Areal Flood Warning 3, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
653 AM AKDT Fri Aug 13 2021

.SHORT TERM...The atmospheric river (AR) that has been affecting
the region continues to deliver copious amounts of moisture to the
panhandle as evidenced once again by the Yakutat 00z sounding,
which yielded an all-time record precipitable water value (for
00z soundings at PAYA) of 1.57 inches. The IVT value of 762 kg/m/s
was the 3rd highest value ever for 00z soundings. A 120 kt-jet
overhead helped provide the dynamics to generate over 7 inches of
rain over the past two days at the airport in Yakutat. A
developing short wave in the western gulf finally moved the AR
east as expected. Rainfall rates around Yakutat decreased
overnight and heavy rain began over the Icy Strait corridor last
evening, including areas around Juneau, with the MIRS rainfall
rate product showing pockets of 0.25 inches per hour through the
central panhandle. The forecast office here received 1.28 inches
of rain in the 6-hour period between 10pm and 4am AKDT, and this
corresponds to a 1-year ARI for this site. Most sites have
remained in the 1-5 year ARI range for this event so far. The
shortwave that kicked the AR eastward will support a surface low
over the gulf that will move toward Yakutat and enhance the
precipitation there again today with an additional 1-2 inches
possible. The central panhandle will continue to receive rainfall,
heavy at times today, while under the influence of the AR. An
additional 2-3 inches of rainfall is expected through tonight with
3 to 6 inches across higher elevations.

For the southern panhandle, after a nice day Thursday, the focus
of the AR will continue to slowly move south with rain developing
during the day today. This morning some fog (locally dense)
developed over the southern inner channels areas (Craig, Klawock,
Hydaburg). This should dissipate as the rain moves in later this
morning. Rainfall amounts through tomorrow night for the southern
panhandle are expected to be 1-2 inches as AR event comes to a
close.

As advertised, rivers and streams are responding to the heavy
rain, see the hydrology section below for details on the flood
products and rivers / streams that are currently being monitored.

This system brought and continues to bring solid small-craft
conditions to the outside waters and the inner channels. Expect
another enhancement of both land and marine winds early Friday
morning as another wave of low pressure pushes into the NE gulf
Coast. Expect small craft conditions to develop in much of the
Central and Northern Inner Channels, peaking late Friday morning
then diminishing Friday night. Due to such a stable atmosphere and
WAA aloft, it will be difficult for strong low level jet winds
(45 kt) to mix down to the surface. Kept strong wind gusts of 40
mph in Skagway as the gradient will remain tight up there; however
do not think other areas across the Northern Panhandle will see
winds that high Friday. Expect a wind shift from SE flow to SW
flow Friday evening as the main front pushes SE through the
region. A brief burst of stronger winds is likely for both land
and marine areas as the front passes through and winds shift
Friday evening. Expect winds to dramatically decrease late Friday
night through Saturday as the gradient and flow aloft weakens.

After a brief break from precipitation during the day on Saturday,
another wave moves into the panhandle Saturday night bringing
another 0.5 to 1 inches of rain.

There continues to be high confidence in the short term forecast,
including the progression of this atmospheric river.

.LONG TERM.../ Sunday to Friday, as of 9 PM Thursday / A wave
which pushed into the area following the departure of the previous
atmospheric river will push out through the day on Sunday. Do not
expect this wave to generate significant precipitation totals as
compared to the previous few systems. Behind it, lingering showers
for parts of the outer coast as low and mid-level westerly flow
steer Pacific moisture into the coastal mountains.

Ridging will likely be able to rebuild through much of the Monday
through Tuesday timeframe. In the lower levels of the troposphere,
this will help cut off the supply of moisture responsible for the
lingering chances of rain in some parts of the area. Consequently,
expect that the panhandle will see an opportunity to dry out,
especially in the southern half of the panhandle. Some offshore
flow may even yield breaks in the cloud cover for the area.

Unfortunately, the return of more pleasant weather will be short-
lived, as a disturbance races up from the Alaska Peninsula into
the area. There continue to be some significant timing differences
from run-to-run, and even between the ensembles, but all agree
that something will race up to impact the area between Tuesday and
Wednesday. Notably enough, given the plume of moisture that looks
likely to accompany it, think this system could bring a
respectable amount of rain to the area. Given the timing
differences, am currently leaning towards the less aggressive ends
for QPF totals, but this may need to be changed going forward.

&&

.HYDROLOGY...The Situk near Yakutat leveled out around 72.4 ft,
which is solidly above minor flood stage of 71.5 ft. A Flood
Advisory is in effect for the Situk. With the additional rainfall
expected today, we will extend the flood advisory to be in effect
until 6am AKDT Saturday morning. The Taiya near Skagway is above
moderate flood stage (17ft) at 17.67 ft. A Flood Warning is in
effect for the Taiya until 1pm AKDT Saturday. Juneau area rivers
have really started to respond to the heavy rain overnight with
Montana Creek and the Mendenhall entering action stage. The
Mendenhall is forecast to rise above minor flood stage today. A
Flood Advisory is currently in effect until 4pm AKDT Saturday for
the Juneau area streams. A Flood Watch remains in effect for areas
around the Chilkat, Salmon at Gustavus, and other streams in the
northern Panhandle. The Salmon at Gustavus as well as the
Mendenhall River and Lake in particular will need to be watched
today as gauge data is showing the river nearing action stage.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Fog Advisory until 10 AM AKDT this morning for AKZ027.
     Areal Flood Advisory until 4 PM AKDT Saturday for AKZ025.
     Flood Watch through this evening for AKZ019>023.
     Areal Flood Warning until 1 PM AKDT Saturday for AKZ018.
     Areal Flood Advisory until 6 AM AKDT Saturday for AKZ017.
MARINE...Small Craft Advisory for PKZ012-013-022-031>035-041>043-051>053.

&&

BFL/GFS

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