Flood in Alaska, September 27 to 28, 2017

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
243 PM AKDT Wed Sep 27 2017

.SHORT TERM...Tonight through Friday...As of 1500 this afternoon,
a ridge of high pressure is positioned over western Canada, and a
deep trough is quasi stationary, spinning out in the western gulf
waters. Additional shortwaves have developed from the parent
trough and are helping to funnel a solid stream of moisture across
the SE Alaskan panhandle. In addition, a weather front, aligned
north to south, is currently pushing east through the panhandle,
giving us very high winds and producing some substantial
precipitation amounts across many locations. Expecting Montana
Creek near Juneau and Taiya River near Skagway to reach minor
flood stage late tonight and into tomorrow before quickly
receding. We're currently monitoring other river and stream gages
for the rest of the Panhandle for any indications of flooding
issues.

For Thursday, we're expecting the front to have finally passed
through the area, so we should see winds decrease and PoPs finally
diminish. Later in the day, a ridge of high pressure will then
begin building in over the eastern gulf, with its axis stretched
just offshore of the panhandle. This should help to initiate
stable conditions and offshore flow beginning late Thursday night
into early Friday morning. Due to the development of this ridge,
we may even witness some clearing sky conditions, and fog may
become a possibility during the Friday morning commute, as the
ground will be saturated after the recent rains. Winds will also
be light to encourage fog growth.

We used the GFS heavily for any major adjustments to the
forecast, especially for PoPs, and we used a combination of the
GFS and the NAM for updating the wind forecasts.

.LONG TERM.../Friday through Wednesday/...Very good upper level
agreement aloft at the start of the long range forecast period.
Main feature at the end of the week will be a very broad upper low
that spans the gulf and extends south into the North Pacific.
Embedded features in the upper levels include a closed low west
of Haida Gwaii and multiple smaller upper circulations vicinity
Kodiak Island. Stepping forward in time, an upper trough north of
the Aleutians will dig south as it rotates around the base of the
large upper low. Models diverge by Saturday with the disposition
of this upper trough with NAM and ECMWF closing a circulation well
to the west while GFS and GEM close the upper low more over the
northeastern gulf. The upper low/trough is displaced southeast by
and upper ridge on Monday. Details vary considerably, but ECMWF,
GEM, and GFS are all depicting this. By Tuesday, EC and GFS are
developing an upper short wave over the southern interior and
track this across the top of the ridge. This will allow for
onshore flow and unsettled weather by mid week next week.

Surface reflections of the two main upper lows are clear in ECMWF,
though the Haida Gwaii feature is not nearly as obvious in GFS,
GEM, or NAM. Very strong surface high pressure builds in from the
northwest Pacific and Bering Sea, spreading across much of the
interior and northwest Canada. Like the situation aloft, the
details vary significantly, but by Monday, all are depicting high
pressure centers southwest of the gulf and over the Yukon/Alaska
border. Over southeast Alaska, an inverted trough will set up
with its axis aligned along the outer coast. This often translates
into better than average weather for the Panhandle in terms of
precipitation and this is reflected in the current long range
forecast. By Tuesday, a surface low associated with the upper
short wave discussed above will move east over either the far
northern gulf or across the southern coast, bringing a front and
increasing the chance of rain by the middle of next week.

Main changes to the extended range forecast were to decrease PoP
Friday and again Sunday night through Monday night. Sky cover was
also diminished for the same time frame.

&&

.AVIATION...Low IFR to IFR conditions for central and northern
Panhandle airports with first wave of rain bands tracked over
through the afternoon. Increasing rain rains and lowering ceilings
as frontal boundary, currently orientated N-s along the Panhandle
coast moves inland. Wind shear drops off this evening following
the track of 825 mb llj as it pulls to the east. Frontal passage
overnight with decreasing winds, rain, winds and improving
ceilings and visibility by the morning. Southern locations will be
last to see these improving conditions.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Areal Flood Advisory until 7 AM AKDT Thursday for AKZ025.
     Strong Wind until 4 PM AKDT this afternoon for AKZ022-023-025.
     Areal Flood Advisory until 4 AM AKDT Thursday for AKZ018.
MARINE...Gale Warning for PKZ041>043.
     Small Craft Advisory for PKZ011>013-021-022-031>036-051>053.

&&

Voveris/Fritsch

Visit us at http://www.weather.gov/Juneau

Special Weather Statement
National Weather Service Juneau AK
1234 PM AKDT Wed Sep 27 2017

AKZ017-019>024-026>029-280700-
Cape Fairweather to Cape Suckling Coastal Area-
Haines Borough and Lynn Canal-Glacier Bay-
Eastern Chichagof Island-
Salisbury Sound to Cape Fairweather Coastal Area-
Cape Decision to Salisbury Sound Coastal Area-
Eastern Baranof Island and Southern Admiralty Island-
Inner Channels from Kupreanof Island to Etolin Island-
Dixon Entrance to Cape Decision Coastal Area-
Southern Inner Channels-Misty Fjords-
Including the cities of Yakutat, Haines, Gustavus, Hoonah,
Elfin Cove, Pelican, Sitka, Port Alexander, Angoon, Petersburg,
Wrangell, Kake, Craig, Klawock, Ketchikan, Metlakatla, and Hyder
1234 PM AKDT Wed Sep 27 2017

...HEAVY RAIN WILL CONTINUE TO AFFECT SOUTHEAST ALASKA
WEDNESDAY NIGHT...

Rainfall amounts of 2 to almost 5 inches have fallen over the
last 24 hours as rain bands moved over the AK Panhandle. Heavier
rain bands shifted to the north Wednesday morning as frontal
boundary aligned from north to south along the Panhandle coast.
This front will progress eastward through Wednesday night with
more heavy rain developing for most of the region. Additional
rainfall amounts of 1 to 3 inches are expected through early
Thursday morning. Rainfall rates will tapper off through Thursday
morning from north to south. Local rivers and streams will show
rises in response to the heavy rain but no flooding is expected
for these areas.

Along with the rain, gusty south winds of 30 to 40 mph will occur
across much of the Panhandle through this evening. Wind and rain
will make travel difficult.

This statement will be updated by 11 PM Wednesday or sooner if
conditions warrant.

AKZ025-280700-
Juneau Borough and Northern Admiralty Island-
Including the city of Juneau
1234 PM AKDT Wed Sep 27 2017

...SIGNIFICANT RISES EXPECTED ON AREA RIVERS AND STREAMS THROUGH
WEDNESDAY NIGHT...

Heavy rain has fallen over the Juneau area with 24 hour
accumulations upwards of 1.5 inches through Wednesday afternoon.
Rain will intensify again through the evening with additional rain
fall amounts around 2 inches expected. Montana Creek is forecast
to rise above minor flood stage, see Flood Advisory. Other local
rivers and streams such as Jordan Creek and Mendenhall Lake will
also show sharp rises but are expected to remain below bankfull.

Along with the rain, gusty south winds of 30 to 40 mph will occur
across much of the area through this evening. Wind and rain will
make travel difficult.

This statement will be updated by 11 PM Wednesday or sooner if
conditions warrant.

AKZ018-280700-
Taiya Inlet and Klondike Highway-
Including the city of Skagway
1234 PM AKDT Wed Sep 27 2017

...SIGNIFICANT RISES EXPECTED ON AREA RIVERS AND STREAMS THROUGH
WEDNESDAY NIGHT...

Heavy rain has fallen over the Skagway area with 24 hour
accumulations upwards of 1.25 inches through Wednesday afternoon.
Rain will intensify again through the evening with additional rain
fall amounts around 1 inch expected. The Taiya River is forecast
to rise above minor flood stage, see Flood Advisory. Other local
rivers and streams will also show sharp rises but are expected to
remain below bankfull.

Along with the rain, gusty south winds of 30 to 40 mph will occur
across much of the area through this evening. Wind and rain will
make travel difficult.

This statement will be updated by 11 PM Wednesday or sooner if
conditions warrant.