Click here to read the report. Here’s an excerpt:
Statewide Water Supply Conditions
￼Snowmelt in the mountains of Colorado is in full swing and now more than half of Colorado’s SNOTEL sites no longer retain snow. In April, statewide snowpack had peaked and it appeared runoff was on an early trajectory. Fortunately, premature runoff was slowed in all of Colorado’s basins by a cool wet May weather pattern with some considerable snowstorms. This increased snowpack at upper and middle mountain elevations and allowed some watersheds to reach greater snowpack peaks. Later peaks were achieved in basins such as the South Platte and Arkansas. May 2016 precipitation, while only half of last year’s accumulation, was integral to the preservation of snowpack of the tributaries that drain the San Juan Mountains. Future streamflow projections do vary across the state. In the Rio Grande and combined San Miguel, Dolores, Animas and San Juan basins forecasts are below average, while conversely, both the North and South Platte basins do have above normal forecasts. At the beginning of June, year to date precipitation, snowpack and reservoir storage are all above normal statewide and have Colorado poised for a positive start to summer runoff.
Favorable mountain weather conditions during the first half of May continued to delay snowmelt at many SNOTEL sites. As of June 1st, many high elevation sites, especially along the northern half of the Continental Divide, continue to maintain snowpacks that are greater than half of their total accumulation for the season. All of Colorado’s major river basins, except the Rio Grande, have an above normal basin-wide snowpack and the statewide snowpack is 201 percent of the median. Overall snowpack trends for winter 2016 were split between the northern and southern basins. All basins had developed above normal snowpacks by January 1st, which were substantial enough to prevent snowpack amounts from dropping too far below normal during an especially dry February. However, warm temperatures and dry conditions persisted in the southern river basins through March, which decreased the snowpack to below normal levels on April 1st in the Arkansas, Rio Grande, and combined San Miguel, Dolores, Animas, and San Juan River (SMDASJ) basins. Snowy conditions returned in April, boosting normals for the Arkansas and parts of the SMDASJ river basins, but the snowpack continued to deplete for all but the highest elevation SNOTEL sites in the Rio Grande River basin, bringing it to 85 percent of normal on June 1st. Additionally, basin-wide snowfall amounts in the Gunnison, Rio Grande, and SMDASJ failed to reach typical peak snowpack amounts. Alternatively in the northern regions of the state, abundant snowfall fell during March through much of May, which has kept snowpack levels above normal this spring in the South Platte, Colorado, and combined Yampa, White, North Platte River basins. These basins reached peak accumulations above normal and continue to hold the most snow in the state.
Precipitation amounts varied widely across Colorado throughout the month of May but statewide ended up above normal, at 118 percent of average. Water year to date precipitation is slightly above normal as of June 1st, at 102 percent of average. The mountains of Southwest Colorado received the most May precipitation in the state relative to their normal amounts. The combined San Miguel, Dolores, Animas, and San Juan basins received 154 percent of average May precipitation and the Upper Rio Grande received 144 percent. The next highest precipitation amounts occurred in the Yampa, White, and North Platte basins of Northwest Colorado, which collectively received 135 percent of average May precipitation. The Gunnison and Arkansas basins received similar amounts of precipitation as they did in April, at 123 and 120 percent of average, respectively. The Colorado River basin received 107 percent of average May precipitation and the South Platte was the only basin below normal, at 90 percent. Across the basins water year to date precipitation varies but not widely compared to the most recent monthly values, with all major basins being near normal; ranging between 96 and 108 percent of average since October 1st. It is interesting to note that while May 2016 did have well above average precipitation across much of the state only one group of basins (Yampa, White, and North Platte) received much more than half of the precipitation that was received during the extremely wet May of 2015.
Percent of average statewide reservoir storage dropped slightly from the beginning of May but is still above normal levels, at 108%. The Gunnison and the Upper Rio Grande are the only basins in the state that currently have below average reservoir storage. The Gunnison is only slightly below, at 97 percent, but the Upper Rio Grande was already the lowest in the state and dropped an additional 12 percent from last month and is now at 79 percent of average reservoir storage. On the opposite end of the spectrum, the Arkansas currently has the highest value in the state at 116 percent of average. This is followed closely by the Yampa basin that is currently at full capacity, which is 114 percent of its average for the beginning of June. Reservoirs of the South Platte basin are currently storing 112 percent of average and 92 percent of capacity, even with Antero Reservoir being very low due to construction being done on the dam. The Upper Colorado and combined San Miguel, Dolores, Animas, and San Juan basins are both storing 110 percent of their average volumes for this time of year. There is however a notable difference in their current storage as a percent of capacity, with the basins of Southwest Colorado being at 96 percent of reservoir capacity while reservoirs in the Colorado basin are storing 82 percent of capacity.
Streamflow forecasts for the remainder of the runoff season continue to follow the same general trend that has persisted this winter since January: streamflows in the northern half of the state have a better outlook than those in the southern portion. Forecasts for the northern streams have largely creeped up or stayed the same each month, while forecasts for southern streams have mostly decreased since January. Due to a lingering snowpack and plentiful May precipitation, runoff volumes for streams in the South Platte, combined Yampa, White, and North Platte, and northern tributaries of the Colorado River basin are largely expected to exceed normal flows. Forecasts are highest for tributaries in the South Platte River basin, where all streams are predicted to have flows above normal, and most are expected to be greater than 110 percent above the average. Many streamflows in the Yampa, White, and North Platte basin are also forecast to be greater than 110 percent of average. The lowest streamflows are currently predicted for the Rio Grande and combined San Miguel, Dolores, Animas and San Juan River basins. The above average May precipitation slightly boosted streamflow forecasts for the April – July period, but this was not enough to make up for the lackluster snowpack experienced in much of these basins. There is a range of variability for streams in the southern basins, but most forecasts range from 60 to 85 percent of average. Forecasts for the Gunnison and Arkansas River basins are mostly predicted to be somewhat below normal, in the 75 to 100 percent of average range, with a few outliers exceeding normal runoff volumes.