Figure 13. Precipitation Verification (November 2009–April 2010)
Source: NOAA-Climate Prediction Center (CPC)

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Comparisons of observed precipitation for November–January to forecasts issued in October for the one-month lead time covering the same period suggest that forecast skill—a measure of the accuracy of the forecast—is only slightly better than equal chances for most of Arizona and New Mexico, and worse than equal chances in northern New Mexico (Figure 13a). Forecasts issued in October for the two-month lead time (the December–February period), however, have been better than equal chances in all of the Southwest, with the most accurate forecasts corresponding to southeast Arizona and southwest New Mexico (Figures 13b). For the three-month forecasts covering January–March, historical forecasts have been most accurate in southeast Arizona and southwest New Mexico (Figure 13c). The four-month lead forecasts spanning February to April have displayed skill in all regions except northern New Mexico (Figure 13d). At all lead times in Arizona, October forecasts have displayed the highest skill in southeast Arizona (Figures 13a–d). NOAA–Climate Predictions Center (NOAA–CPC) forecasts have historically been more accurate than equal chances. However, caution is advised to users of the NOAA–CPC seasonal outlooks for regions where the verification maps display reddish hues.

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.

More Information:
For more information on the Forecast Evaluation Tool, visit:
http://fet.hwr.arizona.edu/ForecastEvaluationTool/

For a CLIMAS publication that explains how to use the Forecast Evaluation Tool, visit:
http://www.climas.arizona.edu/forecasts/articles/FET_Nov2005.pdf