LITTLE ROCK, Ark. – All storms are dangerous and should be taken seriously. Some storms are stronger than others and create more life-threatening situations. These are what we (the meteorologists) call severe storms.
With any thunderstorm there may be cloud-to-ground lightning, hail, flash floods, straight-line winds and tornadoes. When a storm is deemed “severe,” these characteristics of the storm reach certain parameters.
For a thunderstorm to be considered severe, it must produce at least one – but not all – of the following:
- Wind speeds of 58 mph or greater
- Hail that is one inch in diameter or greater
- A tornado
Severe thunderstorm warnings are issued primarily for the wind and hail components of a severe storm. If there is a threat of a tornado, the local National Weather Service office issues a tornado warning.
Arkansas has two severe weather seasons in which severe storms most often occur – spring and fall. It’s during these seasons when warm and cold air masses collide and have the most prevalent ingredients for severe storm formation. Those ingredients include but are not limited to: moisture, wind shear (winds differing in speed and/or direction), instability (energy potential), lifting mechanism (cold fronts).
Let’s talk for a moment about the different components of severe storms.
Hail is formed when droplets of water are carried high enough into the atmosphere, thanks to a storms’ updraft, where temperatures go below freezing. Once water droplets make it to the part of a thunderstorm that is below freezing, they will start to become a solid. In this process, hailstones will drift above and below the freezing line in the thunderstorm melting, collecting more liquid while doing so, then going back up and refreezing again.
Once the hailstone becomes too heavy for a storms’ updraft to keep it aloft, it will fall to the surface. The size of the hail will vary on the strength of the thunderstorm.
Some thunderstorm updrafts can be very strong, keeping hail inside a storm long enough for it to grow to very large sizes. On June 19th, 2019 areas of Polk County reported softball to grapefruit size hail. One stone measured 4.6 inches in diameter which was just short of the state hail size record of 5 inches which fell on January 21st, 1999 and again on April 2nd, 2006.
Wind in a thunderstorm can pack a punch. As a storm is overwhelmed with rain and hail it can create a lot of cold air aloft. This cold air must sink and can do so in a hurry. Once a downdraft makes it to the surface it will spread out. This is where straight-line wind damage comes from. Depending on the strength of the downdraft, wind speeds will vary. Some straight-line wind damage comes from a phenomenon called a microburst.
Tornadoes can form within thunderstorms. Usually that storm is already very strong and producing the components above, but not always. Tornadoes are violent and sometimes unpredictable in their movement.
A tornado is a rotating column of air that extends vertically from a cloud base to the ground. If it does not reach the ground, it is called a funnel cloud. Forecasting for tornadoes has come a long way, though brief, weak tornadoes can still form so quickly that radar barely picks up its signature.
If there is enough evidence on radar or from ground spotters, a severe thunderstorm warning will be upgraded to a tornado warning.
If a severe thunderstorm warning is issued, it is good practice to go ahead and find a safe place to ride through the storm.