERM FORECAST (Days 3 through 7)...
There is decent agreement with the models moving into the extended
timeframe with both deterministic solutions and the ensembles. The
upper low that moves south of the Gulf of Alaska Sunday night
looks to stay around that same location for much of the week. This
will bring more moisture and rain to the southern mainland with
the majority of the rain along the coast. There is also better
agreement with the high that looks to push in over northwest
Alaska Monday night. The agreement comes with the guidance now
favoring a weaker high that pushes off to the west on Wednesday
and will not have as much of a significant impact for the
southern mainland.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...Red Flag 141.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...ML
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
LONG TERM...DK