Submitted to: Environmental and Water Resources Institute World Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: May 19, 2002
Publication Date: May 19, 2002
Interpretive Summary: Seasonal forecasts of precipitation are issued by NOAA's Climate Prediction Center and provide potentially useful information to support water resources planning and management. However, effective use requires assessment of forecast performance in terms immediately useful to managers. This study examines the performance of forecasts for drought conditions in the southwestern U.S. during the 1998-2000 La Niña conditions. Forecast and actual precipitation deficits were expressed as fractions of normal precipitation, and compared over 25 months for southeastern California, Arizona, New Mexico, and western and southern Texas. Season-long and region-wide precipitation deficits did occur during forecasts for such deficits. However, the magnitude of the deficits was under-forecast, the duration was over-forecast, and the timing of the start and end were not accurately predicted. Further, a regional surplus of precipitation during summer 1999 was not forecast at all. The results illustrate recent limitations in forecasts of droughts for the southwestern U.S. resulting from La Niña conditions.
Technical Abstract: Experimental climate forecasts for 3-month total precipitation are issued monthly by the NOAA Climate Prediction Center (CPC), for lead times from 0.5 to 12.5 months. Among these forecasts, the CPC Probability of Exceedance maps and graphs present information on expected shifts in the probability distribution of precipitation amounts, relative to climatological distributions for 102 forecast divisions covering the contiguous United States. These forecasts support a wide range of possible applications in water resource management. However, the forecast techniques are still undergoing rapid development, and forecasts over the past 6 years have been shown to vary in degree of boldness and skill, depending on forecast region and strength of ENSO conditions. In this assessment, the CPC Probability of Exceedance forecasts are examined during the La Niña episode that began in 1998, at the 0.5 month lead time, for 13 forecast divisions in the southwestern United States, covering southeastern California, Arizona, New Mexico, and western Texas. This represents a "best case" scenario of recent Probability of Exceedance forecasts, with relatively bold predictions of ENSO-associated dry conditions. Coherent, season-long and region-wide precipitation deficits did occur during forecasts for such deficits. The magnitude was under-forecast, the duration was over-forecast, and the timing of initiation and end was not well predicted. The regional surplus during summer 1999 was not forecast at all.