A Proposal to Clearly Define Threat and Risk for Weather Events

One of the key components of NOAA’s National Weather Service (NWS) Strategic Plan to build a Weather Ready Nation is to “communicate forecast confidence and risk information for better decision making” by core partners who serve in public safety roles. One definition of risk used by the Federal Emergency Management Agency (FEMA) is “a combination of the probability that an event will occur, and the consequences of its occurrence”. Or, in simple equation form: Risk = Asset Value x Threat Rating x Vulnerability Rating. Variations on the risk equation account for resiliency, exposure, and response, and will be briefly discussed.

For decades, NWS has sometimes provided unclear messages where the term risk is used to describe threat. In 2018, this remained the case for many information sources, including those for most weather events from the national to the local level. The Storm Prediction Center uses the term risk to describe differing levels of threat, based on probabilities for wind, hail, and tornadoes. The Weather Prediction Center also uses the term risk to describe a probabilistic level of threat for flooding from excessive rainfall, and many Weather Forecast Offices (WFO) use the term risk to describe the probability of local events. There are communication successes as well. These include threat-based decision support tools for tropical cyclone wind, storm surge/rainfall flooding, and tornadoes; and threat-based decision support tools for a variety of weather events provided by a number of local NWS offices.

It is proposed that NWS and the Weather Enterprise replace the term risk with the term threat for the communication of probabilities for all weather events. By using the correct terminology to describe forecast confidence in the occurrence of an event, NWS and the Weather Enterprise will be able to match the parameter that describes the threat component used by FEMA and most related core partners at regional, state, and local levels. The uniform definition of threat, when applied to vulnerability to an event, will help the entire community make progress in improving risk communication - a key component of providing Impact-Based Decision Support Services to build a Weather Ready Nation.