FGUS75 KBOI 052135 ESFBOI IDC001-003-005-007-009-011-013-015-017-019-021-023-025-027-029-031- 033-035-037-039-041-043-045-047-049-051-053-055-057-059-061-063-065- 067-069-071-073-075-077-079-081-083-085-087-070600- Water Supply Outlook National Weather Service Boise ID 235 PM MST THU MAR 5 2026 ...IDAHO SPRING FLOOD AND WATER RESOURCES OUTLOOK... Spring Flood Potential... The overall risk for spring flooding due to snowmelt is low across Idaho. Above normal temperatures caused much of our winter precipitation to fall as rain instead of snow, resulting in a snow drought. The lack of low and mid elevation snow has significantly reduced the threat of spring snowmelt flooding. However, some areas in eastern Idaho such as the Wood River Basins and Big Lost River Basin are holding a good mid and high elevation snowpack and the spring flood risk for these areas is about average. Idaho's snowpack as a whole typically continues building through March and peaks in early April, leaving several weeks ahead of us for additional snow accumulation and potential changes to the spring flood risk. 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. Temperature and Precipitation Summary... Average temperatures for the 2026 Water Year have been above normal across the entire state. The Water Year kicked off with near normal temperatures in October but the following months of November and December saw temperature anomalies of 5-10 degrees (F) above normal. The warm temperatures in November and December were critical for our mountain snowpack as much of the precipitation fell as rain instead of snow. Temperature anomalies in January and February weren't quite as extreme as the previous two months but still averaged around 5 degrees above normal for the majority of the state. Preliminary data indicates that some locations in Idaho are on track for their warmest winter on record. Water Year precipitation through the end of February has been normal to above normal for the majority of Idaho. The exception is far southwest Idaho, particularly the Owyhee Mountains where precipitation has only been 60-80 percent of normal. October precipitation was well above normal across southern Idaho with normal to below normal precipitation falling across northern Idaho. Much of the Idaho Panhandle saw well above normal precipitation in November while southern Idaho was quite dry. December brought abundant precipitation to the entire state as a number of very warm atmospheric rivers impacted the region. January was very dry throughout the state. A more active weather pattern in February resulted in well above normal precipitation across far southern Idaho with a mix of normal to below normal precipitation for other regions of Idaho. Snowpack... Snowpack percentages as of March 2nd were well below average for almost all basins in Idaho, generally ranging from 60 to 85 percent of average. The exceptions were across eastern Idaho in the Little Wood Basin, Big Lost and Little Lost Basins, and the Snake River above Heise where snowpack was 90 to 120 percent of average. The lowest snowpack percentages were in southern Idaho near the Nevada border ranging from 20 to 55 percent with the Owyhee Basin being the lowest. The low and mid elevation snowpack is extremely poor across much of the state resulting in a snow drought. This is due to above normal temperatures that caused much of our winter precipitation to fall as rain instead of snow. Snow levels during atmospheric river events in December ranged from 7 to 9 thousand feet at times. Idaho snowpack as a whole typically builds through March and peaks in early April. Reservoirs... The 2026 Water Year began with above average storage in SW Idaho and SE Oregon, though the Upper Snake River lagged behind. Unusual December rainfall significantly boosted storage in the Boise and Payette River systems. Smaller non-federal reservoirs in southern Idaho were generally holding well below normal storage as of early March. Weather patterns, irrigation demand, and flood risk management 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. Observed Streamflows... USGS streamflow conditions as of early March are a mix across the state. Many stream gauge sites have been running much above normal due to rainfall runoff over the winter. Meanwhile, there are some gauging stations showing well below normal streamflows, especially along the mainstem Snake River in southern Idaho. Drought... Drought conditions have improved overall since the start of the 2026 Water Year. However, snow drought has become a significant concern across the state and water supply forecasts continue to trend down. Snow drought has hit the Owyhee Basin in far southwest Idaho particularly hard where little to no snow exists, even at the high elevations. Temperature and precipitation patterns through the spring will determine whether or not drought conditions improve or deteriorate. Water Supply... National Weather Service water supply forecasts for April through September, 2026 indicate well below normal runoff volumes (50-80 percent) for the majority of forecast locations in Idaho. Exceptions are in portions of the Panhandle where the Kootenai River and Clark Fork at Cabinet Gorge are close to 100 percent of normal. In central and southern Idaho the Salmon River at Salmon, the Big Wood at Hailey, and Little Wood River near Carey are close to 100 percent of normal as well. The highest percentages are on the Big Lost River with forecasts indicating around 130-140 percent of normal. Seasonal Outlook... The outlook for the month of March indicates equal chances for either below normal, normal, or above normal temperatures for the northern half of Idaho while above normal temperatures are slightly favored across southern Idaho. The precipitation outlook for March indicates equal chances of either category throughout the state. The outlook for April through June, 2026 favors above normal temperatures across all but far northern Idaho where equal chances of either below normal, normal, or above normal temperatures exists. Odds favor below normal precipitation across all of Idaho during the period. On-line Resources... 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.nrcs.usda.gov/wps/portal/nrcs/main/id/snow Drought Conditions 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/lmap/lmap.php?interface=peakfp Temperature and Precipitation Outlook Climate Prediction Center www.cpc.ncep.noaa.gov/ $$