Flood Potential Outlook
Issued by NWS Boise, ID

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Water Supply Outlook
National Weather Service Boise ID
118 PM MDT Fri May 4 2018

...IDAHO SPRING FLOOD AND WATER RESOURCES OUTLOOK...

The potential for spring flooding due to snowmelt is elevated for
portions of eastern Idaho and northern Idaho. The spring flood risk
for the rest of the state is average or below average.

There remains an elevated risk of spring flooding across portions of
the Upper Snake Basin due to above average mountain snowpack. This
includes the mainstem Snake River above American Falls Reservoir and
the Henrys Fork near Rexburg. An elevated flood risk also exists
across the Panhandle Region where some of the highest snowpack
percentages in the state reside. Elsewhere across Idaho, early May
snowpack conditions suggest a low probability of spring flooding due
to snowmelt.

The primary factors in the development of spring flooding are the
occurrence of persistent above normal temperatures, and rain on snow
precipitation events. Even for areas that have low snowpack, spring
flooding is possible under the right conditions. Additionally,
wildfire burn scars can have a significant impact on local flood
potential during spring snowmelt.

Water Supply

National Weather Service April through September water supply volume
forecasts for northern and eastern Idaho, and the mainstem Snake
River across southern Idaho range from 115 to 150 percent of
average. Elsewhere, water supply forecasts are 85 to 110 percent of
average for most of the Central Mountains and only 30 to 70 percent
of average for south central and southwest Idaho. The lowest are in
the Bruneau and Owyhee River Basins at less than 40 percent of
average.

Temperature and Precipitation

As of May 1, temperatures for the 2018 Water Year have been average
or slightly below average across the northern half of the state
while most of southern Idaho has experienced a little above average
temperatures. Water Year precipitation was above average for the
Panhandle, Spokane, and Clearwater Basins. The Clearwater Basin had
the greatest anomalies in the state at 130 to 150 percent of
average. The Salmon Basin and Upper Snake Basin near the Wyoming
border were generally 100 to 130 percent of average. Elsewhere in
southern and southwest Idaho the Water Year precipitation was mostly
in the 70 to 90 percent of average range with pockets of around 60
percent.

Snowpack

As of May 2, the highest snowpack percentages in the state ranged
from 138 to 145 percent of median in the Clearwater, Spokane and
Northern Panhandle Region. Not far behind were basins in eastern
Idaho such as the Little Lost and Birch Basins, Henrys Fork, Teton,
and Snake Basin above Palisades at 120 to 130 percent. The Payette,
Boise, Salmon, Wood and Lost Basins ranged from 74 to 109 percent of
median. Southside Snake River Basins along the Nevada border were a
mixed bag ranging from a low of 17 percent of median in the Owyhee
Basin to a high of 86 percent of median in the Raft River Basin.
Northern Idaho Basins and those near the continental divide reached
their peak snowpack in mid to late April. The low elevation snow is
gone but melting of the high elevations will increase over the next
few weeks as temperatures warm.

Reservoirs

Reservoir storage across the Idaho is in good shape. As of May 1,
storage in major reservoir systems throughout Idaho was 100 percent
of average or greater, except where systems were heavily drafted to
make space for anticipated snowmelt runoff.  Weather patterns,
irrigation demand, and flood control needs will continue to drive
operations through late spring as reservoirs are topped off. Wet
spring weather or extended periods of above normal temperatures
resulting in rapid snowmelt and large reservoir inflows could result
in significant fluctuations in reservoir discharge and downstream
and river levels.

Drought

Idaho is currently free from any official drought classification
according to the U.S. Drought Monitor. However, below average
precipitation for the Water Year and poor snowpack has put much of
southern Idaho in the abnormally dry category. Weather and
precipitation for the remainder of spring will determine whether or
not conditions improve or deteriorate for these areas experiencing
dryness. Good reservoir storage will ease drought concerns for those
served by major storage projects.

Long Range and Seasonal Outlook

The outlook for May favors above normal temperatures across the
state. The May precipitation outlook favors above normal
precipitation for southeast Idaho and below normal precipitation for
the northwest half of the state. The seasonal outlook for June
through August favors above normal temperatures and below normal
precipitation.

Refer to the links provided below for the latest information on
water resources.

Water Supply Volume Forecasts National Weather Service-Northwest
River Forecast Center www.nwrfc.noaa.gov/ws/

National Weather Service-Colorado Basin River Forecast Center
www.cbrfc.noaa.gov/

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

National Weather Service-National Operational Hydrologic Remote
Sensing Center www.nohrsc.noaa.gov

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

Reservoir Storage Bureau of Reclamation
www.usbr.gov/pn/hydromet/select.html

USDA-Natural Resources Conservation Service
www.wcc.nrcs.usda.gov/basin.html

Drought Conditions U.S. Drought Portal www.drought.gov

U.S. Drought Monitor www.droughtmonitor.unl.edu

National Drought Mitigation Center www.drought.unl.edu/

Peak Flow Forecasts Northwest River Forecast Center
www.nwrfc.noaa.gov/peak/

Colorado Basin River Forecast Center
www.cbrfc.noaa.gov/rmap/peak/peaklist.php

Temperature and Precipitation Outlook Climate Prediction Center
www.cpc.ncep.noaa.gov/

$$

TL



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