Rapid Snowmelt in May Results in an Early Melt Out Across Montana
June 14, 2023
"If it seems to you like the snow disappeared quickly this spring, you are not wrong," said Eric Larson, USDA Natural Resources Conservation Service (NRCS) Water Supply Specialist. Warmer than normal temperatures in May resulted in rapid snowmelt. SNOTEL stations across Montana melted out about 10-20 days earlier than normal.
During the first several days of May and again in mid-month, high mountain elevations in Montana reached 70 degrees. Outside of those dates, daily average temperatures were closer to normal but still warmer than normal. "Many SNOTEL stations across Montana experienced daily minimum temperatures above freezing for extended periods of time during May, which is what accelerated snowmelt to above normal rates," said Larson.
Quicker than normal snowmelt decreased snowpack percentages from May 1. "Snowpack percentages dropped from near normal or above normal to less than 50% of normal in nearly all Montana basins since May first," said Larson.
One exception was the region extending from the northern Kootenai to the Saint Mary River basin, which already had a below normal snowpack and saw further decreases last month. The other exception is part of southwest Montana, which had a near record snowpack along the Idaho border. Several SNOTEL stations in that area still have an above normal snowpack that is currently tilting the basin-wide snowpack up to about 50-70% of normal in the Gallatin, Madison, and Jefferson River basins. Almost 70% of NRCS snow monitoring stations measured on June 1 are snow free. For many, this is an earlier than normal melt out.
"Snowpack percentages represent the current snowpack on an individual day compared to the median value for that given day. They don't necessarily represent the entire snow year. For example, we had a relatively large snow year in much of Montana," said Larson. Snowpack percentages in central and southwest Montana along the Idaho border were 110-140% of normal through May 1. The rest of the region was within about 10% of normal most of the year except part of northwest Montana that lacked snow overall this year. Additionally, snow water equivalent peaks occurred at near normal levels on near normal dates this year in most locations. Snow water equivalent peaks are a strong indicator of overall snowmelt for the season, unfortunately, the bulk of that snowmelt occurred early this year.
Active snowmelt began over a month ago at all elevations in Montana. Mountain locations that held more snow than normal released a substantial amount of water last month and some rivers reported near record monthly streamflows. Rivers near Missoula, Butte, Helena, Bozeman, White Sulphur Springs, and Billings experienced total monthly streamflows in May that were in their 80th percentile or greater. "On May first the NRCS May-July water supply forecasts estimated streamflows to be within about 10% of normal or greater for most of Montana. Since some of the snowmelt that was expected to occur in June already occurred in May, the recently published June-July forecasts have reduced volumes from what they would have been if May streamflows were normal," said Larson.
June 1 seasonal water supply forecasts range from well above normal to well below normal in Montana. The Beaverhead, Ruby, Smith, Boulder (Jefferson drainage), Musselshell, and Madison rivers are expected to have streamflows over 120% of normal through September. Forecasts are lowest in northwest Montana and the northern Rocky Mountain Front where many streamflows are expected to be about 60-70% of normal. Some forecasts within that region, including all forks of the Flathead River, indicate streamflows will be less than 60% of normal through September. Most forecasts across the rest of Montana are expected to be about 80-90% of normal through September. "Regardless of location, above normal precipitation would be ideal in June before heading into the drier months of summer, especially given the rate at which the snowpack melted out this year," said Larson.
A full report of conditions on June 1 can be found in the monthly Water Supply Outlook Report available on the Montana Snow Survey website. In addition, real-time snow survey data can be found at nrcs.usda.gov/montana, scroll to Popular Topics and click Montana Snow Survey Program.