Flood Potential Outlook
Issued by NWS Boise, ID

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FGUS75 KBOI 101210

Water Supply Outlook
National Weather Service Boise ID
507 AM MST Sat Mar 10 2018


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

Good soil moisture recharge from autumn rains and well above average
reservoir storage has resulted in a slightly elevated threat of
spring flooding along the mainstem Snake River in eastern Idaho, and
along smaller tributaries above Idaho Falls. Spring flood risk is
also slightly elevated in the Northern Panhandle Region which is
currently holding some of the highest snowpack percentages in the
state. Elsewhere, the absence of low elevation snow and average to
below average snowpack at higher elevations suggests an average or
below average spring flood threat. One thing to remember is that
snowpack in Idaho generally peaks in early April, leaving a few more
weeks to potentially add to our snowpack and change the flood

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 vary from 88 to 137 percent of average for most of the
Central Mountains north across the Panhandle. In southern Idaho,
forecasts for the Big Lost Basin, the mainstem Snake River and
tributaries above American Falls range from 78 to 123 percent of
average, with the exception of Willow Creek near Ririe which is 66
percent of average. Volume forecasts for the rest of southern Idaho
range from 45 to 107 percent of average with the lowest percentages
in southwest Idaho. These forecasts may change considerably over the
next month or two since seasonal snow accumulation and rainfall
typically occur during March and April.


As of March 6, Water Year precipitation was above average for the
Panhandle, Spokane and Clearwater Basins. The Clearwater 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
around 100 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 a few areas at 50 to 60 percent of


As of March 6, the highest snowpack percentages in the state ranged
from 112 to 125 percent of median near the Wyoming border, Little
Lost and Birch Basins, the Clearwater, Spokane and Northern
Panhandle Region. The lowest snowpack percentages were 43 to 65
percent of median in southwest Idaho along the Nevada border. For the
remainder of central and southern Idaho, snowpack percentages ranged
from 71 percent of median in the Weiser Basin to 107 percent of
median in the Salmon Basin. Mountain snowpack in Idaho typically
builds through March, so early April snow conditions will be pivotal
to water supply conditions through the summer.


Reservoir storage across Idaho is in good shape overall. As of March
1, storage in major reservoir systems from the Panhandle south
through the Clearwater Basin were at 85 to 90 percent of average.
Across the southern half of the state, with the exception of
Brownlee at 84 percent and Mann Creek at 54 percent of average, most
major projects had well above average storage which is great news
considering the below average snowpack in many basins. Weather
patterns, irrigation demand, and flood control needs will drive
reservoir operations over the next several months. Wet spring
weather or extended periods of above normal temperatures resulting
in rapid snowmelt could result in significant increases in reservoir
outflows and river levels.


After record setting precipitation and snowpack last year,
abnormally dry conditions have returned to portions of west central
and southern Idaho due to lagging winter precipitation. Weather and
precipitation through this spring will determine whether or not
conditions improve or deteriorate for areas experiencing short term
dryness. Good reservoir carryover will help ease drought concerns
for those served by major storage projects.

Long Range Outlook

The outlook through the rest of March favors near normal to below
normal temperatures and above normal precipitation for the state.
The seasonal outlook for April through June leans toward warmer and
drier than normal conditions near the Nevada and Utah border. For
the rest of the state, neither above normal or below normal
temperature and precipitation is favored during the April though
June period.

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

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

Reservoir Storage Bureau of Reclamation

USDA-Natural Resources Conservation Service

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

Colorado Basin River Forecast Center

Temperature and Precipitation Outlook Climate Prediction Center



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