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Mountain snowpack, a key water storage system for states like Montana and Wyoming, will decline if winters continue to steadily warm.

April’s weather patterns took their toll on Montana’s mountain snowpack, according to data from the USDA’s Natural Resources Conservation Service (NRCS). Sunny and warm days overwhelmed cooler and wet periods during the month, causing snowmelt to begin ahead of schedule across the state.

“Snowpack peaked during the first week of April in many of the state’s river basins,” said Lucas Zukiewicz, NRCS water supply specialist. “This year’s peak was weeks earlier than average.”

At 14 measurement locations west of the Divide, the amount of snowmelt during April was the most on record. At 10 SNOTEL sites, April’s snowmelt was the second highest on record.

“When you look across the state, higher elevation snow measurement locations continue to have snowpack that is 45 to 95 percent of normal, while lower elevation sites melted by May 1,” Zukiewicz said.

This year’s seasonal peak snow water was 92 to 108 percent of normal in most basins west of the Divide and in southern and central portions of the state. However, in northern basins east of the Divide, snowpack peaked well below average. The Sun-Teton-Marias River basin peaked at 66 percent of normal this year, and is 34 percent of normal for May 1.

This is the second year in a row that the Sun-Teton-Marias River basin has received well below normal winter and spring snowfall, and this year’s snow totals were lower than last year’s. Water users in the basin should be prepared for well below average runoff from snowmelt this year. They should also expect to be more reliant on spring and summer precipitation for water.

“Throughout the winter, it has been difficult to answer questions about El Nino’s impact on our snowpack and spring runoff,” Zukiewicz said. “These questions have been hard to answer because we have experienced near to slightly above normal precipitation for the water year and snowpack totals peaked at near normal.”

The biggest El Nino impact has been the well above average to record-breaking temperatures this spring, which caused the snowpack to transition to melt one to three weeks earlier than normal. Forecasts from the Climate Prediction Center indicate the warm weather will persist, impacting the timing of snowmelt and flows in rivers and streams.

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The timing of runoff is critical to water users in the state. The mountain snowpack is the largest reservoir in the state, typically holding water into late spring and summer. This year’s early runoff means there will be less water in the system where reservoirs are not present, increasing the need for spring and summer precipitation. Fortunately, May and June are favored, with regards to precipitation east of the Divide, and weather patterns will be closely monitored across the state this spring and summer.

Streamflows west of the Divide rose sharply in response to the melt of the mountain snowpack during April, and east of the Divide basins saw increases in flow but smaller in magnitude. “Unsettled weather at the end of the month slowed melt in some locations, but warm temperatures at the beginning of May are causing streams to rise again,” Zukiewicz said. Snowmelt during April, entered the river systems by May 1, and reduced the remaining water available in the snowpack. As a result, streamflow forecasts have decreased from last month.

Basin-wide, streamflow forecasts for the May- July time period are 86 percent of average for the Columbia River Basin, 80 percent of average for the Missouri River basin, and 87 percent of average for the Yellowstone River basin. Forecasts are down, percentage-wise, from April 1. Water users should consult forecasts in the May 1, 2016, Water Supply Outlook Report for rivers of interest.

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