Severe Weather in April

1974 "Super Outbreak"  Source: Dr. Ted Fujita, University of Chicago

There is typically an uptick in the amount of severe weather across the United States during the month of April.  That’s because during spring cold and dry air masses regularly collide with warm and humid air masses leading to showers and thunderstorms.  The United States’ unique geography contributes to the highest annual average of severe weather reports of any country in the world.  Severe weather is defined as the presence of any of the following in a thunderstorm: wind gusts of at least 58 mph; hail at least 1” in diameter or greater; or a tornado.

April, May and June have the highest three-month average of severe weather and tornadoes in the United States, according to NOAA data from 1991 through 2010.  The United States averages 1,253 tornadoes in a year, with an average of 674 occurring in April, May and June.  Severe weather can occur at any time during the year, though, given favorable weather conditions.

One of the worst severe weather outbreaks in U.S. history occurred on April 3-4, 1974.  Known as the “Super Outbreak,” there were 148 tornadoes from Mississippi to Michigan over an 18-hour period.  These tornadoes claimed 319 lives and this event remains one of the deadliest tornado outbreaks on record in the United States.  Approximately 20% of these tornadoes were ranked as either an F4 or F5 on the Fujita Tornado Intensity Scale – as strong as they get.  This outbreak has been widely studied by scientists and severe weather forecasting has improved significantly in the decades since then.

Another significant severe weather outbreak occurred from April 25 – 28, 2011.  During that four-day period, NOAA confirmed 492 tornadoes had developed from Texas to New York.  Nineteen of those tornadoes developed in Maryland and Virginia.  Severe weather forecasting had markedly improved by 2011 as compared to the 1970s.  Nevertheless, there were still more than 200 fatalities during the 2011 outbreak.

Improved severe weather forecasts are only part of the formula for safety.  Effective communication of the latest weather watches and warnings in a timely fashion to the general public remains one of the hardest aspects of forecasting.  Since 2011, the evolution of smart phone technology and increase in the number of weather apps for mobile devices has facilitated effective communication of critical weather information.  That’s especially important during the active severe weather months of April, May and June.