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May precipitation increases snowpack and water supply forecasts

DENVER — The month of May produced an impressive 239 percent of average precipitation across the mountains of Colorado, a welcome change from the two previous months of below average precipitation. This precipitation combined with cool temperatures at high elevations allowed for a prolongation of the snowpack during a time of year when snowmelt is often already well under way. “This substantial addition of moisture, both in the form of snow and rain have notably increased water supply forecasts across the state from a month ago,” comments Brian Domonkos, Colorado Snow Survey Supervisor. While this additional moisture helped to increase water supply for the upcoming summer statewide, it was a particularly welcome addition to the basins of south central and southwest Colorado. The Upper Rio Grande and combined Sam Miguel, Dolores, Animas, and San Juan basins received well over three times the normal amount of May precipitation, at 329 and 349 percent of average, respectively. These amounts were the

highest on record for most SNOTEL sites across the San Juan Mountains in the month of May. Snowpack in the Arkansas River basin is above normal at 282% of the median. Precipitation for May was 254% of average which brings water year-to-date precipitation up to 109% of average. Reservoir storage at the end of May was 108% of average compared to 56% last year. Current streamflow forecasts range from 122% of average for Grape Creek at Westcliffe to 77% of average the Cucharas River near La Veta (Jun-Jul). While the snowpacks in these basins never reached their median peak values these late additions to the snowpack will help provide notably more water to the area, and for longer into the summer than previously expected. While the exceptionally wet May was great news for many parts of Colorado from a water supply standpoint, there is a flip side, particularly in areas with the most snow remaining. Having a substantial amount of snow in the mountains as temperatures continue to rise, and the probability of rain on snow events, increases the risk of flooding resulting from accelerated snowmelt. These events depend on a variety of combined factors but are something to be aware of as a possibility in the coming weeks. Snow and precipitation aside, reservoir storage across the state remains very similar to last month statewide, at 107 percent of average, with most basins showing only modest changes from last month. The Arkansas basin reservoirs were the main exception to this, showing a very large increase from 79 to 108 percent of average over the last month. ­Streamflow forecasts, however, are a different story. While there was a net increase from last month across the state the largest increases were seen in the basins of the northern Front Range, which already had the most plentiful forecasts in the state (not to mention the entire Western U.S.), even further widening the gap between those and forecasts in the Upper Rio Grande and combined San Miguel, Dolores, Animas, and San Juan basins of Southwest Colorado.