Flood Potential Outlook
Issued by NWS Portland, OR

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Water Supply Outlook
National Weather Service Portland OR
1155 PM PDT Wednesday April 5 2017

...OREGON WATER SUPPLY AND SPRING FLOOD POTENTIAL OUTLOOK AS OF
APRIL 5TH 2017...

The water supply forecast for the spring and summer of 2017 is
generally above-average across the state.  April through September
runoff-volume forecasts range from 95 to 200 percent of average,
with above-average forecasts for most of Oregon, except near-average
forecasts for some rivers in northwest and northeast Oregon. Several
rivers in central and eastern Oregon had early-spring flooding in
March due to much-above-average precipitation and some lower and mid
elevation snowmelt over the past several weeks. Given the remaining
mountain snowpack statewide and continued active storm-track
affecting the Pacific Northwest, the potential for additional spring
flooding is above-average for much of Oregon east of the Cascades.
There remains heightened concern for flood potential in the Canyon
Creek basin near John Day in central Oregon. Historically, the
frequency of spring flooding is low, but when it does occur it
typically involves a combination of snowmelt and rainfall runoff, as
was the case in March.

Snowpack as of April 4th remains above-average in higher elevations
statewide, with basin snowpack ranging from 100 to 150 percent of
average. Precipitation in February and March brought a very soggy
end to an already-wet winter. For much of western Oregon, it was the
wettest combined February-March on record.

Significant changes in water supply forecasts are possible through
May. High-elevation snow accumulation is possible through April, and
significant rainfall is possible through May. The April 2017 outlook
by the Climate Prediction Center calls for increased likelihood of
above-average precipitation and below-average temperatures for at
least the first half of April, with continued wet conditions likely
through the end of April. For May through July, there is a slightly-
enhanced chance of above-average temperatures for Oregon, with equal
chances of near, above, or below-average precipitation across the
state. For more details, visit www.cpc.ncep.noaa.gov.

Refer to the sections below and links provided for details regarding
snowpack, precipitation, reservoir conditions, and water supply
forecasts for individual basins.

The Oregon water supply outlook will be updated early each month
through June. Look for the next update by May 3rd.

============================================================
Snowpack across Oregon

Oregon snowpack ranges from 100 to 150 percent of average as of
April 4th. In the Cascades, there is still significant snowpack
above 2500 feet north and 3500 feet south. In central and eastern
Oregon, most snow-monitoring stations below 5000 feet had
significant melt in late March, with a mix of melt and additional
accumulation at higher elevations. Any periods of above-average
temperatures in April would likely result in rapid snowmelt,
especially in central and eastern Oregon.

Peak seasonal snowpack, in terms of water content, typically occurs
between mid March and mid April.

Additional snowpack information:

NOAA National Weather Service - Northwest River Forecast Center
www.nwrfc.noaa.gov/snow/

USDA Natural Resources Conservation Service
www.nrcs.usda.gov/wps/portal/nrcs/main/or/snow/

============================================================
Precipitation and Temperatures across Oregon

Precipitation for the 2017 water year thus far (Oct 1, 2016 through
March 31, 2017) ranges from 120 to 160 percent of average in Oregon.
Adding to what has already been a wet winter, combined February-
March precipitation was much-above average, with record amounts in
parts of western Oregon. March temperatures were above-average in
central and eastern Oregon and near-average in western Oregon.

Details on precipitation and temperatures:

NOAA National Weather Service - Northwest River Forecast Center
www.nwrfc.noaa.gov/water_supply/wy_summary/wy_summary.php

NOAA NWS - California-Nevada River Forecast Center (Klamath basin)
www.cnrfc.noaa.gov/water_resources_update.php

============================================================
Reservoirs

Reservoir storage increased greatly in March. As of early April,
most irrigation reservoirs around the state are 85 to 95 percent
full, and nearly all reservoirs are expected to fill to summer full-
pool levels by late May. Note that Owyhee Reservoir, Oregon`s
largest irrigation reservoir, is filled to 97 percent of capacity.

Reservoir data is provided by the Natural Resources Conservation
Service, the Bureau of Reclamation, and the US Army Corps of
Engineers.

Additional reservoir information:

www.wcc.nrcs.usda.gov/basin.html
www.usbr.gov/pn/hydromet/select.html
www.nwd-wc.usace.army.mil/nwp/teacup/willamette/

============================================================
Observed Streamflow

Observed streamflow in March 2017 was much-above-average for nearly
all Oregon rivers. The combination of above-average temperatures and
precipitation resulted in very high streamflow and some periods of
flooding along several rivers in central and eastern Oregon.

Visit waterwatch.usgs.gov for details on observed streamflow. Water
year and monthly runoff data is available at www.nwrfc.noaa.gov for
several locations in Oregon.

============================================================
Forecast Streamflow and Seasonal Runoff Volumes

Forecasts for April-September runoff volume range from 90 to 200
percent of average, with the highest values generally in southern
and eastern Oregon and the lowest values in northwest Oregon.
Seasonal forecasts have trended upward over the past two months and
still could change significantly based on temperatures and
precipitation in April and May.

The April 4th forecast for the Columbia River at The Dalles, which
is a good index of conditions across the Columbia Basin, is 120
percent of average for April-September, an increase of 13 percent
from the forecast on March 1st.

Details on basin-scale water supply forecasts:

NOAA National Weather Service - Northwest River Forecast Center
www.nwrfc.noaa.gov/ws/

USDA Natural Resources Conservation Service
www.wcc.nrcs.usda.gov/wsf/

============================================================
Spring Flood Potential

It`s already been an active season of early-spring flooding for
several rivers in central and eastern Oregon. With the remaining mid
and high elevation snowpack in Oregon mountains and continued active
storm-track expected through much of April, the potential for
additional spring flooding is higher than usual for central and
eastern Oregon. Also note there remains heightened concern for flood
potential in the Canyon Creek basin near John Day in central Oregon
due to the impacts of the 2015 fire. Historically, the frequency of
spring flooding is low, but when it does occur, it typically
involves a combination of snowmelt and rainfall runoff. Keep an eye
on temperature, precipitation, and streamflow forecasts at
www.nwrfc.noaa.gov through the spring and early-summer.

Bryant
$$



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