Colorado Water Supply Outlook Report - February 1, 2018

The general patterns of snowpack, precipitation, reservoir storage, and streamflow forecasts observed in the first three months of water year 2018 largely continued through the month of January. Southern Colorado has remained abnormally dry and continues to hold well below normal snowpack, which is also reflected in low streamflow forecasts. Snowpack is 31 percent of normal in the Upper Rio Grande basin and only 34 percent in the combined San Miguel, Dolores, Animas, and San Juan basins to the west. Conversely, snowpack in the North and South Platte basins is 82 and 80 percent of normal, respectively, and they hold several forecasts for near average streamflows, but currently no rivers in Colorado are forecasted to have above average summer flows. While snowpack still lags well below normal in southern Colorado, January did bring the most precipitation of any month of the water year to date, which did help in some capacity. Statewide, mountain precipitation in January was 70 percent of average with the South Platte receiving the most, at 85 percent, and the Upper Rio Grande the least at 50 percent of average. This leaves statewide water year-to-date precipitation at 62 percent of normal and snowpack at 59 percent, a slight increase over a month ago. Despite below normal snowpack and streamflow forecasts across the state, the good news is that every major basin in Colorado is still maintaining above average reservoir storage volumes, which may become critical this summer. Statewide reservoir storage remained unchanged from a month ago and resides at 115 percent of average.

The storm track failed to deliver the moisture laden storms that Colorado was hoping for last month. The mountains across the state continue to hold a below normal snowpack, making the possibility of achieving a normal snowpack peak a dwindling prospect. There continues to be a strong gradient in snowpack conditions deteriorating north to south across the state, with the North and South Platte River basins having the most favorable conditions at 82 and 80 percent of median, respectively. However, these two basins both experienced a drop compared to normal conditions since last month after monthly snowpack accumulations in January were below normal, except at a handful of SNOTEL sites. Colorado’s remaining major river basins all experienced a slight boost to percent of normal snowpack, however these improvements are little consolation given the grim snowpack present in the southern river basins. The combined San Miguel, Dolores, Animas, and San Juan River basins are currently at 34 percent of median and the Upper Rio Grande is at 31 percent. In these two basins, more than half of the snow measurement sites (snow courses and SNOTEL) are at or near record low levels. The mountains are already about two-thirds of the way through a typical snowpack accumulation season, so impressive snowfall that is considerably above average is needed to overcome the snow drought that dominated these basins for the first half of the water year. Snowpack amounts in the Gunnison and Arkansas River basins are only slightly better, at 48 and 55 percent of median respectively. The Upper Colorado and combined Yampa and White River basins are in the middle of the range of the current snowpack values across the state, and are at 73 and 70 percent of median respectively. Statewide, snowpack is at a dismal 59 percent of median. A major shift in precipitation patterns is badly needed to bring the state’s snowpack anywhere close to normal levels.

January precipitation amounts were an improvement over the previous monthly accumulations in many of Colorado’s major river basins, however precipitation for the month was below normal across the entire state. Additionally, water year-to-date precipitation is no longer above normal for any basin. The northern basins received precipitation accumulations in January that were nearest to normal amounts and these basins have the highest water year precipitation with respect to normal. The South Platte, combined Yampa, White, and North Platte, and Colorado River basins all received between 80 and 85 percent of average monthly precipitation. The January accumulations resulted in a minimal change to the water year-to-date precipitation for the Colorado and combined Yampa, White, and North Platte River basins, which are at 76 and 77 percent of average respectively. Water year-to-date precipitation fell slightly in the South Platte, but the basin remains near normal at 98 percent of average for the water year. The Gunnison received precipitation that was 73 percent of average for January, which while still below average, boosted the water year precipitation in the basin from 36 percent to 46 percent of average. The Arkansas and combined San Miguel, Dolores, Animas, and San Juan River basins were at 58 and 55 percent of average for the month, while the Upper Rio Grande River basin experienced the driest conditions in the state and received only 50 percent of average January precipitation. The combined San Miguel, Dolores, Animas, and San Juan River basins continue to have the lowest water-year-to date precipitation in the state. Average SNOTEL precipitation for these basins is only 4 inches for the water year, which is 29 percent of average. Statewide, January precipitation was 70 percent of average, while the water year precipitation rose slightly to 62 percent of average.

Reservoir levels have fluctuated minimally throughout water year 2018 so far, relative to normal amounts. Statewide reservoir storage was 115 percent of average as of February 1st, the same value as of a month ago. Storage across the major basins has shown change but generally nothing too substantial and all basins remain at above average levels. The most significant declines over the last four months have been in the Arkansas basin, but these reservoirs still have the highest storage in the state, at 140 percent of average. On the low end, the Gunnison and combined San Miguel, Dolores, Animas, and San Juan basins are holding 104 and 105 percent of average storage, respectively. Storage is 109 percent of normal in the South Platte River basin and 116 percent in the Upper Colorado. The Upper Rio Grande River basin continues to hold well above average reservoir storage, at 123 percent, the same as last month. Given the very low snowpack levels in the Rio Grande this is an especially important year to have good reservoir storage, as it has been below average since 2002. The combined Yampa, White, and North Platte River basins lie slightly lower than the Rio Grande relative to normal, holding 120 percent of their average storage values. Given the current low snowpack and streamflow forecasts across the state, particularly in southern Colorado, these above average reservoir volumes may become a critical resource this summer.

February 1st streamflow forecasts vary widely across the state, largely following the trend of a well below average snowpack in southern Colorado and closer to normal values in the northern Front Range. While there are many streams in the Colorado River headwaters and the northern portion of the South Platte basin forecast to have near average streamflows this summer, there are currently no points in the state forecasted to have above average streamflow at the 50% chance of exceedance level. On the high end, several streams in the South Platte and Colorado River basins are forecast to have 91 percent of average April-July flows, but other forecasts across these basins also range to as low as 59 percent of normal. Across the combined Yampa and White River basins, forecasts range from a low of 48 percent on Elkhead Creek to a high of 70 percent on the Elk River near Milner. Forecasted streamflow volumes generally continue to get lower and lower moving towards the southwest corner of the state. The average of streamflow forecasts across the Gunnison basin is for 55 percent of normal volumes. Forecasts for the rivers and streams of the Rio Grande basin have not changed dramatically from a month ago and currently average out to around 50 percent of normal, but with many substantially lower. The combined San Miguel, Dolores, Animas, and San Juan basins of far southwest Colorado currently have the most consistently low streamflow forecasts in the state. The Dolores, Animas, and San Juan rivers are all forecast to have 43-45 percent of normal flows and the total range of forecasts in these combined basins is 33-56 percent. Forecasts in the Arkansas basin vary the most of any in the state, from a high of 80 percent of average for the Arkansas River at Salida to a low of 20 percent for Grape Creek at Westcliffe.