Flood Potential Outlook
Issued by NWS Portland, OR

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Water Supply Outlook
National Weather Service Portland OR
4 PM PST Friday May 1 2020

...OREGON WATER SUPPLY AND SPRING FLOOD OUTLOOK AS OF MAY 1, 2020...

The water supply forecast for the spring and summer of 2020 is below-
average for most of Oregon, except for near- or above-average for
northeast Oregon basins. Water supply forecasts are particularly low
for basins in central, south-central, and southwest Oregon. Water
supply forecasts decreased by 5 to 20 percent, relative to normal,
for most locations due to dry conditions in April. Note that there
remains some potential for spring flooding in northeast Oregon,
particularly the Grande Ronde basin. Any flooding would likely be
caused by a combination of snowmelt and rainfall. Spring flooding is
very unlikely for all other areas east of the Cascades, and spring
snowmelt flooding has historically not occurred west of the Cascades.

The May outlook from the Climate Prediction Center highlights the
enhanced likelihood of above-average temperatures, with equal
chances of near, above, or below average precipitation for most of
the state. However, the first half of the month looks generally warm
and dry for the state.  For more information on monthly and seasonal
outlooks, visit cpc.ncep.noaa.gov.

Refer to the sections below and links provided for details regarding
snowpack, precipitation, seasonal climate outlooks, reservoirs,
streamflow, and water supply forecasts.

The next update to this outlook will be issued by June 4, 2020.

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

Precipitation for the 2020 water year thus far (Oct 1, 2019 through
Apr 30, 2020) ranges from 40 to 80 percent of average for all of
Oregon, except for 98 percent for the Grande Ronde basin. April
precipitation was very low statewide, with many observing stations
reporting the month as one of the driest Aprils on record.

April temperatures were slightly above average, ranging from normal
to +2.0 degrees relative to average.

Additional Information:

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

AHPS Precipitation:
https://water.weather.gov/precip/index.php?location_type=state&locati
on_name=OR Western Regional Climate Center West-Wide Drought
Tracker: https://wrcc.dri.edu/wwdt/index.php?region=or

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

As of May 1, mountain snowpack is melting rapidly across the state,
with many SNOTEL monitoring stations reporting no snow. Peak
snowpack this year occurred the first week of April for most snow
monitoring stations. The highest snowpack, in terms of percent of
average, was in northeast Oregon, where it was 100 to 120 percent of
average. For the north and central Cascades and mountains in east-
central and southeast Oregon, snowpack was 90 to 100 percent of
average. For the south Cascades and Siskyous, snowpack was about 80
percent of average.  With dry and sunny conditions for most of
April, the snowpack has melted quickly, which could mean more rapid
declines to summer baseflow for many streams that are typically fed
by late spring snowmelt.

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 Temperature Outlook

The Climate Prediction Center produces monthly and seasonal
outlooks, in which there is a weighing of the odds of near-normal,
above-normal, or below-normal for temperatures and precipitation.

The May outlook from the Climate Prediction Center highlights the
likelihood of above-average temperatures across the state. The April
precipitation outlook also shows near, above, or below-average
conditions for most of the state, except for enhanced below-average
chances for southeast Oregon. That said, the first half of May is
likely to be mostly dry across Oregon.

The June through August outlook indicates an enhanced likelihood of
above-average temperatures and below-average precipitation
statewide, which would only increase drought concerns for much of
Oregon.

Visit www.cpc.ncep.noaa.gov for more about seasonal outlooks.

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

Storage for most irrigation reservoirs in central and eastern Oregon
as of May 1 ranges from 80 to 100 percent of capacity, with the
exceptions of Ochoco at 50 percent, Philips at 42 percent, Wickiup
at 63 percent, and Cold Springs at 49 percent of capacity. Storage
for southwest Oregon reservoirs, where reservoir inflow has been
well below average, ranges from 30 to 70 percent of capacity.

Corps of Engineers flood control reservoirs in western Oregon are
refilling slower than the spring refill plan due to dry spring
conditions and are at 74 percent of capacity as of May 1. It appears
likely that most Corps reservoirs in the Willamette basin will not
fill to summer full pool levels.

Owyhee Reservoir, the largest irrigation project in the state, has
storage of 597,000 acre-feet, 83 percent of capacity, as of early
May. This is a 1 percent decrease from a month ago.

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 runoff so far this water year is much below-average for
most of the state and particularly low for central and southwest
Oregon rivers, where water-year runoff ranges from 30 to 60 percent
of average. At the other extreme, water year runoff is above average
for far-northeast Oregon rivers, ranging from 100 to 130 percent of
average, with much of that runoff occurring during February
flooding. For the rest of the state, runoff ranges from 50 to 90
percent for the water year.

April streamflow was near average for rivers fed by snowmelt across
the state. However, for much of western Oregon, and particularly
southwest Oregon, April streamflow was low, with some streams in the
driest areas near record-lows for this time of year.

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.

=============================================================
Water Supply Seasonal Forecasts

Water supply forecasts for April-September runoff volume vary widely
across the state but are mostly below-average. The main exceptions
are northeast Oregon basins, ranging from 90 to 120 percent of
average. Northwest Oregon basins are a little below-average, ranging
from 60 to 90 percent. Basins in southern and central Oregon are
well below-average, ranging from 30 to 70 percent. The dry
conditions in April resulted in significant declines and increased
certainty of below-average forecast volumes for many watersheds.

The forecast for the Columbia River at The Dalles, which is a good
index of conditions across the Columbia Basin, is 96 percent of
average for April-September, no change from a month ago.

Details on basin-scale water supply forecasts:

NOAA/NWS Northwest River Forecast Center: www.nwrfc.noaa.gov/ws/
NOAA/NWS California-Nevada River Forecast Center:  USDA Natural
Resources Conservation Service www.wcc.nrcs.usda.gov/wsf/

$$

Bryant



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