Montana snowpack rapidly melted in May

Throughout May, warm temperatures across Montana led to a rapid melt off of the state’s snowpack, which sits “largely below 50% of median,” as of June 1.

That’s according to the U.S. Department of Agriculture’s Natural Resources Conservation Service, which released its June Water Supply Outlook Report last week. 

“Late spring snowpack percentages rarely tell the whole story,” said Florence Miller, USDA Natural Resources Conservation Service Hydrologist, in a press release. “When evaluating a season’s snowpack, peak snowpack accumulation and snowmelt timing provide a more comprehensive picture.”

Even though some higher elevation sites, including those that feed into the Flathead, Clark Fork and Bitterroot basins, saw near or above normal snowpack accumulation for the season, “snowpack accumulation peaked and transitioned to melt in mid-March, about a month earlier than usual.”

The NRCS report shows that snowpack is around 55% to 70% of the 30-year median in the northwest, but at or below 50% of median in the rest of the state for the start of June, following the rapid melting in May. 

Of the 145 SNOTEL sites that track data for Montana’s river basins, 111 have melted out with 66 experiencing the earliest or second earliest complete melt out on record.

The snowmelt plays a key role in agriculture, hydropower production, water storage, aquatic ecosystem health and recreation such as boating and fishing, as well as flood and drought forecasting.

The St. Mary basin, which stretches into Alberta, shows the highest snowpack percent of normal — 72% — followed by the upper Clark Fork at 69%, the Flathead basin at 66% and the Lower Clark Fork at 56%. 

Due to big storms last fall at the start of the water year, and late-spring precipitation, the state is showing overall precipitation on par with normal values, but both early, and late-season storms occurred in warmer conditions, leading to less snow to act as a buffer for summer streamflows. 

In the final water supply outlook report for the season, forecasts show that stream volume across the state is unlikely to exceed the 30-year median in most rivers, though a high-precipitation summer could help bolster the water supply. 

“Continued June precipitation could help improve these forecasts, though in most regions east of the divide, full recovery to normal streamflows is unlikely,” Miller said. “Additionally, with snowmelt occurring earlier than normal, runoff volumes are likely to be shifted earlier in the season”

The NRCS forecasts streamflows in a range which accounts for uncertainty in the forecast and unknown future precipitation in the summer. Forecatss are described under typical summer conditions, and graphics produced by the NRCS shiw how wetter or drier conditions affecr the forecast on the edges of the scale.

Starting in the state’s northwest corner, streamflow forecasts for the Clark Fork and Kootenai basins are predicted to be near normal; while the Flathead and St. Mary basins are expected to be around 80%- 90% of normal streamflow. 

Moving southeast across the state, forecasts get more grim — rapid snowmelt in the Bitterroot shows forecasts around 60% of normal, while the Jefferson, Madison, Gallatin, Smith and Musselshell basins are forecast to have 70% or less than normal streamflow, according to the report. 

The Upper Yellowstone basin is forecasting 70% – 80% of normal, while the Bighorn and Lower Yellowstone are predicting 60% to 70% of normal. 

An exception is the higher elevation watersheds in the Absaroka-Beartooth and Wind River ranges, which are forecasting 75%-100% of normal, due to retaining better snowpack with cooler alpine conditions this spring. 

Late May precipitation, especially in the northwestern part of the state, has helped fill some reservoirs close to capacity, including Flathead Lake, Hungry Horse Lake, and Mystic Lake Reservoirs showing above 110 to as much as 125% of median for the end of May. Lake Sherburne Reservoir also reported well above normal reservoir levels at 150%. 

On June 2, Energy Keepers Inc., the Confederated Salish and Kootenai Tribes company that operates Se̓liš Ksanka Qĺispe̓ (SKQ) Dam on the south end of Flathead Lake, reported the lake reached its full-pool volume of 2,893 feet. 

The lake’s volume has been the subject of scrutiny in recent low-water years, as it has at times dropped below levels that allow for regular recreational activities including boating, leading to state and federal officials to seek a “path forward” to keep the lake full in future drought condition years.