Small Craft Advisory 699, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
506 PM AKDT Wed Sep 16 2020

.ANALYSIS AND UPPER LEVELS...

A short-wave trough is digging along the central to eastern
Aleutians. This is leading to a strengthening gale force surface
low just head of it. This low, just south of Cold Bay as of mid
afternoon, is tracking toward Bristol Bay. An associated frontal
system east of the low center is making its way into the western
Gulf. Winds and rain with this storm system are spreading
northward toward both Southcentral and Southwest Alaska. Out ahead
of it, weak upper level waves embedded in broad southerly flow
are producing abundant cloud cover and a few showers. Conditions
are quiet over the western to central Bering and Aleutians under
the influence of weak ridging.

&&

.MODEL DISCUSSION...

Models have come into much better agreement with many of the
details of the system along the Alaska Peninsula as it tracks to
Southwest Alaska Thursday. Thus, have refined the wind and rain
forecast based on much of the higher resolution model data.
Models diverge a bit on a fast moving second low tracking across
the southern Bering Thursday night and into the Gulf Friday.
Have generally stuck with inherited forecast for now, so expect
some changes will be needed in future forecasts.

&&

.AVIATION...

PANC...VFR conditions are expected through most of the TAF
forecast, largely thanks to strengthening low level southeast
flow. However, southerly flow aloft will bring in rain and
ceilings may hover around the 5000' threshold at times. There
is uncertainty in the timing of frontal passage on Thursday, but
expect it somewhere around midday. Prior to frontal passage low
level wind shear will prevail, while gusty southeast winds will
move in behind the front.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Wed
afternoon through Friday afternoon)...

Wind and rain are in store for Southcentral beginning tonight and
lasting through Friday. A front associated with a low centered
over Bristol Bay will lift northward Thursday. Gusty
southeasterly winds are expected to begin this evening through the
usual areas including Passage Canal, Portage Valley, Turnagain
Arm, Anchorage Hillside, and Knik River Valley. By Thursday
morning, gusty southeasterly winds will spread across much of the
Anchorage Bowl and Mat Valley with sustained winds around 10-15
mph and gusts up to 35 mph as the low continues to track
northward. These winds are expected to diminish overnight Thursday
into early Friday.

Rain is the second half of this story. As this front lifts
northward, the northern Gulf coast will see the bulk of the rain
beginning this evening and eventually tapering off by Friday
morning. Given the southeasterly winds, rain in Anchorage may be
delayed due to downsloping, though thinking the rain will fill in
late tonight. Areas farther north including the western Susitna
Valley will see continued rainfall as upsloping enhances
precipitation along the mountains. By Friday, the windy
conditions should diminish across much of the area though cloudy
skies and scattered rain showers are likely to continue through
the end the week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Saturday)

A circulation south of Alaska Peninsula (AKPEN) will sweep a lot
of rainfall and gusty winds between 25 to 40 mph over much of
Alaska Peninsula and Bristol Bay tonight through at least Thursday
night. This same system will bring more rain across the region
and winds gusting to 30 mph over the lower Kuskokwim Valley
between Thursday and Thursday night. Computer models favor
weakening this circulation between Thursday afternoon and Thursday
evening as it transitions into a surface trough trekking
northeastward across the Alaska Range into early Friday morning.
Over Bethel and Kusko Delta, clouds and showers passing through
this area will continue moving eastward and across the area
through at least Friday afternoon. Another circulation from the
Aleutians will either bring additional rainfall by Friday night
along Bristol Bay or maybe not, depending on the location and
motion of this expected system. Overall, anticipate a very wet
pattern through the next day or two, with the potential of more
rain entering the forecast area between Friday and Saturday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Saturday)

Hourly observations from this morning through this afternoon
along the eastern Aleutians, buoy 46075 and AKPEN are responding
to the motion of a large scale circulation south of Cold Bay. The
buoy 46075 recorded sustained small craft winds with a peak wind
of 39kt from this afternoon, while other observation sites
continue to pick up gusty winds in the upper 20s to mid 30s. Heavy
rainfall and strong gale are expected mainly over south of AKPEN
and Bristol Bay tonight through at least Thursday night. Further
west, a developing circulation from western Aleutians will bring
a lot of moisture and rainfall across the Aleutians in the coming
days. On Friday night, a broad frontal system from the northwest
Pacific will usher more rain, and the potential for small crafts-
gale conditions are feasible. For now, will monitor this situation
and provide updates in the next package. As a result, anticipate a
very wet pattern and elevated winds across the Aleutians through
most of the period.

&&

.MARINE (Days 3 through 5: Saturday through Monday)...

By Saturday, a strong low pushes into the Bering Sea, bringing
gale force winds to the coastal areas of the Western/Central
Aleutian Chain and small craft elsewhere. Models are in
disagreement on the exact strength and location of this low,
but do agree on the general track as it travels eastward toward
Southwest Alaska. The Canadian tracks slightly more northeast
toward Chignik, whereas the GFS and EC are more in agreement
tracking it more toward Dutch Harbor. It looks like it then
travels to the Gulf by Monday, but there is uncertainty on both
the specific track and the strength of the winds over the Gulf.

&&

.LONG TERM FORECAST (Days 4 through 7: Sunday through Wednesday)...

A strong low pushes into the Bering Sea on Sunday. Widespread
rainfall and gale force winds along the Aleutian Chain are
expected in association with this low. A low amplitude ridge out
ahead of the low moves over the Southwest region at this same
time, providing somewhat drier conditions, although the
possibility of rain can't be ruled out completely. This ridge
moves with the low and will be over Southcentral by Monday. By
Tuesday, the low will travel into the Southern Gulf, with the
heaviest rainfall confined to Kodiak Island and the Coastal areas.
Gale force winds are also possible for the Southcentral Coast and
Northern Gulf.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 119 125 130 131 132 138 139 150 155 160 172.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...KM