Small Craft Advisory 904, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
447 AM AKST Thu Nov 17 2016

.ANALYSIS AND UPPER LEVELS...
A concentric low pressure center aloft is digging south of the
Alaska Range over Southcentral Alaska. The cold-core system doesn't
appear to have really good moisture associated with it due to
its trajectory from the central Interior. At the surface, an
outflow pattern exists with high pressure north of the Alaska
Range and weak low pressure in the Gulf. Gusty winds in favored
gap locations along the north Gulf coast look to continue.

The longwave pattern is anchored by a larger upper low over
Kamchatka. The frontal boundary associated with it has an
impressive moisture feed from the sub-tropics all the way
northward through the Central Bering and Chukchi Peninsula. Winds
along the front are solid gale-force from the south, driving the
accompanying rain.

&&

.MODEL DISCUSSION...

Guidance continues to be locked into the current synoptic pattern.
Little discrepancy exists among guidance for either frontal
position/strength in the Bering or for the upper low position in
the western Gulf. Some slight differences exist with the
precipitation placement and upper low position are present,
however, synoptically the story is the same. The east domain
favors the NAM early, while the western domain used the GFS.

&&

.AVIATION...

PANC...A marginal ceiling this morning will give way to VFR
conditions by this afternoon. Otherwise, weak northerly winds and
VFR conditions prevail thereafter.

&&

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

The main feature impacting Southcentral this morning is a compact
upper level vort max, pushing south across the Kenai Peninsula.
Although the sounding this morning indicates a relatively dry
airmass, there is enough associated cold air aloft to support
steep lapse rates and light local showers along Cook Inlet this
morning. A separate band of showers are forming along Western
Prince William Sound and are associated with the weak surface low
in the Gulf of Alaska. Showers will continue to persist across
Prince William Sound into Friday as the Gulf of Alaska low
intensifies just to the south of Prince William Sound as the upper
level vort max interacts with the surface low.

Cold air will continue to filter into Southcentral as high
pressure remains established inland. Gusty winds will also
continue through local gaps as the current pressure gradient
remains in place. A persistent stratus deck across the Copper
River Basin is keeping temperatures from plummeting under cold
air advection across the region and will likely keep temperatures
plummeting tomorrow night as well.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An amplified upper level ridge builds into the Southwest Alaska
region with continued offshore flow accompanied by drier conditions
in the lower levels. An upstream trough over the eastern to
northern Bering will then move into the Kuskokwim Delta coastline
heading into this weekend. Therefore, look for some snow along the
previous aforementioned region by late Friday evening as the
weather front moves along the Kuskokwim Delta shoreline. As this
system moves through the Bristol Bay region, the precipitation
will be a wintry mix along the Bristol Bay coastline before
changing to all snow in the interior with temperatures below
freezing.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The gale force weather front continues to elongate as it moves
eastward stretching from the northern Bering southward into the
eastern Bering. This system moves inland through the Southwest
Alaska region by Friday evening. Meanwhile, on the backside of
this system the Bering/Aleutians will be under the influence of
westerly flow. As the cooler air filters in with the westerly
flow look for a wintry mix of precipitation with a showery
weather regime heading into the weekend.

&&

.LONG TERM FORECAST (Days 3 through 7)...

By Fri afternoon a pattern change will be in full swing over our
corner of the Northern Hemisphere. The upper level ridge that has
been inhibiting the eastward progression of weather systems will
be slowly moving northeast. On Sat the change from a blocking
pattern to a progressive pattern begins. The location of the upper
high will force these eastward moving systems to remain south of
much of the forecast area. Whether the upper flow becomes
essentially zonal, or if there are several troughs that move east
is still uncertain. This means the timing of each individual low
that moves along the Aleutians remains in question. While the
forecast utilizes some NAM and GFS through Sat, the rest of the
forecast defers to the ensemble solution from WPC.

The relaxation of the blocking upper high means the Bering front
will be able to move across the coast before decaying over
southwest AK Fri night into Sat. A residual area of low pressure
will also gradually weaken along the north Gulf coast during this
period. On Sat an elongated double low moves along the Aleutians
through Sun, before consolidating and moving to Vancouver Island
by Wed. This will be followed by ridging that transits the
Aleutians to the Akpen Sun night into Tue. Another low will move
off Kamchatka Mon with an associated triple point reaching the
Gulf Wed. Finally a third low develops in the extreme western
Bering later on Wed.

In the west this will all result in some snow along the coast
initially Fri night into Sat, and then rain and snow showers
mainly over the Bering and Aleutians through the middle of next
week. In the east, some coastal rain and Chugach Mountain snow
will be associated with the north Gulf low through Sat. Brief
drying will be followed by some coastal rain and coastal mountain
snow as the lows move through the Gulf through midweek. Inland areas
of Southcentral will remain cold and dry.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 127 173-176 178-179 181 185 411-414.

&&

SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHCENTRAL ALASKA...ELN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PLD
LONG TERM...DS