Small Craft Advisory 598, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
436 AM AKDT Sat Aug 4 2018

.ANALYSIS AND UPPER LEVELS...

A vertically stacked upper level low over the Gulf continues to
slowly move towards the southeast this morning. The residual
frontal boundary along the northern Gulf coast is continuing to
produce rain in and around the Prince William Sound area.

Out west, a large upper level ridge is building over the Bering.
At the surface, satellite imagery confirms plenty of low level
marine stratus due to subsidence associated with the ridge. As a
result, foggy conditions are observed across the western and
central Aleutians.

&&

.MODEL DISCUSSION...

Model solutions are in good agreement through Sunday. Disagreement
arises on Monday with the timing of when an upper level trough
over the northern Bering. The GFS currently has a faster solution
and places the trough over Southcentral by Tue afternoon while the
EC has a slower solution and shows the trough moving in by Wed
morning.

&&

.AVIATION...
PAFC...VFR conditions will prevail through the day today with high
pressure building in from the southwest. Winds will begin to pick
up this afternoon from the south/southeast as a low approaches
the west coast leading to up inlet flow as well as inducing a
Turnagain Arm wind.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Today through Monday)...
The Gulf low will continue slowly weakening as it drifts southward
and becomes a cutoff low over the North Pacific low. This should
allow for some gradual improvement in the widespread rain
currently falling from the North Gulf into the Copper River Basin,
but with incoming westerly zonal flow aloft showers and clouds
will continue for most of the day. Behind this rainfall, there is
still an outside chance of some patchy fog/stratus around the Cook
Inlet region early this morning, but with less clearing than
expected overnight it doesn't appear at this point that anything
that develops will be widespread or dense.

Starting Sunday, a shift to a prolonged period of westerly zonal
flow will keep generally cloudy and cool conditions with periods
of rain in the forecast into early next week. As as often the case
in zonal flow patterns, there remains some uncertainty in the
track of shortwaves within the zonal flow that will drive chances
of rain for the South Mainland. The first wave will move into the
region Sunday evening, bringing widespread rains primarily to the
north of Anchorage with more spotty coverage to the south. This
system should then most east of the area by late Monday, leaving
continued cloud cover and showers over the area into Tuesday.

&&

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

Upper level ridging will build in across Southwest today, but this
will be a rather "dirty" ridge with extensive mid and high cloud
cover. This pattern continues through most of tonight before a low
tracking into Norton Sound drags a front into Southwest Saturday
night and Sunday with increasing rain chances, especially across
the Kuskokwim Delta and Valley, and along the Alaska Range. This
will continue through Sunday night before rain chances become
more scattered into Monday as the upper level flow flattens out
and becomes predominantly westerly and onshore.

&&

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

The upper level flow aloft will flatten out as a series of
shortwave troughs moves east across the Bering Sea. The associated
front will move from the western Bering to the eastern Bering Sea
with 25-30 knot south/southwest winds along the front. A series of
weaker lows will trail behind the front Sunday night and Monday.

&&

.LONG TERM FORECAST (Days 4 through 7: Tuesday through Friday)...

The models in the long-term continue to show a lot of
discrepancies, but some broad features are coming together. On
Tuesday, the models are in somewhat better agreement. A longwave
trough with embedded shortwaves will swing eastward across the
Bering, making it to western Alaska on Tuesday morning. The latest
model runs all generally agree that Tuesday will be a wet day for
nearly all of southern Alaska in that pattern. Within that large-
scale agreement, however, the local scale details are far from
certain. As such, when and how much rain will fall on any given
location Tuesday are details that cannot be said with any
certainty. By Wednesday, even the large-scale pattern becomes
more uncertain as the Canadian model holds the longwave trough
axis over the Kuskokwim Delta, while the GFS and EC both continue
swinging the trough eastward. Going with that latter solution,
portions of Southwest Alaska will dry out Wednesday, while the
trough becomes negatively tilted resulting in another wet day for
essentially all of Southcentral except Kodiak and maybe the Homer
area of the southern Kenai Peninsula.

By Thursday, the broad GFS/EC solution continues to suggest the
trough continuing to swing eastward with the trough axis somewhere
over Southcentral. That pattern would result in a somewhat drier
day there, though lots of clouds and showers would persist for
much of the day, particularly through Prince William Sound and the
Copper River Basin. Meanwhile, broad upper-level northwest flow
over Southwest Alaska and Kodiak will make for much drier weather
there, with a good bit of clearing as well for locations away
from the coast. Friday for now should be a generally dry day for
most of the area, depending on the behavior of an upper level low
over northern and/or western Alaska. By Friday evening another low
moves out of the Pacific and into the Bering coast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...KO/KVP
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...JW

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
118 PM AKDT Sat Aug 4 2018

AKZ111-145-051800-
Matanuska Valley-Susitna Valley-
Including the cities of Palmer, Wasilla, Sutton, Chickaloon,
Talkeetna, Willow, and Cantwell
118 PM AKDT Sat Aug 4 2018

...HEAVY RAIN IN THE MATANUSKA AND SUSITNA VALLEYS SUNDAY MORNING
THROUGH MONDAY MORNING BRINGS POTENTIAL FOR FLOODING...

Heavy rain will fall across much of the Matanuska and Susitna
Valleys Sunday morning through Monday morning, leading to rapid
rises in area rivers and streams along with potential for
flooding.

Rain will develop first over the northern Susitna Valley Sunday
morning then spread to the eastern Susitna Valley and Matanuska
Valley Sunday afternoon and evening. The heaviest rain is expected
Sunday evening through early Monday, bringing up to an inch of
accumulation with locally higher amounts. Heavier localized
rainfall amounts of up to 3 inches are possible in the Talkeetna
Mountains, including the Hatcher Pass Recreation Area.

The heavy rain will cause rivers and streams in the Mat-Su to
rise to near bankfull. If the rainfall totals are higher or if
heavier rain is more widespread than currently anticipated, river
levels could go over bankfull and produce minor flooding.

Another round of heavy rain is on tap for Tuesday. With elevated
river levels from the first round of rain, the threat of flooding
will increase.

Anyone heading into the mountains and backcountry should be
prepared for heavy rain and rapidly rising water levels on rivers
and streams. Those who live along Mat-Su rivers and streams should
monitor the latest forecasts and information on flooding potential
at www.weather.gov/afc.