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Forecast Verification
Published November 23, 2010

Precipitation Verification

 (December 2010–May 2010)
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 December–February to forecasts issued in November for the same period suggest that forecasts in all regions of Arizona and northern New Mexico have not been substantially more accurate than an equal chances forecast  (Figure 14a). Forecast skill—a measure of the accuracy of the forecast—has been highest in southwest New Mexico for this period. Forecast skill for the two-month lead time shows improvements in accuracy on all of Arizona and most of western New Mexico (Figure 14b). For the three- and four-month lead times, forecasts in Arizona have been better than equal chances (Figures 14c–d). While bluish hues denote more accurate forecasts, caution is advised to users of the seasonal forecasts for regions with reddish colors.

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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