Diagnostic verification of the Climate Prediction Center long-lead outlooks, 1995-98

Abstract

The performance of the Climate Prediction Center's long-lead forecasts for the period 1995-98 is assessed through a diagnostic verification, which involves examination of the full joint frequency distributions of the forecasts and the corresponding observations. The most striking results of the verifications are the strong cool and dry biases of the outlooks. These seem clearly related to the 1995-98 period being warmer and wetter than the 1961-90 climatological base period. This bias results in the ranked probability score indicating very low skill for both temperature and precipitation forecasts at all leads. However, the temperature forecasts at all leads, and the precipitation forecasts for leads up to a few months, exhibit very substantial resolution: low (high) forecast probabilities are consistently associated with lower (higher) than average relative frequency of event occurrence, even though these relative frequencies are substantially different (because of the unconditional biases) from the forecast probabilities. Conditional biases, related to systematic under- or overconfidence on the part of the forecasters, are also evident in some circumstances.