San Juan Basin

Observed flows in the San Juan basin were much above average during the April-July period. Early in the forecast season, during February and March, streamflow also was much above average. By April and May, streamflow had dropped to near 85% of average due to lack of snowmelt because of colder than normal temperatures. Monthly precipitation during the snow accumulation season never fell below average. For January through May, precipitation was reported at 135%, 120%, 170%, 105%, and 190% of normal. On March 1, the average basin-wide snowpack was the lowest for the season at 110% of normal. By April 1, the snowpack increased to 125%, one month later, to 140%, and, by June 1, to 315% of normal.

Most of the January through April forecasts underestimated observed flows. January forecasts started at above the average flow for all points. Furthermore, as the season progressed, forecasts either remained nearly the same or continued to increase. These forecasts represented the most probable or 50% exceedance probability estimates. For many points, observed flows exceeded even the reasonable maximum forecasts (only 10% chance of exceedance.) The May forecasts were improved over previous forecasts, although the reasonable maximum forecasts for a few stations still were just at or slightly below the final observed flows. Early forecasts did not account for the unusual late spring conditions of colder than normal temperatures and much above average precipitation that combined to create much above average flows. Given these known conditions, June 1 forecasts were the most accurate.