Here’s what makes a thunderstorm strong enough for a severe thunderstorm warning

Learn what NWS meteorologists look for

Dark storm clouds over Central Florida. (Copyright 2022 by WKMG ClickOrlando - All rights reserved.)

ORLANDO, Fla. – If you’ve lived in Central Florida for any amount of time, severe thunderstorm warnings don’t come as a surprise. Summers are full of severe thunderstorm warnings, but they can happen heading into the cooler months too. The question we hear often is why one thunderstorm can have a warning, but not another one that may seem just as strong?

While both storms may seem equal, there’s certain criteria that must be met for a warning to be issued. Let’s get started.

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We reached out to Justin Gibbs, a meteorologist instructor with the National Weather Service Warning Decision Training Division. A few key items meteorologists look for include a storm producing winds of 58 mph or greater and hail 1.00″ in diameter or larger.

“Winds of that speed can knock down trees and powerlines and hail to 1.00″ can start doing some damage to things it hits as well,” Gibbs said.

Comparing hail size to common objects. The hail that fell in Sanford Thursday evening was about the size of a tea cup or large apple. This puts it in the exceptionally rare category, especially for Florida standards. Hail becomes severe when it is 1" in diameter or larger.

The more intense the updraft of a storm is, generally the more significant the impacts on the ground will be, such as stronger winds and bigger hail. The height of the storm plays a big role in this, too.

You may often hear a meteorologist on television talk about how high or tall the storm is before relating it to the storm’s strength and what it’s capable of producing. The higher the storm is and the denser the material being lifted into the storm, the brighter and stronger the return on the radar will be, hence the bright reds and purples that show up on radar imagery like the image below from a storm in May of 2020.

3-D volume radar scan of the storm that produced tea cup-size hail near Sanford Thursday night. The deep purple in the radar slice represents the hail core of the storm within an intense thunderstorm updraft. The hail core was approximately 30,000 feet tall while the storm itself about 50,000 feet tall.

“We also combine what we see on the radar with environmental conditions like thermal instability and wind shear,” Gibbs said.

Then, there’s the human intuition element.

“We also rely on our experience or intuition too, after looking at a few thousand storms you generally start to pick up on the ones that are more dangerous or when something’s just not right,” Gibbs explained.

Additionally, there are a number of other things that are looked at to determine if a warning should be issued, ranging from near-storm environment to rapid intensification to societal and population concerns.

While watching all this data and imagery, the trained meteorologists at NWS offices across the nation also are looking at rotational characteristics in the storm’s velocity data, and it’s analyzed in eight to 14 imagery slices up to 40,000-50,000 feet in the air. Why does this matter? One word: tornadoes.

Of course, tornado watches and warnings are issued separately with additional criteria, but it’s still something closely monitored in severe storms.

The length of time a warning is issued is determined by the degree, hazard and forecastability of the event. Location plays a big role, too.

“On a summer pulse storm over Central Florida for example, I might only warn for 25 or 30 minutes because that storm is going to poof out pretty quickly in most cases. Conversely, on a long-track tornado that is significant and showing no signs of stopping like in the Midwest, those warnings might go for 45-60 minutes,” Gibbs said.

National Weather Service logo.

Each individual NWS Weather Forecast Office (WFO) issues warnings for the area they provide service for. In Central Florida, there are three WFOs for the 10 counties covered on local television: Melbourne, Jacksonville, and Tampa.

The Melbourne office covers a good chunk of eastern Central Florida, providing service to Volusia, Seminole, Orange, Osceola, Brevard and Lake counties. The Tampa office covers Sumter and Polk, while the Jacksonville WFO covers Marion and Flagler counties. These offices cover a lot more counties than what we mentioned, but we kept it easy and local for this article.

Within the three offices mentioned, there are certain meteorologists who issue warnings, but overall it’s a team effort.

“We tend to make decisions as a team and very often discuss what we are seeing with our teammates during an event so no one is ever really operating alone,” Gibbs said.

To issue warnings, it requires around 200 hours of training in addition to much on-the-job experience. Gibbs said forecasters mentor and train each other following their formal training before the individual can operate on their own if needed.

Yes, there is a lot of information, training and thought behind each warning issued, but the overall objective is simple: to keep people safe.

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About the Author:

Emmy Award Winning Meteorologist Samara Cokinos joined the News 6 team in September 2017. In her free time, she loves running and being outside.