Small Craft Advisory 578, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
444 AM AKDT TUE JUL 19 2016

.ANALYSIS AND UPPER LEVELS...
An upper level ridge currently extends from the Pacific north
across the Alaska Peninsula, Bristol Bay and Southwest Alaska
coast. Upper lows are centered south of the western Aleutians,
north in the Arctic Ocean and offshore of Washington and Oregon.
Currently a shortwave trough is swinging through Interior Alaska
just north of the Alaska Range and spreading clouds into northern
southcentral Alaska. Additionally northwesterly flow aloft has
been bringing breezy west to northwest winds as well as supporting
the formation of a lee-side trough along the northern Gulf Coast.
Out to the west a front bringing east to northeast gales is
lifting north through the west and central Aleutians.

&&

.MODEL DISCUSSION...
Models are in good agreement through Tuesday night. Timing
differences in the low tracking north into the western Bering then
swinging east start appearing Wednesday and become more
significant Wednesday Night and Thursday. The NAM is currently the
fastest with this system and being considered an outlier.

&&

.FIRE WEATHER...
Warm, dry and locally breezy conditions will continue over the
area today. Clouds associated with an upper level disturbance
moving through interior Alaska will however continue to spill
south over the Alaska Range providing a slight moderation in
temperatures and RHs this afternoon and a bit more so on
Wednesday. A frontal system moving in from the west will begin
spreading precipitation over the Southwest coast Wednesday
morning, inland Southwest Alaska Wednesday evening and
Southcentral Alaska Thursday. While the initial frontal system
will weaken as it progresses eastward (with rainfall amounts
ranging from around 0.40 in on the Southwest coast to 0.10 to 0.20
in further inland and to the east) southwesterly flow behind the
front will continue to push showers across the area Thursday night
and Friday. Additionally, an upper level trough swinging through
will catch up with the surface low over southcentral Alaska on
Friday bringing a very good chance for widespread wetting rains
across much of southcentral Alaska Friday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A deep upper low over the Arctic Ocean that is centered north of
the Yukon Territory will slowly drift east, with the flow aloft
remaining from the west-northwest and slightly cyclonic through
early Wednesday. At the surface, high pressure inland is
combining with a lee trough along the coast to produce offshore
flow. Gusty offshore gap winds will begin to weaken later today
and continue to abate through Wednesday as the low moves off to
the southeast and allows the gradient to relax. On Wednesday the
upper ridge to the southwest will push eastward into southern
Alaska. This marks the beginning of a pattern change as a front
associated with a large upper low in the Bering Sea moves into
southwest Alaska. Late Wednesday night this front will move into
south central Alaska, although it will be quite weak by then.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
Ridging over the Eastern Bering will hold through today keeping
rain isolated to areas of the Lower Kuskokwim Valley. On Wednesday
a front will push in from the Bering bringing widespread rain to
Southwest Alaska. Behind the front, southwest flow will bring more
rain, albeit lighter, Thursday and Friday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
The front pushing into the Southern Bering stretches south
through the Central Aleutians with its parent low just south of
the Western Aleutians. The main low will continue to slowly track
north as its front quickly pushes north and east spreading across
the Bering tonight into Wednesday. By early Wednesday it will be
well into the Eastern Bering and draped south over the Eastern
Aleutians. The low will then track across the Northern Bering
keeping westerly flow over the remainder of the Bering and along
the Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The primary weather story for the long term will be the pattern
change expected over southern mainland Alaska. On Wednesday
afternoon, an impulse of energy in the northern jet will flatten
the ridge over southern Alaska. This will allow the subtropical
low that has been drifting over the western Bering to begin moving
with an associated jet streak between the low and the upper ridge.
The jet streak will become more east-west oriented, helping move
the upper low eastward. On Thursday morning, the subtropical upper
low will begin merging with another impulse of energy on the
northern jet. The merger will invigorate the low to dig
southeastward across Southwest Alaska by Friday morning.

There are some model differences on the speed at which this
pattern change will happen. General agreement on a faster
solution exists between the Canadian and GFS, with the EC staying
slower/further west. Regardless of which solution pans out, the
effect on the weather will be the same by Friday; much cooler and
wetter weather will return to southern mainland Alaska. The
weekend will remain cool and wet as the upper trough locks in
place over most of the mainland. As a result, frequent waves of
energy rounding the southern periphery of the trough through
Southwest Alaska and into the Gulf will bring several rounds of
wet weather to the area.

The subtropical ridge currently over southern Alaska will move
westward and reestablish itself this weekend over the western
Bering/northwest Pacific.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES...130 131 176 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JER
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...JW