Small Craft Advisory 629, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
522 AM AKDT Fri Aug 25 2017

.ANALYSIS AND UPPER LEVELS...
A very active pattern continues for most of southern mainland
Alaska as a broad upper level trough currently extends from the
Arctic Ocean down to the Gulf of Alaska, which is being supported
by a southward diving jet stream with a 110 kt jet streak around
the base of the trough. The upper level trough is reflected down
at the surface with a 991 mb low centered over the AKPEN moving
southeast into the Gulf. Widespread rain associated with this
system is falling across most of Southcentral and Southwest
Alaska, though increasing easterly flow is leading to some downsloping
off the Kenai and Chugach mountains with lighter, more occasional
rain seen in the the Anchorage Bowl and the western Kenai
Peninsula. An occluded front is currently moving ashore along the
north Gulf Coast leading to heavy rain as well as an easterly
coastal barrier jet, with gale force winds reported between
Montague Island and Cape Suckling.

Further west, a large area of high pressure continues to build in
across the western and central Bering. A tightening pressure
gradient between the Bering high and the low over southwest
Alaska is leading to gusty northerly winds from the southwest
coast into Bristol Bay and down across the AKPEN. A surface low is
approaching the Western Aleutians from the southwest.

&&

.MODEL DISCUSSION...
Models are continuing to handle the low entering the Gulf fairly
well in terms of placement and intensity, increasing overall
forecaster confidence. Some minor differences crop up over the
weekend as the low center moves northeast across the Gulf and over
the coast, with the NAM moving the system a touch quicker than
the other models.

&&

.AVIATION...
PANC...Easterly flow aloft will act to dry out the lower levels
over the terminal keeping conditions VFR through the period. A few
rain showers may reach the terminal this morning but precipitation
should begin pushing out of the area later this afternoon as the
low over the Gulf shifts south.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

Radar data this morning still shows widespread showers across much
of Southcentral. Currently, downsloping has weakened across the
area and showers are filling in across the Kenai PEninsula and
Anchorage Bowl. The sounding this morning indicates steep lapse
rates at the mountain top which is once more allowing for
spillover precipitation. The HRRR model is capturing the
development of these showers the most accurately and continues
widespread showers into the afternoon. Besides the models under
estimating the precipitation this morning, they are in fairly good
agreement for the short term. Showers will remain in the forecast
for much of Southcentral as the low near Kodiak Island drifts into
the eastern Gulf of Alaska. The gusty winds that are currently
occuring along the northern Gulf coast will gradually weaken today
as pressure gradients relax across the region.

&&

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

Rainfall associated with an area of low pressure moving across
Bristol Bay toward the Alaska Peninsula has continued across
Southwest Alaska through the overnight hours. Rainfall will
steadily taper off from north to south through early this
afternoon as the low moves into the western Gulf of Alaska. In the
mean time, gusty winds upwards to 45 kts will be observed along
the western capes of the Bristol Bay area as well as gaps throughout
the Alaska Peninsula. Pressure rises coupled with cold air
continuing to advect southward on the backside of the low pressure
is creating a favorable environment to ageostrophically force
winds through the channeled terrain. Winds will begin to relax by
Saturday morning as the gradient weakens. Showers will remain in
the forecast for Southwest Alaska through the weekend as the semi-
permanent arctic trough still shows no sign of shifting off to
the east.

&&

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

High pressure has begun to shift from the western Bering Sea into
the central Bering Sea as a front quickly moves into the western
Bering Sea this morning. Latest satellite observations show a good
bit of marine stratus/fog across the central Bering Sea under the
high pressure which will likely continue through Saturday morning
when the high pressure dissipates. Models are in pretty good
agreement in bringing in a near gale force front through the
western half of the Bering Sea late today through Saturday before
stalling over the central bering Sea and weakening.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Thinking is largely unchanged in the long term portion of the
forecast, with a wet and active pattern persisting over mainland
Alaska into next week after a relative lull towards the beginning
of the week. The extended period begins on Sunday evening with a
longwave trough over the mainland anchored by a weak upper low in
the vicinity of the AK Peninsula and Kodiak Island. This will
coincide with benign showery conditions across southern mainland
and Gulf regions through Monday. The main feature of interest in
the long term still looks to be a deep low developing at the base
of the longwave trough and moving north towards Kodiak Island
Tuesday into Wednesday as shortwave energy racing across the
southern Bering interacts with the Akpen low. This will send a
strong front towards the Gulf coast Tuesday afternoon, accompanied
by potentially heavy rains and strong winds. Some uncertainty
remains concerning some of the details that will impact strength,
timing, and placement of these features, however confidence is
increasing that a significant system could impact the Gulf Coast
region early to mid next week. Rain is also expected to spread
inland and then remain in the forecast for the southern mainland
through much of next week as the low slowly weakens in the
vicinity of Bristol Bay and broad onshore flow remains focused
over Southcentral AK.

Looking farther west, the Bering looks to remain in a much quieter
pattern next week in northerly flow between the mainland trough
and high pressure centered over eastern Russia. The previously
mentioned disturbance crossing the Bering will bring a chance of
rain to the Aleutian chain early in the week. Otherwise, a benign
cloudy regime will prevail across much of the region until the
next system enters the western Bering late in the week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale warning 150 155 160 165.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KVP
SOUTHCENTRAL ALASKA...ELN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MC
LONG TERM...CB