Get to know FEMA’s weather risk assessment for your area so you know what to expect in the aftermath of a natural disaster.
The Federal Emergency Management Agency (FEMA) has developed the National Risk Index (NRI), “which provides a relative measurement of community-level natural hazard risk across 50 U.S. states and Washington, DC.” Natural hazards are defined as environmental phenomena with the potential to impact societies. Natural disasters are defined as the negative impact following an actual occurrence of a natural hazard. Human-caused disasters aren’t included. For example, floods caused by excessive rainfall are natural hazards, but floods caused by dam failures aren’t.
The NRI is an interactive tool. Of the 18 natural hazards included in this risk assessment, most are weather events or weather-related events.
In terms of picking which natural hazards to include, if a natural hazard had a significant impact in the past – i.e., a natural disaster that was enough to a warrant a government mitigation plan – it was included. FEMA says, “State mitigation plans from all 50 states were reviewed, and the most common natural hazard types profiled in all plans were identified. If a hazard type was profiled by at least 25 states, it was included in the NRI. Regionally significant hazards (e.g., hurricanes, etc.) were also included.”
Expected Annual Loss (EAL) Factors
The actual value for the NRI is a function of three separate factors: Expected Annual Loss (EAL), Social Vulnerability, and Community Resilience. Social Vulnerability and Community Resilience have to do with the local population, their vulnerability to natural hazards, and their ability to cope with them. Expected Annual Loss deals with the actual occurrence of a natural hazard event.
FEMA states that EAL “represents the average economic loss in dollars resulting from natural hazards each year. [It’s] calculated for each hazard type and quantifies loss for relevant consequence types: buildings, people, and agriculture.” The EAL is a combination of three components.
Exposure
“Exposure” represents a community’s monetary value – including the fiscal value of agriculture, buildings, and population – potentially exposed to a hazard. This factor is lesser if there’s little population or property exposed, regardless of the actual risk of a severe weather event occurring.
Historic Loss Ratio
“Historic loss ratio” represents the percentage of exposed buildings, people, or agriculture that may be lost in an event. This is dependent on the size of the event; for example, hurricanes are much larger than tornadoes and can threaten a much larger area.
Annualized Frequency
“Annualized frequency” represents “the average number of recorded hazard occurrences per year over the period of record or the modeled probability of a hazard occurrence each year.” It’s the actual hazard component of the NRI. The annualized frequency, of course, varies by location and type of event. For example, southern coastal areas would have a higher frequency of hurricane occurrence than more inland, northern locations. Just the opposite would be true of ice storms. A higher annualized frequency value results in higher EAL and NRI scores. Much of the specific weather data comes from the National Oceanic and Atmospheric Administration (NOAA), specifically the National Weather Service (NWS), and the National Centers for Environmental Information.
Duration of Events
The NRI also allows for the duration of impact for various events. Lightning strikes occur within fractions of seconds, and resulting fires may last for only minutes or hours (with the exception of lightning-caused wildfires). Hurricanes and floods can produce impacts lasting days or weeks. Drought losses can add up over months or even years. For short-term events, the number of events per year is calculated. For long-term events, the number of event-days per year is used.
There are also situations where an area may not have been affected by a natural hazard within the given period of record but still is in a region that could be. Rather than a “No Risk” rating, an “appropriate minimum annual frequency” is assigned based on expert opinion. Hazards using this approach include avalanche, hurricane, ice storm, landslide, riverine flooding, and tornado.
Understanding Local Risk
Not all consequence types are considered for all hazard types. Building and population exposure are modeled for all hazard types except drought, which is only modeled for agriculture exposure. Agriculture exposure was also modeled for cold wave, hail, heat wave, hurricane, riverine flooding, strong wind, tornado, wildfire, and winter weather.
Some natural weather hazards are defined locally, such as heat waves and cold waves. The definition is based on standard local temperatures. Impacts will vary depending on local resilience, which is often based on normal or average conditions. What would constitute a cold or heat wave in Minnesota would be different than for Florida.
The NRI was designed for counties to determine their risk and better prepare to deal with severe natural events at the county level. Even if the actual risk of a natural hazard is great, the NRI will be low if there’s little population or development that would be affected. For individuals, only the “annualized risk” will tell you your actual risk, but the combined NRI will certainly give you an idea of what you may have to deal with.
Keep in mind, this isn’t a forecast; it’s the overall risk over time. Real-time weather hazard warnings will be issued by the NWS. Also, the three components of the NRI were all derived from past data. The exposure factor can change with increased or decreased development in an area. Annualized frequency can change with long-term variances in weather patterns, such as those occurring with climate change. All statistical guidance is limited by what’s happened in the past. Unprecedented events do occur. For example, the risk for hurricane damage in western North Carolina is given as “Relatively Low” or “Very Low.” But last September, this region was devastated by Hurricane Helene.
Reduce Your Risk
Many of my articles in the past have included things property owners can do to reduce property damage due to individual weather events. Your local government may use the NRI to determine the risk of natural hazards and take steps on a local basis to mitigate the impacts of such events. Some of these mitigations can impact individuals, such as building codes that are designed to reduce potential damage. In addition, risk data can be used “to find safe areas for potential evacuation, sheltering, and emergency supply distribution.”
Although designed for communities, the NRI can help individuals reduce their risk of significant impacts from natural hazards.
For more than 30 years, Ed Brotak taught thousands of college students about weather, and he’s helped many pursue careers in meteorology. He lives in Asheville, North Carolina, with his wife, Liz.
Originally published in the May/June 2025 issue of Grit magazine and regularly vetted for accuracy.
