Small Craft Advisory 380, NWS Juneau, AK

Southeast Alaska Forecast Discussion...CORRECTED
National Weather Service Juneau AK
1133 PM AKDT Thu Aug 26 2021

.SHORT TERM.../ Through Saturday night / Thursday night a ridge
of high pressure over the eastern gulf will build and help to
decrease shower activity. The ridge will then shift over the inner
channels through Friday morning, bringing fog potential for the
southern areas. Most lingering showers look greatest along the
Coast Mountains for daytime Friday. Models often overspread these
a bit too much, but still cannot rule out the potential so left
some scattered activity in the forecast.

The big story of the week will be a low pressure system moving
into the area for the second half of Friday. An upper level trough
will be digging out of the Bering Sea and cause a surface low to
develop over the NW gulf. This will merge with another arm of the
jet stream moving across the north Pacific bringing with it
tropical moisture. Jet dynamics from the 2 jet streaks merging at
the base of the upper level trough will develop the surface low
even more. Models are currently supporting 2 separate low centers
until they merge together over the NE gulf late Friday night. The
deepening system will bring with it heavy rain and strong winds.
Have increased both factors with this forecast update. The outer
coast and southern inner channels in particular will see wind
gusts of 40+mph Friday night. Some model solutions support wind
gusts of 50mph. The initial frontal band will move through
quickly, meaning the peak winds should be short-lived. Over the
outside waters a swath of 35-40kt winds will move through with the
front Friday evening, then track east over the central/southern
inner channels with widespread 25-35kt S-SE winds. Almost all
parts of the panhandle should see a period of moderate to heavy
rain Friday night.

Rainfall totals form Friday night into Saturday have remained
fairly stable, although with some of the new model runs did adjust
totals for the event, it is now looking like somewhere between 1
and 2.5 inches in the 24 hour block. Most of the rain will be near
the frontal passage so expect majority of it fall in about a 12
hour period. This expected amount of rain on the already wet
ground will mean most of it will go straight to runoff and to the
streams and rivers of Southeast Alaska. Expect to see some quick
rises on streams and rivers, however, we are not anticipating
flooding along them.

The system is expected to be quick moving through the panhandle,
and with cooler air spilling in behind aloft. Models are
suggesting a pocket of instability behind the front moving from
the southeast gulf and into the southern half of the panhandle
Saturday to Saturday evening. A result switched the precip type
over to showers quickly behind the front and added a small area of
slight chance thunder to the forecast. Will need to keep an eye
out in case the instability changes or moves location.

.LONG TERM.../Saturday through Wednesday as of 10PM Wednesday/...The
Gale force low that will be impacting the Panhandle with gusty
winds and appreciable rainfall will begin to weaken and drift
southeastward through Sunday night, giving a progressively more
relaxed pressure gradient causing the winds to begin diminishing.
Gales up to 35 to 40 knots will still be possible over the Gulf
waters Saturday morning and 20 to 30 knots remain likely over
some of the Inner Channels.

This system has ample moisture to work with. It brings with it a
long fetch of deep moisture, originating all the way from the
central Pacific region. Model forecast Integrated Water Vapor
Transport(IVT) values of up to 1,000 kg/(ms) are possible over the
eastern Gulf and Panhandle as the system is impacting the area.
Since the low pressure system is quickly moving, major hydrologic
issues are not anticipated. Forecast rainfall totals of 1 to 2
inches are possible over the Panhandle.

The remainder of the period will be relatively nice with a general
ridging pattern being dominant over the eastern Gulf and southeast
Alaska giving decreased cloudiness and only slight chances for
rain showers. The pressure gradient will be progressively more
relaxed for the rest of the period, yielding light winds over the
forecast area. With the broken to scattered clouds, light winds,
and saturated soils from recent rainfall, radiation fog is
possible over land areas, primarily on Monday night into Tuesday
morning, as cloud cover looks to be only in the scattered category
across the majority of the Panhandle. As for high and low
temperatures, they look to be pretty non-impactful for the
entirety of the long-term forecast period.

Forecast models were in good agreement. Therefore, confidence in
the forecast is average to above average for the long-term
forecast period. A GFS/NAM/CMC/ECMWF blend was used until the NAM
ran out. The blend was phased to WPC for the last day and a half
of the forecast period.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-041>043-051-052.

&&

Bezenek/JLC

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