The National Risk Index - Frequency

The National Risk Index (NRI) is a dataset updated annually by FEMA that predicts the expected annual frequency of natural weather events and associated annual losses for every county in the United States. It can also be analyzed visually using FEMA’s Resilience Analysis and Planning Tool (RAPT). Since weather patterns do not dramatically change across South Central Wisconsin, for this Metro Metrics we will focus primarily on Dane County data.  

Of the 18 different event types covered by the dataset, Dane County has an expected annual frequency of zero events for four types: avalanche, coastal flood, tsunami and volcanic activity.  

As Fig. 1 shows, risks tend to be highest in coastal areas because of major events like hurricanes and coastal flooding, while elevated wildfire risk is concentrated in parts of the western United States. All have increased in frequency and intensity in recent decades. The Midwest’s risk is largely limited to cold waves, tornadoes and associated events, which have not increased at the same rate as the disasters affecting the coasts.   

Table 1 shows the expected annual frequency of the 18 different event types in Dane County, as well as Dane County’s percentile ranking relative to all counties in the United States. Note that some expected values are calculated on a per-event basis, while others are calculated as expected number of days an event will occur. For example, Dane County’s annualized frequency of 37.12 for lightning indicates there are approximately 37 days each year when lightning will occur somewhere in the county and not 37 individual lightning strikes. 

A few different event types stand out where Dane County has a greater expected frequency compared to the national average. These include days of cold wave per year (ranked above 87.7% of U.S. counties), days of inland flooding per year (81.2%), strong wind events per year (77.1%), tornadoes per year (96.3%), and days of winter weather per year (78.4%). Dane County ranks above the national median in only five of the 18 hazard categories. For four of the 18 event types that FEMA tracks, Dane County and surrounding counties experience no events at all.

The National Risk Index - Severity

While frequency data can help identify strengths and vulnerabilities compared to the rest of the country, it doesn’t tell us how damaging these events are likely to be. The expected annual loss rate measures how much loss is expected from a specific event type compared to the total value in a specific county. This provides a useful measure that incorporates both how frequent a type of event is (more events in a year cause more damage) and how severe that event is on average (more severe events cause more destruction). The annual loss ratio combines expected building, population and agricultural losses into a single measure.   

When looking at the expected annual loss ratio for natural disaster events in Dane County, it becomes apparent that even though some events are more frequent than the national average, the impacts are generally less severe.  

The table below shows Dane County’s percentile ranking among all counties in the United States. Many of the events that are more frequent in this region, such as cold waves, inland flooding, strong winds and winter weather, are actually less damaging than the national median. While tornadoes can occur more frequently in this region, their impact is also lower than in many higher-risk areas. The only three events that are both more common than the national average and more damaging over the course of a year are hail, landslides and tornadoes. The other 15 types of events – 83% of those that FEMA tracks – are less damaging in Dane County and most surrounding counties than the national average. Other counties surrounding Dane mostly fall around the same ranking nationally as Dane does, often with slightly lower annual loss ratios.  

With half of the counties in south central Wisconsin being classified as “relatively moderate” risk and the other half being “relatively low” risk, the data reinforce the Madison Metro’s reputation as a resilient region. Although no community is immune to extreme weather events, this region faces fewer threats than much of the country.