Click here to read the report. Here’s the introduction:
Summary
Unlike last year, the month of March continued to bring snow to Colorado. Unfortunately the state’s snowpack, as of April 1, only showed a nominal increase from last month’s report. Beneficial storms and relatively cool temperatures helped maintain the snowpack in the mountains but were not able to bring it closer to normal conditions. This marks the fourth consecutive month of below average snowpack for the state as well as the third consecutive month of below average precipitation in the mountains. Forecasts for spring and summer streamflows are well below average across the entire state with many streams expected to see volumes that are below 50 percent of average. We are also still feeling the ill effects of the previous year’s paltry snowpack and the resulting streamflow runoff, in our reservoirs. The majority of the state’s major river basins are reporting well below average reservoir storage. Judicious use of our existing supplies will be critical in minimizing impacts this season and there is always the potential for unexpected late season snowfall and above average spring precipitation to help ease impacts.
Snowpack
Overall the snowpack percentage for the state inched up during March. Current readings are now are 74 percent of median; up 1 percentage point from the report on March 1. On the bright side, the current snowpack is 130 percent of last year’s April 1 snowpack report. By this time last year the statewide snowpack had already peaked and was melting rapidly. This year snow continued to accumulate in the mountains during March; however the storms were more focused on the northern mountains and the Front Range and missed the southwest portion of the state. While the snowpack percentages did increase in most basins, not all watersheds showed improvement, and some saw a substantial decline. The Upper Rio Grande basin dropped 9 percentage points from the March 1 readings and the combined San Juan, Animas, Dolores, and San Miguel basins lost 10 percentage points. The lowest snowpack percentages in the state occur in the Upper Rio Grande and the South Platte basins which are both at just 69 percent of median. The state’s best snowpack percentage occurs in the North Platte basin which is at 81 percent of median; the Yampa and White basins are close behind with a snowpack at 78 percent of median. All basins currently have snowpacks that are better than those reported last year at this time; but comparing bad to worse could be considered a moot point.
Precipitation
Precipitation totals for the month of March, measured at SNOTEL sites across the state, were mostly below average. Statewide precipitation for the month was just 76 percent of average. The South Platte and the Colorado basins actually recorded precipitation in the mountains that was close to average. The South Platte reported totals at 97 percent of average and the Colorado was at 96 percent of average. The driest basins this month were the Upper Rio Grande and combined San Juan, Animas, Dolores, and San Miguel basins with precipitation totals at only 47 percent and 56 percent of average respectively. Precipitation in the remaining basins ranged from 84 percent of average in the Yampa, White and North Platte basins to 63 percent of average in the Arkansas. Water year to date totals range from only 66 percent of average in the Upper Rio Grande basin, to 78 percent of average in the Yampa, White and North Platte basins. Water year to date precipitation for the state is at 73 percent of average.
Reservoir Storage
Statewide reservoir storage was reported to be 71 percent of average at the end of March. The combined Yampa and White River basins boast the highest storage amounts in the state, currently storing 105 percent of average volumes for this date; though the basin has the fewest reservoirs and lowest capacity for storage. Storage volumes in the other major basins range from 84 percent of average in the South Platte basin, to 54 percent of average in the Upper Rio Grande basin. Statewide storage volumes are way below where they were this time last year. Last year at this time, the reservoirs in the state were storing 3,651,000 acre-feet of water which was 108 percent of the average volumes. This year the reservoirs are storing 2,421,000 acre-feet of water for agricultural and municipal use this season.
Streamflow
Colorado’s water users can anticipate very low streamflow volumes this summer. Due to extremely poor snowpack conditions spring and summer streamflow volumes may approach the minimum volumes on record in some areas. The projected inflow into Dillon Reservoir calls for 66 percent of average flows; forecasts for some areas of Colorado River basin are even lower. Clear Creek at Golden is expected to flow at 72 percent of average from April to July and the forecast for the Big Thompson River is only 52 percent of average. The Gunnison and Upper Rio Grande River basins have some of the lowest forecasts in the state. Tomichi Creek at Gunnison is only expected to see volumes that are 34 percent of average and Sangre de Cristo Creek’s forecast calls for flows that are 29 percent of average. All of these forecasts assume normal precipitation amounts throughout the forecast period.
Special Note on Interpreting Forecasts
According to the National Water and Climate Center (NWCC), “a water supply forecast is a prediction of streamflow volume that will flow past a point on a stream during a specified season, typically in the spring and summer. These forecasts are given not as a single number, but as a range of numbers to reflect risk and forecast uncertainty. Each month, five forecasts are issued for each forecast point and each forecast period. Unless otherwise specified, all forecasts are for streamflow volumes that would occur naturally without any upstream influences.”
The forecasts we typically emphasize in this report are the 50 percent exceedance probability forecasts because they are in the middle of the range of forecasts with 50 percent chance that actual volumes will be above or below the predicted volume. The 50 percent exceedance forecasts assume that typical weather patterns will prevail into the forecast season. In a water year such as this one, when conditions have been anything but typical, it is important to pay attention to the other forecasts provided. If dry conditions prevail into the rest of this spring and summer it may be prudent to use the 70 or 90 percent exceedance forecasts for management purposes this season. If we continue to receive snowfall late into the season or above average precipitation this spring, actual streamflow volumes may be more in line with the 50 or 30 percent exceedance forecasts.
Much below average spring and summer streamflow volumes are forecast throughout north central Colorado. twitter.com/NWSBoulder/sta…
— NWS Boulder (@NWSBoulder) April 5, 2013