Home » April 2011 Southwest Climate Outlook » Precipitation Verification
Forecast Verification
Published April 27, 2011

Precipitation Verification

 (May 2011–October 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 May–July to forecasts issued in April for the same period suggest that forecasts are slightly more accurate than equal chances in all of Arizona and New Mexico (Figure 16a). While all regions have positive Rank Probability Skill Score (PRSS) values, they are very low, indicating that the historical accuracy of the forecasts have only been marginally better than equal chances. For the June–August period, forecasts for both states have historically been no more accurate than equal chances; most RPSS values are close to zero (Figure 16b). For the July–September period, which covers the monsoon, forecasts have less accurate than equal chances, especially in regions most influenced by the monsoon, such as southwest New Mexico and southeast Arizona (Figure 16c). This implies that the current forecasts for this period may not be a useful tool for decision-making. For the four-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 16d). Regions with bluish hues suggest that the NOAA–Climate Prediction Center (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.

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.

Related Links: