Small Craft Advisory 1038, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
408 PM AKST Wed Dec 28 2016

.ANALYSIS AND UPPER LEVELS...

Two impactful storms to discuss today. The first is the upper
level trough and associated surface low complex over southwest
Alaska to south central Alaska that is supporting the snow that
moved through Southcentral Alaska today. The other major
impactful storm is the large deep low over the western Bering Sea
with the associated front extending through the western Aleutians.
The Bering Sea storm has a significant push of warm air into the
Bering Sea today that will move over western Alaska tonight.
Currently over the western mainland the low level air is quite
cold and will support a brief period of freezing rain that will
rapidly change to rain before changing back to snow as the cold
air sweeps in with the cold front. Between the Bering storm and
the low over the mainland is a ridge of high pressure that is
resulting in clearing skies.

&&

.MODEL DISCUSSION...

The models are in excellent agreement with the major synoptic
level features. Differences occur with respect to the QPF, though
these are minor. The models are in good agreement with respect to
inhibiting the push of warm air beyond Cook Inlet, holding it back
over southwest Alaska and the Kenai Peninsula to Prince William
Sound.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Lingering snow showers over Southcentral Alaska will begin to
taper off this evening from west to east, as the surface low over
Prince William Sound weakens and tracks southeast into the gulf
tonight. A blast of cold air will move in quickly behind this
system tonight, causing favored gap wind areas along the coast to
ramp up quickly and temperatures to decrease over inland areas.

The focus of the forecast then shifts to Thursday, as a strong
warm front currently surging along the southwest coast moves into
southern Alaska. The warm front itself is expected to bring a
brief period of light snow to most areas with little or no
accumulation, but by Thursday night a strong cold front races
through Cook Inlet and is expected to bring a period of widespread
and heavy precipitation to much of the Mat-Su Valleys and
Anchorage area. Models have trended a bit colder with this storm,
and points from Anchorage northward will remain as snow for the
duration of the storm. There will be an upslope/orographic
component to the storm that will cause the front range of the
Chugach and especially the southwest facing slopes of the
Talkeetna mountains to see the higher snowfall amounts. Snowfall
near Hatcher Pass should be the most significant, with lesser but
still substantial accumulations at the lower elevations of the
Mat-Su Valleys. Even though expected snowfall amounts may not hit
warning criteria for all areas for this event, the impacts should
be very similar. Given the similar impacts, a winter storm warning
for snow has been issued for the Matanuska Valley Thursday evening
through Friday morning, and a winter weather advisory for snow has
been issued for the surrounding Anchorage area and Susitna Valley
(from the parks highway eastward).

One area of continued uncertainty centers on whether parts of
Anchorage will see a period of wind off Turnagain Arm Thursday
night, popping temperatures up above freezing and at the very
least limiting snowfall totals even if precipitation stays as
snow. Meanwhile, down on the Kenai Peninsula, a rain and snow mix
north of Kenai should significantly limit accumulations while
areas closer to Kachemak Bay should see things quickly switch over
to rain.

&&

.AVIATION...
PANC...Light snow showers and patchy fog with marginal conditions
will continue through this evening, followed by partial clearing
and more VFR conditions overnight. However with a lack of mixing
tonight and light northerly low level winds, there is some
potential for patchy fog to remain over the inlet and possibly
drift over some of the surrounding inland areas. A decaying front
arriving from the Bering Sea via western AK will cause ceilings to
lower Thursday morning.

&&

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

A strong warm front moves onshore tonight, with snow developing
through the evening especially along and north of the Kuskokwim
River where significant snow accumulations are possible.
Immediately upon warm frontal passage, temperatures are expected
to warm well into the 30s, even for inland locations. The period
of rain in the Delta region may reach 12 hours before the cold
front moves through and changes rain to snow before ending by late
Thursday afternoon. For inland areas, rain will change to snow
before ending from west to east Thursday evening. Given the
expected heavy snow conditions preceding the rain, have opted to
change the Freezing Rain Advisory for Kuskokwim Delta and Valley
to Winter Weather Advisories for snow and freezing rain, and for
the Bristol Bay region enough warm air (including latent heat
release from heavier rain falling on exposed frozen surfaces) will
be present to lessen the expected severity, though there could be
brief and isolated impacts during the day on Thursday.

&&

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

Strong cold front continues to strengthen as it moves from the
western Bering to the central Bering. High winds are occurring
near the Central Aleutians as a mountaintop inversion has
resulted in mountain wave activity, and this should continue into
the mid-evening hours before ending as the cold front nears. The
front will begin to weaken as it reaches the eastern half of the
Bering late tonight, though it will still produce strong gales
with storm-force gusts as it moves toward Bristol Bay and the
Alaska Peninsula coastal waters later Thursday.

Ridging will build briefly Friday and Friday night over the
central and eastern Bering as the next strong low pressure system
spins up over the western Aleutians. With a continued strong
pressure gradient between these two features, this system will
likely produce storm-force winds from the Western Aleutians north
and east through the northwestern half of the Bering Sea.

&&

.LONG TERM FORECAST (Days 3 through 7/Friday Night through Wednesday)...

On Friday night the trough and surface front (expected to bring
heavy snow to portions of Southcentral) will rapidly exit to the
east, with a quick and strong push of cold advection winds on the
back side. Have increased winds, particularly for Whittier/Passage
Canal, Prince William Sound and the northern Gulf. The westerly
flow is most favorable for Whittier, so may see some wind gusts
approach warning level (75 mph).

Meanwhile, a rapidly strengthening low moving up from the Pacific
to the far Western Aleutians, along with a separate downstream
wave, will quickly amplify the pattern. A ridge will build over
the eastern Bering Friday night and slide eastward over the
mainland Saturday, phasing with a well established ridge over the
Northeast Pacific. Previous guidance had indicated this ridge
would hold strong over Mainland Alaska, relegating the active
weather and impacts from deep lows to the Bering Sea and
Aleutians. There is now a very definite trend toward weakening the
ridge over Mainland Alaska as short-wave ride over top of it. This
will allow a front sitting off the west coast Saturday to move
onshore across Southwest Alaska Sunday and possible into
Southcentral on Monday.

Out west, deep storm systems will provide a steady dose of gale
to storm force winds, with hurricane force gusts possible in cold
advection near the primary low center. It looks like there will be
a break in between systems early next week which will allow winds
and seas to diminish, but the pattern will remain favorable for
strong systems moving up from the Pacific as we head through next
week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Heavy Snow 145, High wind warning 187, Winter Weather
Advisories 101 111 152 155.
MARINE...Storm 175 176 177 178 185. Gales 130 131 155 165 170 172
173 174 179 180 181. Heavy freezing spray 130.

&&

SYNOPSIS AND MODEL DISCUSSION...SA
SOUTHCENTRAL ALASKA...TP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CC
LONG TERM...SB