Flood Potential Outlook
Issued by NWS Boise, ID

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Water Supply Outlook
National Weather Service Boise ID
330 PM MST Tue Feb 13 2018

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

The potential for spring flooding due to snowmelt is slightly
elevated for portions of eastern and north central 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 Clearwater Basin which is currently
holding one of the greatest snowpack percentages in the state.
Elsewhere, the absence of low elevation snow and areas of below
average mid elevation snow suggest an average or below average
spring flood threat.

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,
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 90 to 135 percent of average for most of the
Central Mountains north across the Clearwater and Panhandle regions.
In southern Idaho, forecasts for the Big Lost Basin, the mainstem
Snake River and tributaries above American Falls range from 80 to
125 percent of average, with the exception of Willow Creek near
Ririe with a forecast of 38 percent of average. Forecasts for the
rest of southern Idaho range from 39 to 73 percent of average with
the lowest percentages in southwest Idaho. These forecasts may
change considerably over the next couple of months since seasonal
snow accumulation and rainfall typically occur during February,
March, and April.

Precipitation

As of February 12, Water Year precipitation was near normal or above
normal for the Panhandle, Spokane, Clearwater, and Salmon Basins, as
well as the Upper Snake Basin near the Wyoming border. The
Clearwater Basin had the greatest anomalies at 130 to 150 percent of
average. Aside from the Snake River headwaters region, Water Year
precipitation across southern Idaho stood at 60 to 80 percent of
average with south side Snake River Basins having the lowest
percentages.

Snowpack

As of February 13, the highest snowpack percentages in the state
were 113 and 115 percent of median in the Clearwater Basin and the
Upper Snake above Palisades. Percentages were 89 to 105 percent of
median for basins along the Montana border in eastern Idaho, and the
Salmon, Spokane, and Panhandle Basins. Elsewhere in south central
and southeast Idaho the snowpack was 56 to 80 percent of median,
dropping to 35 to 48 percent of median in the Owyhee and Bruneau
Basins in southwest Idaho. Daily snowpack readings indicate record
low levels for a handful of SNOTEL locations in southern Idaho.
Mountain snowpack in Idaho typically builds through March, so early
April snow conditions will be pivotal to water supply conditions
through the summer.


Reservoirs

Reservoir storage across Idaho is in good shape. Major reservoir
systems across the northern half of the state were holding near
average or above average storage as of February 1. Across the
southern half of the state, with the exception of Brownlee at 85
percent and Mann Creek at 45 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.

Drought

After record setting precipitation and snowpack last year,
abnormally dry conditions have returned to portions of west central
and southern Idaho. Weather and precipitation for the remainder of
winter and 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 end of February favors below normal
temperatures, and normal or below normal precipitation for the
state. The outlook for March, April, and May suggests below normal
temperatures for the Panhandle, and normal or above normal
temperatures for the central and southern parts of the state. The
precipitation outlook for March, April, and May slightly favors
wetter than normal conditions for the Panhandle, but does not shift
the probabilities one way or another for the rest of the state.

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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