Spring Flood Potential
The risk for spring flooding due to snowmelt is near normal across most of Idaho. The exception is the Big Lost Basin and Wood River Basins where the risk is below normal due to well below normal snowpack and low soil moisture. Relatively warm winter temperatures and a rather dry February have limited the snowpack in the low elevations which significantly reduces the snowmelt flood risk during late winter and early spring.
The primary factors in the development of spring flooding are the occurrence of persistent above normal temperatures, and rain on snow precipitation events. Even if mainstem rivers do not reach flood stage, smaller creeks and streams can still overflow their banks. Under the right scenario, spring flooding is possible even for areas that have low snowpack. Additionally, wildfire burn scars can have a significant impact on local flood potential during spring snowmelt.
Water Supply
The water supply runoff volume forecasts for the spring and summer of 2020 are near average for northern Idaho watersheds. Meanwhile, forecasts vary considerably across central and southern Idaho. Forecasts for the Snake River headwaters region and along the Utah and Nevada border generally indicate near normal runoff volumes. Forecasts for the Middle Snake River and Central Mountain watersheds indicate below or well below normal runoff volumes. The lowest runoff volume forecasts are for the Big Wood and Little Wood basins, ranging from 33 to 55 percent of normal. Low streamflows across portions of the Central Mountains could lead to water supply concerns for those relying on natural flows. These forecasts may change considerably since seasonal snow accumulation and rainfall typically occur during March and April.
Temperature and Precipitation
A wet September closed out the 2019 Water Year, but very dry conditions dominated through the first quarter of the 2020 Water Year. Precipitation for October through December was only 40 to 70 percent of average for most of the state with the driest conditions across the Central Mountains and southern Idaho. The January storm track brought above normal precipitation to most of Idaho with the exception of the Wood River Basins and Big Lost Basin where precipitation was below normal. In February, the Clearwater and Lemhi Basins, the Salmon River Basin, and areas along the Wyoming border had near normal or above normal precipitation while the rest of the state was below normal. Water Year to date precipitation in general is normal to below normal for northern Idaho and areas along the Wyoming, Utah, and Nevada borders. For the Snake Plain and Central Mountains, Water Year precipitation is below or well below normal. After a cold October where mean temperatures were 5 to 10 degrees below normal for most of the state, above normal temperatures have dominated for the 2020 Water Year.
Snowpack
As of March 5, mountain snowpack was near normal for most of Idaho. The Boise River Basin was lagging at 84 percent of normal. The Big Wood and Little Wood River Basins and Big Lost River Basin had the lowest snowpack percentages ranging from 56 to 68 percent of normal. Idaho snowpack as a whole typically builds through March and peaks in early April.
Reservoirs
Reservoir storage across Idaho was in good shape as of early March with major reservoir systems holding near average or well above average water. Across southern Idaho, reservoir storage in the Bear River Basin and Southside Snake River Basins was 144 to 153 percent of average. Storage in West Central Basins, Wood and Lost Basins, and the Upper Snake River Basin ranged from 110 to 168 percent of average. Meanwhile, reservoir storage in the Clearwater and Panhandle Regions was 96 to 104 percent of average. 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 a drought free start to the 2020 Water Year, dry fall conditions followed by low snowpack led to the development of drought conditions across portions of the Central Idaho Mountains. Weather and precipitation for the remainder of winter and this spring will determine whether or not drought conditions improve or deteriorate.
Long Range Outlook
The outlook for March through May favors above normal temperatures across the south half of Idaho while chances for either below, above, or normal temperatures are equal across the north. The precipitation outlook slightly favors above normal precipitation along the Wyoming border, otherwise the probabilities are not shifted one way or another for the rest of the state.
On-line 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/
USDA-Natural Resources Conservation Service
www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/
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 Reservoir Storage
www.usbr.gov/pn/hydromet/select.html
USDA-Natural Resources Conservation Service
www.wcc.nrcs.usda.gov/basin.html
Drought Information…
U.S. Drought Portal
www.drought.gov
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/
