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Forecast Verification
Published August 25, 2010

Precipitation Verification

 (September 2010–February 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 September–November to forecasts issued in August for the same period suggest that forecasts have been slightly more accurate than forecasting equal chances in southeast Arizona (i.e., 33 percent chance that rain will be above-, below-, or near-average). This largely reflects the area of Arizona most influenced by the monsoon (Figure 16a). Currently, the NOAA–Climate Prediction Center (CPC) forecasts slightly below-average precipitation for the southern region. This forecast has never been issued for this period for southern New Mexico, reflected in the black “no data” color. Outside of southeast Arizona, forecast skill—a measure of the accuracy of the forecast—is similar to an equal chances forecast. For the October–December period, forecasts have been better than equal chances only in southeast Arizona, while forecasts have been less accurate than equal chances in northern Arizona and all of New Mexico (Figure 16b). As the year progresses into the winter, forecast skill is either less accurate than equal chances or only a slight improvement upon an equal chances forecast in most of New Mexico and northern Arizona (Figures 16c–d).

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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