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Forecast Verification
Published May 26, 2011

Precipitation Verification

 (June 2011–November 2011)
Data Source(s): Forecast Evaluation Tool

For a thorough description of the interpretation of these maps, see the feature article, “Evaluating forecasts with the RPSS,” in the April 2009 issue of the Southwest Climate Outlook

Comparisons of observed precipitation for June–August to forecasts issued in May for the same period suggest that forecasts are slightly more accurate than equal chances in all of Arizona and New Mexico (Figure 17a). While most regions have positive Rank Probability Skill Score (PRSS) values, they are very low, indicating that the historical accuracy of the forecasts has been only marginally better than equal chances. For the July–September period, which covers the monsoon, forecasts have been less accurate than equal chances, especially in regions most influenced by the monsoon, such as southwest New Mexico and southeast Arizona (Figure 17b). This implies that the current forecasts for this period may not be a useful tool for decision making. For the three-month lead time, which also spans part of the monsoon period, forecasts have been similar to or less accurate than equal chances in both states (Figure 17c). The four-month lead time forecast has shown better accuracy than equal chances in Arizona
(Figure 17d). Regions with bluish hues suggest that the NOAA–Climate Prediction Center (CPC) forecasts historically have been more accurate than equal chances. However, caution is advised to users of the CPC seasonal outlooks for regions where the verification maps display reddish hues.

Precipitation Verification
Notes:

These maps evaluate the historical performance of the one- to four-month long-lead forecasts made by NOAA’s Climate Prediction Center (CPC). The maps convey the historical accuracy of the CPC forecasts in relation to the reference forecast, which assigns a 33 percent chance to the three CPC categories, “above,” “below,” and “neutral.”  These categories indicate whether conditions are predicted to be similar to the wettest, driest, or normal precipitation for 1971 to 2000. The maps are generated from the Forecast Evaluation Tool, which was developed by The University of Arizona in partnership with NOAA, NASA, NSF, and the University of California-Irvine. The maps display the Ranked Probability Skill Score (RPSS). The more the forecasts and actual weather match, the bluer the color. A bluish or reddish RPSS indicates the forecast is more accurate or less accurate, respectively, than assigning a 33 percent chance to each of the three CPC categories. The RPSS is calculated by comparing all the forecasts made since December 1994 for particular seasons and specified lead times to the actual weather of the season.

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