Small Craft Advisory 584, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
501 AM AKDT Wed Aug 9 2017

.ANALYSIS AND UPPER LEVELS...
The last vestiges of the robust ridge will try to hold on today
over the ALCAN border while a strengthening upper level low makes
slow progress from the North Pacific towards Kodiak Island.
Satellite shows the moisture associated with the low streaming up
from about 40N. This is a fairly deep moisture tap with the total
precipitable water running about 150% of normal within it. The
moisture has been rung out over SW AK as light rain for the last
18 hours or so and will continue to do so today. As the ridge is
shifting slightly to the east, cloud cover is overspreading much
of the Cook Inlet with some on and off light rain making it in to
the Kenai Peninsula this morning. The ridge does look to hold on
long enough to make for another dry and gusty day for the Copper
River Basin.

Meanwhile over the Bering/Aleutians, another smaller low is
dropping south towards Kiska. This is bringing some rain showers
to the far western Aleutians. There is some induced ridging
between this low and the one near Kodiak. This is allowing for yet
another morning of fog from Dutch Harbor up through the Pribilof
Islands.

&&

.MODEL DISCUSSION...
Models are in fair agreement through the short term. The biggest
discrepancy is with the timing and amount of rain over the
Anchorage area later today into Thu morning. The NAM had been a
slow and somewhat dry outlier when compared to the other models,
but now those other models are trending towards it. The result is
low-moderate confidence in rainfall from the Northern Cook Inlet
towards the Matanuska Valley today. All models do agree in
bringing rain into the area as the upper level low starts to move
overhead early Thu morning. So there is moderate to high
confidence that just about all areas from the Prince William Sound
up through the Susitna Valley will see wet weather by Thu morning.
See the long term discussion regarding model uncertainty with the
more significant upcoming pattern change.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The front in the western Gulf of Alaska is progressing northward
today bringing in rain to coastal areas and the Kenai Peninsula.
Inland areas from Anchorage northward will still see most of the
rain downsloped out today. Tonight the upper level support increases
and the cross-barrier flow diminishes which should bring the rain
over the Anchorage and Mat-Su areas.
The Copper River Basin remains the anomaly as the push of moisture
associated with the front will remain west of that area. This will
bring in another warm day today before the cooler and more moist
weather makes it into there.
The increased pressure gradient along the north Gulf Coast will
also bring in some gusty winds to Turnagain Arm and the Knik River
valley. The Copper River will also get some gusts along it this
afternoon but tomorrow will be windier yet.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A low pressure system anchored over the Alaska Peninsula will keep
the mainland in an active pattern for the next few days. A strong
disturbance and warm front will continue to spread rain over the
area today, though downslope drying will likely affect areas just
west of higher terrain. Gusty winds will continue behind the
front, especially in the afternoon hours where gusts into the 30
to 40 mph range can be expected.  Once the front and stronger
disturbance pass this afternoon, rain will become much more
spotty and showery over most areas. Showers will be enhanced in
the afternoon and evening hours for the next couple days as the
moist air mass feeds on diurnal instability. The upper level
pattern will begin to shift on Friday morning as the current
system weakens and a new trough digs into the area from the north.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A strong front and disturbance is entering the Bering from the
northeast Pacific today. While the front is weakening, it should
still spread rain from east to west toward the Pribilof Islands
late tonight. The change in wind direction should get the central
and eastern Bering Sea out of the low stratus and fog pattern that
we've been in for a couple weeks now. Not to say that there won't
be low clouds though, they just won't be AS low. High pressure
sets up over the western Bering Sea keeping a light northerly flow
over most of the area which should bring a drying trend over the
next few days.

&&

.LONG TERM FORECAST (Days 3 through 7 / Friday through Tuesday)...

On Friday the upper level pattern will feature the upper low that
had been near the Alaska Peninsula beginning to slide eastward
into the Gulf. While for the most part associated precipitation
remains near the coast, instability showers inland will be
possible.

The next feature of great interest will be another
digging trough that will bring the area, particularly Southwest
Alaska, its first real taste of autumn on Saturday. Broad
northerly flow directly out of the Arctic pushes into the region,
causing widespread shower activity, particularly the further north
you go. The cold air moving in aloft may even be enough to touch
off a round of thunderstorms Saturday afternoon as the instability
increases, particularly across the Susitna Valley.

Model continuity rapidly degrades from Sunday onward regarding how
the strong trough/upper level low over northern Alaska interacts
with a compact upper level low over the western and central
Aleutians. The spread ranges from the GFS, which depicts no
interaction, causing the low over the Aleutians to be ejected
southeastward into the north Pacific. The result is broad
northerly flow over the mainland, and very pleasant, sunny
(outside of mountain showers), seasonable weather for much of
southern mainland Alaska. The Canadian is the middle ground,
forecasting the Aleutian low to interact and move into Southern
Alaska on Monday, allowing a brief break in the rain and one day
of nice weather on Sunday. The EC is the wettest solution, showing
the low interacting with the trough over the central Aleutians on
Sunday, then moving into Southwest Alaska and cutting off into a
newly reinvigorated low that stalls out and leads to area wide
rain from Sunday evening right through Tuesday and beyond.

Thus, a small difference in the models as to how the trough over
northern Alaska interacts (or doesn't) with a small low over the
Aleutians makes all the difference in the world as to how early
next week's weather plays out. Stay tuned.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 130.
FIRE WEATHER...NONE.

&&

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