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Southeast Alaska forecast discussion 
National Weather Service Juneau Alaska
418 PM akdt Thursday Jul 10 2014

Short term...low pressure will deepen and move north into the far
western Gulf tonight. Occluded front over the west-central Gulf will
move to the eastern Gulf Friday...but begin to slow its eastward movement
late in the day.

Main concerns are the developing flood situation for Mendenhall
Lake and river...precipitation potential/amounts...and winds. Will
discuss the flood situation in the hydrology section below. As
for precipitation potential...models have trended slightly further east
with the precipitation through daytime Friday. Still some differences on how
far east it gets by late Friday afternoon. Decided to use a blend of
the 12z GFS/ec/Gem to handle the progression of the precipitation to the
east and for rainfall amounts. Should have precipitation increasing in the
paya area tonight...and it will likely be heavy at times late
tonight as strong southerly low level jet moves into that area. Precipitation
should reach the pael area early Friday morning...and will likely
get as far east as the pajn area by late Friday afternoon. The heavier
precipitation should gradually shift toward the pael area Friday afternoon
as low level jet axis shifts eastward with the front. Rainfall amounts
could be over 3 inches in the paya area through Friday afternoon...and
1-2 inches around pael by late Friday afternoon. Amounts should drop
off rapidly to the east...with generally less than a tenth of an
inch in the pajn area by early Friday evening.

As for winds...looks like gale force winds will spread NE into
the central and northern Gulf tonight as front moves in. The winds
ahead of the front should slowly weaken as it moves further east and
away from the main low center over the far western Gulf...bu still
expect Small Craft Advisory level winds over much of the remainder of the Gulf.
Does not look like the stronger winds will reach the Inner
Channels through Friday afternoon. Lynn Canal will see strongest winds
over the Inner Channels due to thermally enhanced S flow...with
20 knots winds during the afternoon and early evening periods...but
diminishing to 10 knots during the overnight hours. The 15-20 knots northwest
winds over the far southeastern area should weaken to about 10 knots

Otherwise...will have a thin layer of stratocu clouds hang
around over the northwest 2/3rds of the area tonight. The southeastern third
will likely scatter out cloud wise by early evening...although
some thin high clouds will drift in from the west overnight. Should
see clouds increase slowly over the western part of the southern Panhandle
Friday. Looks like a fairly warm day over the southeastern area Friday...with
many areas getting into the 70s. Hyder looks like it could get
into the lower 80s. Did lower maximum temperatures some over the more eastern
part of the northern Inner Channels Friday due to faster onset of precipitation.


... Basin is currently emptying out into the
Mendenhall Lake and river system. Given the amount of water that
was in the basin and what the lake and river levels were when
event started...record flooding is expected for both the lake and
river. Current forecast maximum stage is 11.7 feet for the lake and
14.6 feet for the river. Event is unfolding faster than past
years...and it looks like the peak of the flood will be sometime
Friday night. Current forecast has water levels dropping rapidly on
Sat. Will update the Flood Warning to include other impacts since
the water is expected to be so high. The banks of the Mendenhall
river will be a dangerous place to be when the flooding begins as
they will be vulnerable to erosion due to the high and swift
water. Will also have to keep an eye on the chilkat and taiya
rivers for Friday night through Sat night with the rain coming into
that area.


Long term...main effort today was on the beginning of the
extended period Friday night to sun. Events to watch during this
peiod are the Mendenhall flood and wavy frontal system causing
tricky pop/quantitative precipitation forecast details in the graphic forecasts. Like short term
forecast... I tended to use a blend of the 12z GFS/Gem/ec.

Flood Warning details covered well in Hydro section above. The
front approaching the coast tonight will have a tight gradient of
rainfall rate with it. Along and just ahead of the front there
will be heavy precipitation. The wavy nature of the front makes it hard to
say but decided to bring a brief period of moderate rain as far
east as Juneau to Angoon to Sitka line late Friday night and early
Sat morning then pull heaviest precipitation back to the west
closer to the coast. The period of heaviest rains late Friday to early
Sat coincides with the predicted peak water levels and flow with
the Mendenhall flood. Rain totals in the Mendenhall basin from Friday
afternoon to Sat afternoon should be around one inch. Also in
that same time frame will be high tide /130 am Sat/ which will
likely back the water in lower Mendenhall river. The good news is
there is a minus tide Sat morning so flood waters should recede
quickly Sat.

Little change made to the rest of the extended period for next
week. I did gradually reduce the chance of rain from Monday to Wednesday
than the previous forecasts had.


Ajk watches/warnings/advisories...
public...areal Flood Warning until 10 PM akdt Saturday for akz025.
Marine...Gale Warning for pkz052.
Small Craft Advisory for pkz042-043-051.
Small Craft Advisory due to seas for pkz041.




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