Stage is set for spring, summer runoff

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BOZEMAN – After an erratic winter brought a little bit of everything weather-wise to the Treasure State, spring followed suit with another change-up. In stark contrast to March, which yielded record low precipitation in some parts of the state, April precipitation was above average at many mountain and valley locations in western Montana, except along the southwest Idaho border. An active storm pattern delivered most of this month’s precipitation during the first three weeks of April, falling as both rain and snow in mountain and valley locations.

“Some monitoring sites in valleys and mountains of the Bitterroot, Madison, Gallatin, and Upper Yellowstone River basins just experienced the ‘wettest’ April on record,” said Lucas Zukiewicz, who measures snowpack and makes streamflow forecasts for the USDA Natural Resources Conservation Service (NRCS).

Precipitation during April continued to build the mountain snowpack and added to the water contained within it at mid and high-elevation monitoring sites. Low elevations melted throughout the month, starting the seasonal runoff for this spring and summer and causing early rises in rivers and streams. Following the wet weather pattern, abundant sunshine and above average temperatures started the transition toward snowmelt at all but the highest elevations in the state, with many snowpack monitoring sites experiencing their seasonal peak sometime during April.

Peak snowpack, or maximum amount of water contained within the snow for the year, was below normal in the northwest river basins, and near to above normal in many central and southern river basins. “Snowpack continues to hold on at the high elevations in some areas and the cool weather during the end of the month continues to prolong the storage of water,” Zukiewicz said. “At some point, when we return to more seasonal weather patterns, we’ll start to see those elevations move.”

Basin-wide snowpack totals on May 1 are near to above normal for this date in all basins except the Kootenai and St. Mary River basins, where the snowpack is slightly below normal.

SEASONAL VOLUME forecasts issued for the May 1-July 31 period indicate near to above average volumes for the spring and summer in central and southern basins, with only a few outliers. Some forecasts in headwaters of the Beaverhead River indicate possibilities of below normal volumes this spring and summer. River basins in the northern part of the state, which have long-running deficits in precipitation totals going back to last summer, may receive below normal volumes of water this runoff season. “A wet May and June could help to offset the deficits snowpack and water-year precipitation we have in some areas but given the uncertainty in the weather patterns this winter and spring it’s a guessing game as to what will actually happen,” Zukiewicz said.

May and June are the last two “wet” months before summer weather patterns set in and are an important part of the total amount of water we get for any given year, Zukiewicz said. East of the Divide, precipitation during these months makes up nearly 20 to 25 percent of the water year total (Oct. 1–Sept. 30) at mountain locations.

“This uncertainty in future weather, and just how much precipitation we will receive, is why our forecasts are issued as a range of outcomes and not just one hard number,” he said. “Even though we now have a good idea of the potential snowmelt component to this year’s runoff, it seems like we’re always playing the waiting game.”

The last Water Supply Outlook Report for the 2019 season will be issued on June 1 and will summarize May precipitation and provide an update on summer streamflow prospects.

Monthly Water Supply Outlook Reports can be found at the website below after the fifth business day of the month: https://www.nrcs.usda.gov/wps/portal/nrcs/mt/snow/

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