Small Craft Advisory 612, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
822 PM AKDT Sat Aug 19 2017

.ANALYSIS AND UPPER LEVELS...

A cold upper level trough continues to encompass much of mainland
Alaska and into the north Pacific Ocean. A bowling ball upper
level low is currently centered over the Seward Peninsula. With
no prominent forcing features, generally weak scattered showers
have been occurring across the Southcentral and Southwest mainland
this morning and the region is enjoying a brief break from the
soggy weather pattern this week. Meanwhile, surface high pressure
continues to prevail across much of the west and central Bering
with a broad stratus deck.

&&

.MODEL DISCUSSION...
Models are in good agreement for the short term and are all
indicating that the low will progress into the western Gulf on
Sunday afternoon and support the development of a coastal low
level jet. By Tuesday, models begin to diverge with the NAM
indicating a much more potent low in the southern Gulf of Alaska,
while the GFS and ECMWF indicate a weaker low.

&&

.AVIATION...
PANC...The main concern for this afternoon will be ceiling height.
Currently diurnal heating is partly mixing out the cloud deck
across the Anchorage airport, but ceilings may lower into the
evening hours. Very light southeasterly winds are expected out of
Turnagain Arm starting later tonight.

&&

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

The majority of Southcentral Alaska is experiencing a brief break
in the persistent August rainfall this evening as the shortwave
responsible for last night's rain has moved northeast. While some
stubborn showers will remain this evening over the Alaska Range
and Talkeetna Mountains, the majority of the precipitation has
shifted east to the eastern Chugach and Wrangell Mountains.

Overnight tonight, the next low to impact the area will enter the
southwestern Gulf of Alaska. This system consists of the
reinvigorated remnants of Tropical Cyclone Banyan phased with an
upper-level arctic low dropping south over the Alaska Peninsula.
The low will push a front through the Gulf tonight, reaching the
northern Gulf Coast by early tomorrow morning. This front will
bring moderate to heavy rain to the Gulf Coast Sunday morning
through Sunday night. In addition to the rainfall, a gale force
coastal jet will develop along the northern Gulf, peaking in
intensity Sunday evening. As the low pressure system retreats
southward to the North Pacific Monday and the front along the
northern Gulf Coast weakens, expect rainfall and winds to taper
off throughout the day Monday.

&&

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

The upper low itself will continue to descend into Southwest
Alaska. With the models still pinging into a due south trajectory
for the low, expect the cold pool directly over southwest with
only a hint of re-curvature on Monday. The jet will track over
the AK Peninsula. Late Sunday into early Monday, a tight pressure
gradient will materialize as the low departs for the Gulf of
Alaska and a ridge builds in resulting in gusty winds over
Bristol Bay.

&&

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

High pressure over the northern Bering will result in widespread
stable conditions north of the Aleutians. Meanwhile, the remnants
of an occluded front south of the archipelago will bring showers
into the vicinity of the Aleutians...but the ridge is acting as a
buffer blocking much of the precipitation shield. Expect the
surface low to track eastward into the Gulf Of Alaska. Sunday
night and Monday expect a gap wind event south of the AK
Peninsula.

&&

.LONG TERM FORECAST (Days 4 through 7)...

Beginning the extended period forecast Tuesday night, there will
be a closed upper low over the far southeastern Gulf of Alaska
with a mobile shortwave trough over the Central Bering Sea. In
between these systems there will be a shortwave ridge axis nosing
into Southern Alaska with an anchoring trough centered over the
North Slope of Alaska. This pattern will be the end of a short
quiescent break in the weather. By Wednesday afternoon, the trough
over the Bering Sea will merge with the anchoring trough over the
North Slope with increasing southwest upper level flow ushering in
more clouds and rain to Southern Alaska. This cloudier and wetter
pattern looks to persist through Friday, although uncertainty
grows by this period as the GFS and ECMWF differ with respect to
the position of the trough and the degree of interior downslope
flow. Coastal locations, especially near the Gulf of Alaska, will
be wet, but by Thursday and Friday it looks like there is a
possibility that downslope drying dramatically limits interior
precipitation, especially in the ECMWF solution. For now a middle
ground solution will be favored, depicting coastal rain into the
weekend and more showery conditions inland by the end of the week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales 119 120 130 131.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EN
SOUTHCENTRAL ALASKA...RD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
LONG TERM...JA