Gale Warning 304, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
238 PM AKST Wed Dec 13 2017

.SHORT TERM.../Through Thursday night/ Wavy frontal system over
the eastern gulf will move across SE AK Thu. An upper level
shortwave trof will move NE across the gulf Thu and into the N
panhandle Thu night. Most adjustments were generally localized,
due to system coming through over next 24 hrs. It appears that
the 12z GEM/EC were best models handling the waves on the front.

Main forecast concerns will be precip amounts with front, TSTM
potential with shortwave,  and winds. Still looking like locally
heavy precip especially over the NW half of the area through Thu.
Low level jet is fairly strong ahead of the front and this will
keep strong moisture advection into the NE gulf area. Wavy nature
of front will cause some shifting E-W of heaviest rain over the NW
half of the area into early Thu morning, then precip band should
move E through entire area later Thu morning into Thu evening.
Rainfall could reach 3+ inches with highest amounts along the NE
gulf coast. See hydrology section for impacts on local watersheds.

Shortwave moving in for Thu and Thu night will bring more showery
weather, and temps aloft at 500 mb are going to be around -35C, so
also added in slight chance of TSTMS for much of the E gulf waters
Thu, with slight chance of TSTMS also brought into the central
coast NWwd for Thu night.

Expect gale force winds ahead of the frontal system tonight over
the E gulf, with SCA to gale force winds over the inner channels.
Kept strong wind headlines for the outer coastal zones for tonight
as well. Winds will diminish over much of the inner channels Thu
afternoon, except over the far N where increasing S wind to SCA
level is expected. Band of SCA level SW winds are expected over
the E gulf and ocean entrances to inner channels with shortwave
moving through Thu afternoon and night.

.LONG TERM.../Friday through Tuesday/ As of 10pm Tuesday...The
upper level jet that has been providing the warm weather slides SE
of the region by Friday morning. Meanwhile, a separate arm of the
jet stream will move into the Gulf of Alaska from the west,
however the upper level pattern quickly becomes amplified with a
ridge building over the eastern gulf and a trough over the
Aleutians/western gulf. This would cause a short period of NWLY
flow aloft over the panhandle Friday. This will help to break up
some of the lingering showers over the area. Temperatures at 850mb
cool enough for some of these showers to be a mix, but these pre-
frontal showers look to be light, so not expecting much in the
way of accumulation. Timing of the breaks will be critical for how
much the surface cools ahead of the next front. If the breaks
occur after sunset then things starting off as snow is more
likely.

The front will move across the gulf Friday afternoon as the upper
level ridge shifts to the east. Expect SE gale force winds with
this front over the outside waters. Flow aloft will increase out
of the SW causing temperatures to rise again as the front moves
inland Friday night into Saturday. Therefore expect a transition
to predominantly rain, especially as a stronger wave forms along
the front and intensifies the SWLY flow with moderate to heavy
rain Saturday. Expect a wind shift to the SW once the main part of
the front moves inland. The front then moves off the the SE
with colder air behind it, causing a transition to showers
Saturday night into Sunday.

Latest models have a weak low over the north-central gulf on
Monday that tracks to the S-SE through Tuesday. Meanwhile, the
upper level jet looks to finally reach up into the arctic and draw
down some colder air to the region next Tuesday. Both the upper
level and surface pattern would favor a shift to a northerly
outflow pattern which tends to be a dry one for us, although not
all models agree on this pattern change. With many differences
amongst models its too early to say this with much confidence 7
days out.

Overall preferred the GFS and Canadian models as they were in good
agreement through the extended period along with the NAM earlier
on.

.HYDROLOGY...With the locally heavy rain expected for the NW half
of the area tonight and Thu morning, rivers and streams will rise
somewhat. Current thinking is that none of them will get above
bankfull level. Only expect Jordan Creek to get to bankfull right
now, and that would occur late tonight and Thu morning. The Situk
River may get close to bankfull late tonight. Some heavier precip
will move into the S area Thu which will cause those streams and
rivers to rise, but rainfall there is expected to be much less
than over the N so not expecting any issues there.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind through Thursday morning for AKZ027.
     Strong Wind until midnight AKST tonight for AKZ017-022.
     Strong Wind through late tonight for AKZ023.
MARINE...Gale Warning for PKZ036-041>043-051.
     Small Craft Advisory for PKZ012-013-021-022-031>035-052-053.

&&

RWT/Ferrin

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