Evaluation of Official Western U.S. Seasonal Water Supply Outlooks, 1922–2002

Abstract An analysis was conducted of almost 5000 operational seasonal streamflow forecast errors across the western United States. These forecasts are for 29 unregulated rivers with diversity in geography and climate. Deterministic evaluations revealed strong correspondence between observations and forecasts issued 1 April. Forecasts issued earlier in the season were more uncertain yet remained skillful. The average change in forecast performance between January and April was primarily linked to the climatological seasonal cycle of precipitation: regions with climatologically wet winters and dry springs (e.g., California) showed much more forecast improvement between January and April than did regions with dry winters and wet springs (e.g., western Great Plains, Colorado Front Range). Other climatological factors played a secondary role; for example, mixed rain–snow basins in the Pacific Northwest did not show as significant an improvement in skill versus lead time as might otherwise be expected. Mixed t...