Flood in Alaska, August 14 to 15, 2016

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
320 PM AKDT SUN AUG 14 2016

.SHORT TERM...A low over the NW gulf will continue to send frontal
bands across the region over the next 24 hours. Showers become
moderate to heavy with each band along with gusty winds of
25-30mph. The strongest band has moved into the central panhandle
with heavy rain in Sitka and will affect the Juneau area through this
evening. Winds through the inside waters will peak this evening
with 25-30kt sustained winds. Higher gusts have been reported on
the south side of Douglas Island. Over the gulf the persistent SW
flow will continue to build up a swell through tomorrow.

Have transitioned precip to showers since the rates have been
highly variable between the frontal bands. On Monday flow at the
low to mid levels will become more westerly which will allow for
some rain shadowing on the lee side of the mountains (ie. Chatham
Strait). Also expect the greatest forcing with subsequent frontal
bands to be confined closer to the low center. This will keep
Yakutat under the gun for moderate to heavy rain while the far
southern panhandle will see showers becoming scattered.

Models were in good agreement in the short term. Used an ensemble
approach for pressure and an emphasis on the NAM12 and HiResARW
for winds. Otherwise adjustments were made for local effects and
based on current conditions.

.LONG TERM...By Mon night the low pressure center over the gulf of
Alaska begins to weaken in vicinity of Prince Wm Sound.
Precipitation extending southeast from the low Mon night will
begin to dimininsh over the panhandle Tue. Rain showers diminish over
the southern panhandle first then gradully drier weather will
move north to approx Frederick Sound Tue afternoon and finally
into the northern inner channels Tue night. A risk of showers will
persists over the NE Gulf including Yakutat Tues night.

Models agree short-lived high pressure will ridge into SE AK Wed.
Surface pressure gradients should be weak so surface winds finally
calm down. The ridge aloft is not very sharp so mid and high level
clouds will spill over the ridge keeping skies somewhat
cloudy...particularly over the northern panhandle. Sunny breaks are
more likely over the southern inner channels Wed which will result
in mild day time temps there as well. Radiation fog is possible
Wed night where cloud cover does break up.

A weather system currently over the far west end of the Aleutians
is forecast to travel across the Gulf and take aim at the NE Gulf
coast Wed night and early Thur. The probability of rain increases
again Wed night north of a line from Sitka to Juneau. WFO
Anchorage and I used the GFS model to refine the existing
forecast grids for this feature. Doing so bumped up winds over the
offshore waters to 25 kts along this weather system. Other models
have different timing on this system so this will have to be
watched the next few days for possible forecast updates. Beyond
Wed night and early Thurs model forecasts begin to diverge and
forecaster confidence goes down.

&&

.HYDROLOGY...The Taiya River near Skagway fell through the noon
hour after rain subsiding this morning coincided with its normal
diurnal lull. Expect that the burst of rain moving into the area
this evening will cause it to rise back above flood stage this
evening and likely above moderate flood stage overnight. Current
thinking is that the river will fall through the noon hour again
on Monday and the flood threat will end.

Smaller creeks and streams around the Juneau area will likely
spike this evening with the burst of moderate to heavy rain. Some
may approach bankfull but no flooding is expected.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Areal Flood Warning until midnight AKDT Monday night for AKZ018.
MARINE...Small Craft Advisory for PKZ011>013-021-031>036-041>043-051>053.
&&

Ferrin/TA

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