Small Craft Advisory 1026, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
515 AM AKST Sun Dec 25 2016

.ANALYSIS AND UPPER LEVELS...
The pattern is dominated by a large trough over the Bering pushing
moisture into a transient ridge located just east of the ALCAN
border. The parent low over the Bering continues to fill and
weaken, but it has allowed a few additional weak surface lows to
take shape along its old frontal remnants. The most pronounced of
these lows is in Bristol Bay and another one is starting to take
shape on the windward (east) side of the Aleutian Range. This
second low is one of the key features for the forecast today. With
continued moist onshore flow into the eastern portion of the Kenai
Peninsula, snowfall has been steady overnight. Warm air has tried
to mix inland with strong southeasterly winds and modified air
from the Gulf of Alaska. This has likely limited accumulations to
some extent as the snow has either been quite wet or even mixed
with rain at the lowest elevations. Now the track of the next low
will be important for exactly where and how the winds shift from
southeasterly to more easterly. It will also dictate how much warm
air makes it inland. For much of Western AK, the same warm
occlusion is draped from the Aleutian Range up through the YK-
Delta Coast. This is bringing some gusty southeast winds to the
area, especially over higher elevations. This will continue to
help to keep some of the main population centers such as Bethel
and King Salmon in the "shadow" of the terrain with some
downsloping. Places that are seeing precipitation are mostly
seeing a mix or just plain rain.

&&

.MODEL DISCUSSION...
Models are generally in good agreement, so forecast confidence is
moderate to high. Some of the finer details are nearly impossible
to discern. This would include, but is not limited to, exact
freezing levels through Whittier up to Girdwood and exact snow
amounts. A few degrees in temperature one way or the other could
make a dramatic difference in accumulations. This package leans on
the hi-res models and quite a bit of manual edits. The one
development worth noting in the models is that they all linger the
low near Prince William Sound through the early portion of the
week now (as opposed to just Mon). This could lead to longer
duration snowfall for areas just inland of the coast and could
spell some wrap- around precipitation to locales along the Cook
Inlet as the downsloping shuts off.

&&

.AVIATION...
PANC...In the low levels, the offshore pressure gradient will
continue to increase today. This will lead to some gustier
northerly winds this afternoon. In the upper levels, stronger
southeasterly flow is keeping the ceilings well above VFR and
this should continue through the day. Any resulting wind shear
conditions will remain well above 2,000'.

&&

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

The advertised storm on the eastern Kenai Peninsula is underway,
with storm total snowfall estimates so far through Turnagain Pass
in the 8-12 inch range and lesser amounts of 3-6 inches around
Portage due to warmer temperatures and a rain/snow mix. Slight
changes in the numerical models overnight continue to suggest the
heavy snow will continue through mid-morning through Turnagain
Pass. However, as the general wind flow becomes more easterly
today the heaviest accumulations should shift away from Turnagain
Pass and into the mountains further north in western Prince
William Sound. By midnight tonight the front will be rapidly
falling apart and the steady orographically-enhanced precipitation
will be diminishing with it.

The Anchorage area and the other population centers around Cook
Inlet will see a very uneventful day, with gradually warming
temperatures through the 20s and gusty northerly winds the main
impact from the storm. As the strong front impacting the coast
today falls apart a weak secondary low pressure center will spin
up near Seward and drift around near or just outside of Prince
William Sound for the start of the week. This weakening of the
cross-barrier winds will allow increased chances for some light
snow shower activity to manifest in the typically downsloped areas
of Cook Inlet. It looks like Monday afternoon/evening could bring
a dusting of snow to Anchorage and the Matanuska Valley, but
confidence is pretty low at this point given the uncertainty in
the exact position of the low pressure center.

&&

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

A broad area of low pressure will slowly weaken over the Bering
into Mon morning before moving over western AK Mon night.
Southeast flow will bring some rain showers to the Bristol Bay
zone, with snow easing over the Ahklun mountains through the
period. During the same time the Kuskokwim Valley and Delta will
have a mix of rain and snow. Lingering easterly winds will
diminish later this afternoon across the region.

&&

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

The continued presence of a consolidated low over the Bering will
keep gale force winds in both the northern and southern quadrants
of the low today. As the low migrates east tonight into Mon, gales
will be limited to the western Bering and Aleutians, with some
storm force winds brushing the central Aleutians. Gales will
persist over the western half of the region into Mon night. Rain
and snow showers can be expected across the chain and southern
half of the Bering through Mon night, with snow showers across the
northern portions.

&&

.LONG TERM FORECAST (Days 3 through 7)...
On Monday evening, a weakening compact Gulf of Alaska low will
provide chances for snow showers inland, with likely rain/snow
showers along the coast. Any frontal boundary will have dissipated
by this point so any precipitation left over will be showers,
enhanced along western Prince William Sound in upslope flow. As
this system slides eastward, mostly cloudy conditions will remain
with generally light winds except for the Gulf coast where weak
gap flow will linger.

On Wednesday, a pattern change begins out west as an energetic
east Asian jet amplifies a trough over Kamchatka. The result is a
frontal boundary and large push of warm air into the arctic. As
this ridge builds into the Bering middle-late next week, the
pattern will switch from a weak outflow pattern to southwest flow.
The uncertainty lies in how far northward/eastward the warm air
progresses, and whether or not a triple point low forms to our
south, the two will go hand in hand. What is higher confidence is
it does look like we will get a trough passage from the west along
with a frontal boundary.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 125
MARINE...Storm Warning 119 174 176
         Gale Warning 120 125 129 130 139 140 150
         155 172 173 175 177 178 185.

&&

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