Trends in Severe Local Storm Watch Verification at the National Severe Storms Forecast Center

Abstract

Trends of tornado and severe thunderstorm watch verification for the period 1967?1990 are presented. Over the past 10 years the annual number of reported severe thunderstorm events has increased substantially. In comparison, the number of tornado events reported has remained relatively constant from year to year. During the period 1967 to 1990, the percentage of watches verifying has increased from 45% to 85%, while the probability of detection (POD) of severe local storm events has increased from about 0.35 to 0.50. Recent results show that the number of severe thunderstorm watches has steadily increased since 1985, the size of watches has slowly decreased, and lead time has slightly decreased. Yearly fluctuations in tornado watch verification statistics appear to be best related to the number of outbreak tornadoes. Data suggest the Severe Local Storm Unit (SELS) performance in forecasting outbreak episodes and strong/violent tornadoes is improving at a faster rate than in forecasting isolated tornadoes. Improving trends in severe thunderstorm watch verification for the period 1967 to 1990 are also documented. In general, the results suggest a gradual improvement in forecast performance since the late 1960s. Several factors that impact the verification scores are also identified and discussed.