Flood Potential Outlook
Issued by NWS Portland, OR

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Water Supply Outlook
National Weather Service Portland OR
625 PM PDT Thursday May 3 2018

...OREGON WATER SUPPLY AND SPRING FLOOD POTENTIAL OUTLOOK AS OF
MAY 3RD 2018...

The water supply forecast for the spring and summer of 2018 is below-
average for much of southwest, central and southeast Oregon and near-
average for the Willamette basin and most coastal and northeast
Oregon watersheds. April through September runoff-volume forecasts
range from about 20 to 130 percent of average, lowest in southeast
and south-central Oregon and highest in northwest Oregon. The
potential for spring flooding in central and eastern Oregon is low,
due to the below-average mountain snowpack and relatively-dry
conditions this spring.

Speaking of Oregon snowpack, it is actively melting as of early May.
The seasonal snowpack peaked in March at about 80 percent of average
statewide but was only 40 to 60 percent of average for most of
southern Oregon and the John Day basin in northeast Oregon. Snow has
completely melted at most SNOTEL monitoring stations below 4000 feet
in the north and 6000 feet in the south. Higher-elevation locations
will see snowmelt continue through May.

Expect relatively dry and warm conditions for the first half of May,
especially for southern Oregon. The May 2018 outlook by the Climate
Prediction Center calls for a enhanced likelihood of above-average
temperatures and below-average precipitation, with similar
expectations for the summer months, June through August. For
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 next update will be issued by June 5, 2018.

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

The seasonal snowpack peaked at about 80 percent of average
statewide but was notably low in the mountains of southern Oregon
and in the John Day basin in northeast Oregon. Seasonal snowpack was
near-average in the north Oregon Cascades, the Wallowas, and
northern portions of the Blue Mountains.

Snow is melting fairly rapidly around the state as of early May, and
significant snowpack only remains above about 4000 feet north to
6000 feet south.

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 2018 water year thus far (Oct 1, 2017 through
May 3, 2018) ranges from 70 to 100 percent of average in Oregon,
highest in northwest and far-northeast Oregon and lowest in south-
central Oregon. Temperatures were notably above-average November
through January, below-average February and March, and near average
in April.

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 as of early May is generally near average for
northwest, central, and eastern Oregon and below-average in
southwest and south-central Oregon. Most reservoir statewide are 75
to 100 percent of capacity, except for southwest Oregon reservoirs
at 50 to 80 percent of capacity. Storage at Owyhee, the largest
irrigation reservoir in the state, is 80 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 April 2018 was below-average in southwest and
south-central Oregon and average to above-average for much of
western and northern Oregon. Rises in many rivers were driven by a
combination of snowmelt and periods of heavy rain.

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 widely around the
state. Most western and far-northeast Oregon basins are between 80
and 130 percent of average, while forecasts for much of central and
southern Oregon are much lower, generally 40 to 60 percent of
average but as low as 20 percent, primarily due to the low mountain
snowpack and dry conditions for much of southern Oregon this spring.

The forecast for the Columbia River at The Dalles, which is a good
index of conditions across the Columbia Basin, is 119 percent of
average for April-September, basically unchanged from a month ago.
This forecast for The Dalles reflects above-average snowpack in
northern Washington and the Rocky Mountain portion of the Columbia
River basin.

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

The potential for flooding along rivers in central and eastern
Oregon is low due to the below-average snowpack in most of the
state. Historically, the frequency of spring flooding is low, but
when it does occur, it typically involves a combination of snowmelt
and rainfall runoff. Snowmelt-driven spring flooding doesn`t occur
in western Oregon. Stay tuned to snowpack conditions and streamflow
forecasts through the spring at www.nwrfc.noaa.gov.

Bryant
$$



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