Small Craft Advisory 633, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
301 PM AKDT SAT OCT 15 2016

.SHORT TERM...Complex pattern is in place across SERN AK/GOA
Saturday afternoon. A NRN stream trough is digging S across the
NRN gulf, where cold air aloft and strong vertical motion are
aiding in local thunderstorm development. Farther S, a weak upper
low was nearly stationary over the NERN PAC. A deep layer of
convergence/deformation was occurring in between the trough to the
north and upper low to the south. This convergence zone was
augmenting precipitation rates during midday Saturday, in the
form of rain at Sitka, and snow in Angoon.

By Saturday evening, strong 850-700 mb frontogenesis is forecast
to move north across the central inner channel region, which will
focus strong upward vertical motion through the dendritic growth
zone and deep-layer saturation over Juneau. Temperatures currently
reside in the mid/upper 30s, but dewpoints in the upper 20s/low
30s will support wet bulb cooling to near freezing as
precipitation rates increase this evening. Thus, expect a 6 hour
window of moderate snowfall will exist from late this afternoon
through Saturday night, resulting in borderline advisory level
accumulations.

The deformation zone and associated rain/snow showers advance N
toward Haines/Skagway during Sunday, while a second zone of
showers develops over the CNTRL/NERN gulf coast. However, of more
significance, will be strong outflow winds developing in response
to deepening low pressure over the CNTRL/WRN gulf. Gale force
winds will be probable over Cross Sound and portions of the NERN
gulf tonight into Sunday...while NELY winds to 30 kt are expected
over Yakutat Bay. Elsewhere, a tight NLY pressure gradient will
favor solid small craft winds for Lynn Canal through much of the
short term period.

.LONG TERM...Wetter pattern will be underway as upper trof
continues to deepen over the gulf into midweek. A fairly deep low
will set up over the western gulf and send a couple of occluded
fronts or surface trofs into the area through Tue. Another low and
occluded front should move into the gulf for midweek. After
midweek, the upper trof may move out only to be replaced by
another trof by the weekend. Used blend of the 12z GFS and ECMWF
for Sun night through Tue evening, then went with mainly WPC after
then.

Increased winds over the north gulf a bit as occluded front moves
into that area and low deepens over the western gulf. Should be a
period of gale force east winds there Sun night. Band of precip
will move across the north Sun night with a trof, then another
band of precip should push onshore Mon with yet another trof. May
see some thunderstorms over the north gulf with the occluded front
moving through them, fairly cold air aloft and decent PVA going
on. Milder air should move in as well in lower levels as flow
becomes more southerly.

Next significant low and front will bring more wet and windy
weather to the area for midweek. Temps should moderate enough that
precip should be mostly rain, although some wet snow could occur
over the far northern area with wind trying to become more
northerly. Models suggest gale force winds will be possible, if
not likely, with the front. Once the front moves through winds
should drop off some but onshore flow will likely persist so wet
and mild weather will be the rule for late week.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Weather Advisory until 4 AM AKDT Sunday for AKZ025.
MARINE...Gale Warning for PKZ022-043.
     Small Craft Advisory for PKZ012-013-034>036-042-051>053.
     Small Craft Advisory due to Seas for PKZ041.
&&

Garner/RWT

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