LITTLE ROCK, Ark. – March 1-7, 2020 has been declared Severe Weather Awareness Week by the National Weather Service (NWS) and Arkansas Department of Emergency Management (ADEM). Today’s topic of discussion is severe thunderstorms.
Let’s start with this important note, all thunderstorms are dangerous and should be taken seriously.
Thunderstorms can strengthen and become severe. For a thunderstorm to be considered severe is when it produces wind speeds of 58 mph or greater, hail that is >one inch in diameter, and/or a tornado.
Let’s talk about one of the elements that a thunderstorm can produce: Hail. 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.
When it comes to wind, 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.
As mentioned above, thunderstorms can have tornadoes. 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. For more on where a safe place may be for your home, check out our tornado story from earlier this week.