The Most Tornado-Prone Counties in the U.S.

At a Glance

• We dug through NOAA's Storm Data to find which counties are the most tornado prone in the U.S.
• Some counties on the lists aren't surprising. Others you may not have thought of.
• We feature three different lists based on raw numbers, number per unit area, and number per unit population.

You can probably guess the nation's most tornado-ravaged states, but a NOAA dataset allows us to zero in on the most tornado-prone counties in the U.S.

(MORE: Tornado Central)

Using the Storm Events database from NOAA's National Centers for Environmental Information, we compiled tornado data for each county in the U.S. from 1950-2016. 

Since a given tornado may travel over multiple counties, the dataset isn't strictly the number of tornadoes, but rather number of tornado segments. For example, a tornado tracking over three counties has three tornado segments, one per county, in the database.

Raw Numbers

If simply considering the raw number of tornado segments over that 67-year period, regardless of a county's size or population, you may be surprised to learn the number-one county is in Colorado.

Weld County, Colorado, just north of the Denver metro area, has tallied 262 tornado segments, an average of roughly 4 segments each year, from 1950-2016. Another Colorado Front Range county, Adams County, ranked third with 173 tornado segments. 

Thunderstorms are often an almost daily occurrence from late spring into summer along the Front Range of Colorado. A north-south area of converging air known as the Denver Convergence Vorticity Zone, frequently forms east of the Interstate 25 corridor from southern Weld County to east of downtown Denver, serving as a source of spin which thunderstorms can then tilt and stretch to form tornadoes.

(MORE: Tornadoes Influenced By Mountainous Terrain?)

Other than Colorado, the other top tornado counties were either in Oklahoma or near the Gulf Coast. 

Runner-up was Harris County, Texas, which includes Houston, with 225 tornado segments.

It was no surprise to have a pair of Sooner State counties – Caddo and Oklahoma – make the list, but neither cracked the top five. Five Florida counties each cracked the top 10 list.

Near the Gulf Coast, tornadoes can strike any time of year, and even on summer days when an outbreak of severe weather isn't expected, benign thunderstorms along the sea-breeze front can spawn brief tornadoes. 

(MORE: Different Types of Tornadoes)

Landfalling tropical storms and hurricanes also typically spawn tornadoes, likely also contributing to the higher totals in some of the counties near the coast.

Of 35 total counties with at least 100 tornado segments, 15 were in Oklahoma, eight in Florida, six in Texas and four in Colorado. McLean County, Illinois (103 segments) and Sherman County, Kansas (100 segments) were the other triple-digit segment counties.

Correcting For County Area

These raw tornado segment statistics may not tell the whole story.

It only makes sense that a larger county would tend to have more tornado segments.

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When we did that, the general layout of the most tornado-prone counties, from the Gulf Coast to Oklahoma and Colorado was roughly the same.

However, there was one county that rocketed to the top of the list, head and shoulders above all others.

Pinellas County, Florida's second smallest county, racked up 130 tornado segments, for a density of roughly 46 segments per 100 square miles, in the 67-year period. Galveston County, Texas, was a solid second place.

Three notorious Oklahoma counties, including the Oklahoma City and Tulsa metro areas, shot up in the list when tornado segments were normalized by county area. Six of the top 10 counties were in the Sooner State.

(MORE: The Future of Tornado Warnings)

Despite only 39 segments, Lafayette Parish, Louisiana, made this top 10 list given it's the third small parish in the state.

Correcting For Population

You may have noticed the most-prone counties covered so far are predominantly near or in large metro areas, such as Denver, Houston, Oklahoma City, Tampa/St. Petersburg, or Miami. 

Before the era of social media, increased interest in storm chasing and Doppler radar, weaker tornadoes in sparsely populated parts of the U.S. may have gone unreported if they hadn't produced any damage.

(MORE: Why Rating Tornadoes in Rural Areas is Challenging)

Despite that, we wanted to take another pass at the NOAAA data normalizing by population (per 100 residents in the 2010 census). This highlighted several other tornado-susceptible counties in the High Plains from West Texas to the Nebraska panhandle.

(Note: This does not imply the number of tornadoes to affect 100 residents of each county in the dataset's time period. It's merely a method of removing population from the data.)

With a 2010 population of 82, Loving County, the least-populated county in Texas, popped to the top of the list, despite only six tornado segments in 67 years.

(INTERACTIVE: Experience the Formation of a Tornado)

Among counties with at least a population of 1,000, Kiowa and Cheyenne Counties on Colorado's eastern plains lead the way, not to mention also leading in the number of tornado segments due, in part, to their rather large size.

Most of these counties lined up along the dry line, the boundary between dry air flowing eastward off the plateau of the Southwest U.S. and moist air from the Gulf of Mexico, along which severe thunderstorms spawning tornadoes can flare up.

Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7. Follow him on Facebook and Twitter.

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