Small Craft Advisory 470, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
511 AM AKDT FRI JUL 22 2016

.SHORT TERM...Strong upper trof will move into the far western gulf
by late this afternoon...then reach the central gulf by late
tonight. surface low will develop over the northwest gulf this
afternoon...then move to the north-central gulf tonight while
slowly deepening. Ahead of the upper trof and low...several
shortwave trofs aloft and surface trofs will move across the area
through this evening. Models were generally close on main
features...with the 00z NAM and ECMWF in best agreement...so those
models were used for pressures and winds.

Main forecast concerns will be precip amounts and winds. Precip
potential will be high across the area through tonight...even
though there will probably be some breaks in between the various
shortwaves. There is potential for locally heavy rain in spots
through tonight with the best potential along the west and south
facing terrain. More organized heavy rain will likely affect a
larger part of the area late tonight as a stronger trof moves into
the far eastern gulf. Rainfall up to 1.5 inches will be possible
at lower elevations along the Coast Mtns and northeast gulf coast.
Higher elevations along the Coast Mtns could see up to 3 inches of
rain.

Winds will fluctuate to some degree today with several trofs
moving through the area. A more general increase in winds will
occur tonight as low deepens and moves to the north-central gulf
and stronger trof moves into the eastern gulf. Think most inner
channels will rise to SCA levels tonight with much of the gulf
also getting to SCA levels. Before that though...Lynn Canal will
see south winds increase to SCA levels this afternoon as enhanced
west to southwest flow in mid-levels lowers pressures near
Skagway. Much of the remaining channels will increase to 15-20 KT
by late this afternoon with a more general pressure gradient
increase.

.LONG TERM...Period begins with a near gale low pressure
dissipating as it moves across the northern gulf pushing a
frontal boundary into the panhandle Sat night into Sun. Model
solutions remain in good agreement with this and 00Z guidance
depicted the upper and surface features a little stronger.
Strongest winds with this system will be from early Sat through
the afternoon with most inner channels experiencing advisory level
winds, except for PKZ036 where gales are expected late Sat morning
into the afternoon. The slightly stronger model solutions raised
potential for some tstms as instability increases. Mid level
lapse rates steepen to 8-9C/km and CAPE values increase to
500-600J/kg as the upper low moves across the N gulf. Given the
good model agreement added a slight chance of thunder over the
gulf Sat. This system is also tapping into some subtropical
moisture and will result in locally heavy rainfall Sat diminishing
from NW-SE Sat afternoon into Sun. Rain will transition to showers
as the system moves inland late Sat. Continued onshore flow Sun
will maintain shower activity across the panhandle with additional
precip amounts generally 1/4-1/2" expected Sun.
Model solutions remain in fair agreement with upper trough
deepening s of the Aleutians into midweek, then begin to diverge.
SE Alaska will remain in SW flow aloft with ridge axis centered
over Yukon/BC. With the strengthening low over the W gulf, models
hint at some ridging building into the southern panhandle. This
could result in a better chance of dry and mostly cloudy days
early next week, but confidence in strength of ridge is too low
attm to drop mention of precip.
Inherited forecast represented this thinking well and used EC/NAM
to update Sat-Sun pressure fields depicting the stronger surface
low in the N gulf.

&&

.HYDROLOGY...Salmon River near Hyder continues to rise due to
glacial dammed lake release. As of 3:30 AM, the river has risen
to 25.1 feet. Still expect it to peak in the 26.5 to 28 foot range
Sat before falling slowly afterward. Other area streams and rivers
will also rise due to the rainfall, but not expecting any
flooding through tonight, although some streams could approach
bankfull levels where heaviest rain occurs.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-013-021-031>036-041-042-051-052.
&&

RWT/BC

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SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE JUNEAU AK
104 PM AKDT FRI JUL 22 2016

AKZ029-230915-
MISTY FJORDS-
INCLUDING THE CITY OF...HYDER
104 PM AKDT FRI JUL 22 2016

...GLACIER DAM RELEASE ON THE SALMON RIVER CONTINUES...

GAUGE DATA FOR THE SALMON RIVER NEAR HYDER SHOWS THAT A GLACIAL
DAM RELEASE FROM SUMMIT LAKE ON THE SALMON GLACIER IS ON GOING. THE
RIVER HAS RISEN TO AROUND 25.7 FEET AS OF NOON FRIDAY. BASED ON 5
YEARS OF PAST DATA THE BEST ESTIMATE FOR THE CREST HEIGHT IS
BETWEEN 26.5 TO 28 FEET. THERE IS POTENTIAL OF WATER GOING OVER
THE ROADWAY AT 9 MILE WHEN THE RIVER NEARS ITS HIGHEST LEVELS.
DEBRIS HAS BEEN REPORTED FLOWING DOWN THE RIVER INTO PORTLAND
CANAL.