Flood Potential Outlook
Issued by NWS Portland, OR

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Water Supply Outlook
National Weather Service Portland OR
125 PM PDT Wednesday April 4 2018

...OREGON WATER SUPPLY AND SPRING FLOOD POTENTIAL OUTLOOK AS OF
APRIL 4TH 2018...

The water supply forecast for the spring and summer of 2018 is near-
average for western and far-northeast Oregon watersheds and below-
average for central, southern, and eastern Oregon. April through
September runoff-volume forecasts range from 20 to 130 percent of
average, lowest in south-central and southeast Oregon and highest in
western Oregon. The potential for spring flooding in central and
eastern Oregon is low, due to the below-average mountain snowpack.

Speaking of Oregon snowpack, it is notably low in the central and
southern Cascades, the Blue Mountains, and mountain ranges in
southeast Oregon. Snowpack is near-average in the northern Cascades
and Wallowa Mountains. Basin-average snow-water content has
increased 10 to 30 percent, in terms of percent of average, since
late January mainly due to the below-average temperatures in
February and March.

The April 2018 outlook by the Climate Prediction Center calls for a
enhanced likelihood of below-average temperatures through at least
the first half of the month, with enhanced likelihood of above-
average precipitation through the month. Looking at May through
July, there are enhanced chances of above-average temperatures and
below-average precipitation for Oregon. 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 next update will be issued by May 4, 2018.

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

As of early April, basin snowpack ranges from 45 to 90 percent of
average, in terms of snow-water content. Values are lowest in south-
central and southeast Oregon.

Although precipitation for the past couple months has been a little
below average for much of the state, temperatures have also been
below average, so most of the precipitation at higher elevations has
fallen as snow, leading to some modest increases in snow-water
content. Note that snowmelt has already started at lower and mid
elevations and will likely increase through April, especially for
the southern half of the state.

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
April 3, 2018) ranges from 60 to 95 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 but below-average February and March.

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 March is generally average to a little
above-average for this time of year and 60 to 100 percent of storage
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 March 2018 was generally near-average, except
for below-average in parts of southwest and south-central Oregon.
Some rivers saw minor rises due to spring snowmelt, along brief
responses to rainfall runoff.

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 120 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. These forecasts are updated daily and have generally been
trending upward since mid February, especially for coastal and
northern Oregon basins.

The 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 10 percent 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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