Abstract
A form of the critical success index (CSI) is used by the National Weather Service to indicate the value of warnings. This verification statistic assumes that the times when an event was neither expected nor observed are of no consequence. It can be shown that the CSI is not an unbiased indicator of forecast skill but is proportional to the frequency of the event being forecast. This innate bias is demonstrated theoretically and via example. An unbiased verification statistic appropriate for forecast of rare events is presented and applied to severe convective weather warnings. Comparisons of this score to the CSI show the extent of the penalty the CSI extracts from forecasters who work in areas that are not climatically prone to given events.
