Snowpack and streamflow outlook

BOZEMAN, Mont., May 6, 2019 – 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 towards 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 1st 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 1st – July 31st 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.”

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 (October 1st – September 30th) 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 1st 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 5th business day of the month: https://www.nrcs.usda.gov/wps/portal/nrcs/mt/snow/waterproducts/basin/

May 1, 2019, Snow Water Equivalent

River Basin % of Normal % Last Year Columbia 101 61 Kootenai, Montana 89 60 Flathead, Montana 97 55 Upper Clark Fork 108 62 Bitterroot 111 67 Lower Clark Fork 106 70 Missouri 110 75 Jefferson 102 74 Madison 115 90 Gallatin 115 78 Headwaters Mainstem 118 65 Smith-Judith-Musselshell 117 84 Sun-Teton-Marias 103 53 St. Mary-Milk 90 60 Yellowstone River Basin 102 69 Upper Yellowstone 116 68 Lower Yellowstone 90 72 West of the Divide 101 61 East of the Divide 104 71 Montana State-Wide 106 67