Small Craft Advisory 495, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 AM AKDT Mon Jun 26 2017

.ANALYSIS AND UPPER LEVELS...
The pattern of the future is here! After a stagnant pattern and
weak flow aloft for the last week, the longwave trough over the
Bering Sea is now propagating eastward. The jet stream is still
well to the south of the Aleutians, near the mid-latitudes. A
weak, decaying front stretches from the Kuskokwim Delta through
the Alaska Range into the Gulf of Alaska. Weak ridging precedes
the front over Southcentral Alaska, while the previous trough
exits into Canada. While the first front is weakening, it is still
spreading light rain from the southwest mainland through the
Gulf. The surface low pressure system encompasses most of the
Bering Sea, with gusty winds along a more potent front in the
eastern Bering Sea. Middle and high level clouds are spreading
over Southcentral Alaska ahead of the first front, followed by
lower conditions behind.

&&

.MODEL DISCUSSION...
Guidance remains in lockstep with each other with the shifting of
the longwave pattern to the east. The timing of the front is well
agreed upon with the first front washing out over Southcentral
today, followed by a stronger shortwave and front on Tuesday.

&&

.AVIATION...
PANC...While VFR conditions and light winds will persist, a
mountain top inversion has trapped moisture around the 4000 ft
level around the northern Cook Inlet region. This moisture should
stick around until the inversion lifts by mid-morning. The
Turnagain Arm wind should begin to pick up early this evening,
bending into Anchorage before turning down-Inlet during the
overnight hours.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The area of instability that has been triggering some afternoon
and evening thunderstorms in the Susitna Valley and Copper River
Basin has moved eastward. The northern Copper River Basin along
the Alaska Range is the last remaining section of the area that
could potentially still see an isolated thunderstorm today. The
front that is moving through the Gulf and into Southcentral Alaska
looks to be weakening as it progresses northward. This will keep
most of the rainfall fairly light today as it moves into the area
and many of the leeward sides of the mountains will not see any
rain at all. There is a reinforcing shot of moisture moving in
behind this front which will arrive tomorrow and bring more rain
to the coastal areas. For the Anchorage and Matanuska Valley there
will be a good amount of downslope keeping most of the rain out
of the area. However the reinforcing moisture may bring enough to
these areas to get a little to spill over the mountains so the
sprinkles term was kept in the forecasts. Winds through Turnagain
Arm and along the Copper River will increase tonight through the
middle of the week as the pressure gradient along the coast
increases.

&&

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

A warm front extending out from a low pressure system in the
Bering Sea will continue to move ashore and push eastward over the
day today. The front will bring widespread rain across the
Kuskokwim River Valley and Delta, making it into interior
sections by this afternoon. Strong southeasterly winds along the
coast will accompany the frontal passage this morning with winds
gusting up to 35 MPH in spots. Downsloping off the AKPEN will
limit precipitation today to places along the eastern Bristol Bay
coast, including King Salmon. Strong cross barrier flow will lead
to winds being funneled through terrain gaps across Kamishak Bay
and down through Lake Iliamna beginning Monday evening through
Tuesday, with gusts reaching over 40 MPH in places. The front will
stall and weaken before the main upper trough supporting the
front moves toward the coast Tuesday morning. This trough will
support waves of moisture moving across the region with widespread
rain and showers present across most of the area through the day
on Tuesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

The low in the Bering Sea will slowly deepen and track east along
and just to the north of the Aleutian chain over the next 24
hours. High pressure over northwestern Canada will block the low
from progressing further east and begin to push the low center
north and then back to the west by the middle of the week,
bringing continued unsettled weather to the region through at
least the end of the week.

&&

.LONG TERM FORECAST (Days 3 through 7)...

The extended forecast beginning Thursday is a progressive pattern
with a long-wave trough over the Bering and Gulf of Alaska which
continues clouds streaming inland along with higher chances for
rain into the weekend. Models are struggling with the multiple
shortwaves moving through the expansive trough leading to higher
uncertainty through the long range forecast. One of the main
challenges on Thursday into Friday is the development of the next
low in the North Pacific which quickly moves into the southern
Gulf. Models differ with how they build a ridge ahead of this
system over the southern mainland, which has large impacts on the
progression of the gulf low. Kept the inherited forecast intact
for this package as models still lack in consistency from run to
run. There is a better signal over the Bering that the broad
trough remains focused over the central waters while weakening
through the end of the week.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...KH