The National Weather Service defines a severe thunderstorm as any storm producing one or more of the following elements:
A tornado. Damaging winds or speeds of 58 mph (50 knots) or greater. Hail 1 inch in diameter or larger.
The Storm Prediction Center further defines significant severe thunderstorms as any storm producing one or more of the following elements:
A tornado that produces EF2 or greater damage. Wind speeds of 75 mph (65 knots) or greater. Hail 2 inches in diameter or larger.
There are five severe thunderstorm risk categories.
The first is Marginal. The most frequently used category by the Storm Prediction Center. It means isolated severe thunderstorms are possible, limited in duration, area of coverage and intensity.
Second is Slight Risk. Scattered severe thunderstorms are possible. Usually, short-lived with isolated intense storms possible.
Third is Enhanced. Numerous thunderstorms may develop. More persistent and widespread with a few intense storms possible.
Fourth is Moderate Risk. Widespread thunderstorms are likely. These can be long-lived, widespread and intense.
The Fifth and strongest category is High Risk. These are less frequently issued by SPC. If you are in a High Risk area, stay alert and tuned to WTWO. This level means widespread severe thunderstorms are expected. These storms will be long-lived and extremely intense.
While local National Weather Service offices issue warnings, it’s the Storm Prediction Center that issues severe weather outlooks. Here are examples.
A Moderate Risk in March of 2021 produced twenty-one tornadoes in the Texas panhandle.
Go back to March, 2012. 160 tornadoes on March second. The Wabash Valley was under Moderate Risk. Just south of us, High Risk. This was the day Henryville, Indiana was hit with an EF 4 tornado.
From 2019, all five risk categories included in this SPC Outlook.
(High Risk days are rare. There were none in 2020. The last High Risk for the Wabash Valley was on November 17th, 2013.)