Flood Potential Outlook
Issued by NWS Boise, ID

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Water Supply Outlook
National Weather Service Boise ID
1030 AM MDT Fri Apr 14 2017

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

The spring flood risk due to snowmelt is above average to well above
average across most of southern Idaho, with areas of greatest
concern being the Central Mountains and southeast Idaho. For
northern Idaho, the spring flood risk is generally above average.

Mid and high elevation snowpack remains exceptional across much of
southern Idaho. In fact, snowpack is at record or near record levels
across portions of the Central Mountains. Meanwhile, early April
snowpack is around normal across northern Idaho. Wet conditions
through the winter and early spring have left soils very moist and
streamflows running high, which sets the stage for above average
flood risk across the majority of the state through the spring
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 scenario. Additionally, 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 across southern Idaho vary from 100 to 260 percent of
average. However, volume forecasts for the majority of southern
Idaho locations exceed 150 percent of normal and several rank within
the top 5 based on historical record from 1971 to 2016. Across the
northern half of Idaho, volume forecast percentages are generally
100 to 150 percent of average for the April through September
period. Mountain snowpack in Idaho typically peaks in early April,
and these forecasts will fluctuate some over the next couple of
months based on spring precipitation.

Precipitation and Temperature
As of April 11, Water Year precipitation was well above normal
across most of Idaho. Percentages were highest across the Panhandle,
Central Mountains, south-central and southeast Idaho, where 150 to
300 percent of normal precipitation has fallen since October 1,
2016. Lowest percentages in the state were generally across west-
central and far southwest Idaho at 110 to 150 percent of normal.
Average temperatures have been a little cooler than normal across
northern and west-central Idaho. Elsewhere across the state,
temperatures have generally been average or a little above average
for the Water Year.

Snowpack
As of April 12, snowpack across the southern half of Idaho was above
normal, with exceptional snowpack remaining across portions of the
Central Mountains and southeast Idaho. According to NRCS SNOTEL
data, portions of the Central Mountains were holding record or near
record amounts of high elevation snow. Basin snowpack percentages
ranged from 103 to 189 percent of median across southern Idaho, with
the Big Wood, Little Wood, and Big Lost Basins carrying the highest
percentages at 180 to 189 percent of median. Across northern Idaho,
snowpack percentages were around 100 percent of median. Mountain
snowpack for Idaho as a whole typically peaks in early April.

Reservoirs
Basin-wide reservoir summaries as of April 1 vary from near average
storage to well above average storage across Idaho. Large inflows on
the Owyhee System in February and March boosted reservoir levels
such that water was being released through Owyhee Dam`s Glory Ring
Gate. The last couple of times this occurred were in 2011, and prior
to that it was 2006. Weather patterns, irrigation demand, and flood
control operations will drive reservoir operations over the next few
months. With the large snowpack remaining across much of southern
Idaho, above average reservoir outflows and high river levels are a
good bet through the spring.

Drought
Abundant autumn rain and good winter snowpack have erased drought
conditions across the state according to the U.S. Drought Monitor.
Weather and precipitation through the spring will determine whether
or not conditions continue to improve before heading into the warm
and dry season. The U.S. Seasonal Drought Outlook suggests that
drought conditions are not likely to return to Idaho through the
spring.

Long Range Outlook
The outlook for April, May, and June indicates equal chances of
above normal, normal, or below normal temperatures across Idaho,
except for extreme southeast Idaho where above normal temperatures
are slightly favored. Probabilities slightly favor above normal
precipitation during the period for areas of Idaho near the Montana
border, with equal chance of above normal, normal, or below normal
precipitation elsewhere.

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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