Reflecting on a Decade of Formalized Tornado Emergencies

TL;DR

This study reviews a decade of formalized tornado emergencies, analyzing their issuance criteria, and suggests updates to improve forecaster adherence and warning effectiveness.

Contribution

It provides the first comprehensive analysis of tornado emergency issuance patterns and proposes enhancements to training based on empirical findings.

Findings

Most tornado emergencies did not follow the suggested VROT and STP criteria.

70% of tornado emergencies were issued for EF-3+ tornadoes.

Tornado emergencies covered over half of EF-4 tornadoes and many fatal events.

Abstract

In 1999 the NWS began using the phrase "tornado emergency" to denote tornado warnings for storms with the potential to cause rare, catastrophic damage. After years of informal usage, tornado emergencies were formally introduced to 46 weather forecasting offices in 2014 as part of the impact-based warning (IBW) program, with a nationwide rollout occurring over the following years. In concert with the new tiered warning approach, the Warning Decision Training Division (WDTD) also introduced suggested criteria for when forecasters should consider upgrading a tornado warning to a tornado emergency, which includes thresholds of rotational velocity (VROT) and significant tornado parameter (STP). Although significant research has studied both tornado forecasting and tornado warning dissemination in the decade since, relatively little work has examined the effectiveness of the tornado emergency specifically. Our analysis of all 89 IBW tornado emergencies issued from 2014-2023 found that forecasters do not appear to follow the suggested criteria for issuance in the majority of cases, with only a handful of tornado emergencies meeting both the VROT and STP thresholds. Regardless, 70% of tornado emergencies were issued for EF-3+ tornadoes, and tornado emergencies covered 55% of all EF-4 tornadoes as well as 41% of all tornadoes resulting in 3 or more fatalities. Based on these results, we propose several updates to the current NWS training materials for impact-based tornado warnings.