TIME COST OF TORNADO WARNINGS AND THE SAVINGS WITH STORM-BASED WARNINGS

Tornado warnings issued by the NWS and investment in a national network of Doppier (WSR-88D) weather radars in the 1990s have helped reduce casualties from tornadoes in the United States. As tornadoes have become less deadly, the value of time spent under warnings has become an increasing proportion of the societal cost of tornadoes. Our research estimates the cost of time spent under county-based tornado warnings, and we have found it to be substantial: 234 million personhours with a value of $2.7 billion annually between 1996 and 2004. Time under warnings, in fact, has become the largest component of the societal cost of tornadoes. Not all of this time under warnings was lost to society, as not everyone receives every warning when issued, and some people will disregard a warning. After adjusting for warning response, perhaps half of this time - almost 120 million person-hours a year - might be spent sheltering.

Tornadoes are small relative to counties, with the average warned county 3,000 times the size of the average tornado damage path. Thus, county-based warnings overwarn substantially for tornadoes, affecting many people not in immediate danger. In October 2007, the NWS introduced Storm Based Warnings (SBWs) for severe weather, designed to warn only the threatened area as opposed to entire counties. SBWs are expected to reduce the area warned for tornadoes by 70%75%, representing a substantial savings in the value of time spent under warnings - a clear benefit to society. It is anticipated that, over time, SBWs will reduce the time spent under warnings by more than 160 million personhours annually, with a value of $1.9 billion. Even after adjusting for warning response, SBWs should save an estimated 66 million person hours spent sheltering annually, with a value of $750 million. A sensitivity analysis demonstrates that the time savings from SBWs for tornadoes exceeds $100 million annually with a probability of 0.95. SBWs still warn the area in the path of the tornado, and thus should not directly compromise safety.

Several indirect channels for the new warnings to affect casualties exist, such as the potential for a tornado to veer out of the smaller warning area or confusion regarding the exact area warned, and research will be necessary in the future to determine if an increase in casualties offsets the time savings of SBW.- DANIEL SUTTER (UNIVERSITY OF TEXAS - PAN AMERICAN), AND S. ERICKSON. "The Time Cost of Tornado Warnings and the Savings with Stormbased Warnings," in the April Weather, Climate, and Society.

[Author Affiliation]

DANIEL SUTTER (UNIVERSITY OF TEXAS - PAN AMERICA), AND S. ERICKSON.