Small Craft Advisory 20, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
549 AM AKST Thu Jan 12 2017

.SHORT TERM...Multiple changes made to previous forecast due to
latest model guidance and overnight observations. Changes were
most notable in temps, precip type, snow amounts and winds
today/tonight. The NAM and Hi- res NMM were the models of choice.

Synoptically the most important features for Southeast Alaska over
the next 24 hours will be a weak low over the northern gulf,
sharpening surface ridge through the inner channels, and a gale
force front moving into the east central gulf late tonight. That
front will be covered more in the long term section.

Increasing low level southerly flow has led to moderating
temperatures overnight panhandle wide. This will continue through
the morning as flow continues to increase from the south. Before
it does so, some light snow accumulation is possible as the
boundary layer moisture continues to deepen. Unfortunately for
snow lovers, the southerly flow is going to win out with most sea
level locations transitioning over to rain by early afternoon. It
will take a bit longer in Haines and Skagway but the sharpening
ridge will increase winds in those areas later in the afternoon.
Some snow will linger in areas such as the Mendenhall Valley and
Petersburg through the afternoon. Based upon all of this,
precipitation was transitioned over to rain quicker and snow
amounts were significantly reduced in this forecast package.

Strong southerly winds will progress northward as the surface
ridge sharpens over the panhandle through the day and into
tonight. Winds were increased 5-10 kt in many of the inner
channels and over several land areas. Expect small craft
conditions to develop this afternoon in Lynn Canal and Stephens
Passage. Southwesterly gusts will increase to 40-45 mph in
Skagway late in the afternoon/early evening and persist through
late tonight.

As mentioned above, temperatures have risen overnight and will
continue to do so. Thursday afternoon highs were raised 3-5
degrees in many areas especially over the northern and central
panhandle. Lows for Thursday night/Friday morning were also
raised 5-10 degrees as we expect them to remain steady from the
afternoon highs with persistent southerly flow.

.LONG TERM...Rather active weather pattern through the long
range. Series of strong low pressure systems move in to the AK
gulf following similar tracks. These system bring in strong winds
and heavy rain. Warming trend continues through the weekend
keeping precip mostly as rain at lower elevations before potential
transition back to cooler temps mid week.

Kept with the trend to increase winds for the Friday system with
associated gale force front. As the parent low tracks
northeastward into the Yukon strong southerly flow continues into
Saturday. For now held off on the wide spread gales for the inner
channels but there is the potential. If MOS guidance is correct,
which is sometimes is, temperatures will be steady or have a
slight increase into Saturday. Getting min and max temps in the
upper 30s to low 40s for almost all of the panhandle from the
southerly flow. Large fetch of tropical moisture seen in model
sounding deep saturated layer so expect heavy rain, especially
along the coast. Backed off on snow at lower elevations with the
warmer air mass but have possible snow amounts of 6 to 12 inches
along the Haines and Klondike highways and a winter weather watch
From Friday afternoon into Saturday morning. Just wasn't getting
snow amounts near Hyder to keep them in the winter weather watch,
but still expect a few inches of accumulation.

Next system moves into the western gulf Sunday with its gale
force front reaching the eastern gulf Sunday night into Monday.
Brief break in weather over the panhandle between these two
systems. Since models still show large spread and run to run
differences held off on making wholesale changes to the later
grids. Yet another low moves in mid week, and if it has a more
southerly track could have a shift in winds to more northerly and
a drop in temps. But more indication that it will follow a
similar northeasterly track to the previous lows.

Used a blend of NAM and Hi res NAM initially to refresh grids.
Slight increase to winds but main changes was to bump up temps and
snow levels due to new model data incorporating the south flow.
Later periods decided to use the ECMWF since GFS, while close in
position, was on the stronger side. Expect day 4 forecast and
beyond to have even more changes with the next packages once
models begin to fall in line. Forecast confidence is low due as
models continue shift run to run.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Storm Watch from Friday afternoon through Saturday
     morning for AKZ018-019.
     Strong Wind from this evening through late tonight for AKZ018.
MARINE...Small Craft Advisory for PKZ012-013-031-041>043-051-052.
&&

TPS/PRB

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