Spring Flood Potential
The risk for spring flooding due to snowmelt is low across Idaho. Snowpack in the lower valleys has melted out and the remaining mid and high elevation snowpack is well below normal for this time of year with the exception of northern Idaho where snowpack percentages are near normal. Snowpack in most basins peaked early and is on pace to melt out several weeks earlier than normal.
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.
Water Supply
N
ational Weather Service water supply forecasts for April through September 2022 call for near normal runoff volumes for northern Idaho watersheds. Meanwhile, forecasts vary considerably across central and southern Idaho and are generally well below normal. Forecasts for the Snake River Headwaters near the Wyoming border and the Bear River in far southeast Idaho are generally 50 to 70 percent of normal. Volume forecasts for the Lost River Basins and Salmon River Basin are 60 to 80 percent of normal while forecasts for the Wood River Basins are 35 to 55 percent of normal. Forecasts for the Middle Snake River and the Boise, Payette, and Weiser Basins range from 50 to 75 percent of normal. Across south central and southwest Idaho near the Nevada border runoff volumes are only expected to be 40 to 45 percent of normal.
Temperature and Precipitation
In October, southern Idaho and the Panhandle Region had average to well above average precipitation while the Clearwater and Salmon River Basins in central Idaho were below average. November precipitation was average to above average across the north while southern Idaho received well below average precipitation. Autumn was relatively warm statewide, especially in November when temperatures were 3 to 6 degrees (F) above average. December moisture varied considerably across Idaho but the majority of the state received average or above average precipitation. December temperatures were on the cool side across northern Idaho and a little above average across southern Idaho.
January precipitation was below average across most of the state while temperatures were near average across northern Idaho and below average across the south. Northern regions of Idaho were a mix of below average and above average precipitation in February while record dryness occurred across central and southern Idaho. February was a cold month with below average temperatures throughout the state. March precipitation was below average with the exception of some of the Idaho Panhandle where precipitation was near average. March temperatures were average to below average across northern and eastern Idaho and a little above average elsewhere across the state. The three month period from January through March, 2022 was one of the driest on record for many locations across central and southern Idaho.
Snowpack
Mountain snowpack was well above normal across most of the state in early January. However, snowpack percentages trended down afterward and by the end of February had decreased by 50 to 60 percentage points across much of central and southern Idaho. As of April 6, mountain snowpack was well below normal with the exception of northern Idaho where snowpack percentages are near normal. April 6 snowpack percentages were the highest in the Spokane and Clearwater Basins at 99 and 91 percent of median, while the lowest was in the Owyhee Basin at only 48 percent of median. Idaho snowpack as a whole typically builds through March and peaks in early April.
Reservoirs
April 1 reservoir storage in northern Idaho, far southeast Idaho, on the Henrys Fork arm of the Upper Snake, the Payette Basin, and on the main stem Snake River was generally around normal. Elsewhere across central and southern Idaho, reservoir system storage ranged from 39 to 88 percent of normal. Reservoirs on the Snake River above Heise were at a combined 57 percent of normal. Magic Reservoir in south central Idaho was only at 39 percent of normal while Owyhee Reservoir was only 63 percent of normal. Weather patterns and irrigation demand will drive reservoir operations over the next several months. 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 river levels.
Drought
Drought continues to plague much of Idaho. Very dry conditions across much of central and southern Idaho from January through March have led to intensification of drought in some areas. Temperature and precipitation patterns for the remainder of spring and this summer will determine whether or not drought conditions improve or deteriorate. The U.S. Seasonal Drought Outlook indicates that drought is likely to persist across central and southern Idaho at least through the end of June.
Long Range Outlook
The outlook for April through June favors below normal temperatures across the Idaho Panhandle and above normal temperatures for most of southern Idaho, and equal chances of either below, above, or normal temperatures elsewhere. The precipitation outlook favors below normal precipitation across central and southern Idaho, and equal chances for either below, above, or normal precipitation across the Idaho Panhandle.
Online 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/
