Severe Weather Awareness Week: Thunderstorms

Weather Knowledge

LITTLE ROCK, Ark. – February 28- March 6, 2021, has been declared Severe Weather Awareness Week by the National Weather Service (NWS) and the Arkansas Department of Emergency Management (ADEM). Today’s topic of discussion is thunderstorms.

Let’s start with this important note, all thunderstorms are dangerous and should be taken seriously.

Thunderstorms are capable of producing lightning, flash floods, hail, straight-line winds, and tornadoes.

Thunderstorms can strengthen and become severe. For a thunderstorm to be considered severe is when it produces wind speeds of 58 mph or greater and or hail that is >1″ in diameter, and or a tornado.

CHANGES TO SEVERE THUNDERSTORM WARNINGS:

Starting April 28th, 2021, the National Weather Service will be implementing a new severe thunderstorm warning structure based on the threat of damaging wind and or hail.

The threshold to issue a severe thunderstorm warning will not change that is mentioned a few paragraphs above. Rather, this change is to help highlight different levels of severity that a severe thunderstorm is capable of producing.

We spoke with Warning Coordination Meteorologist, Dennis Cavanaugh with the National Weather Service Little Rock about these changes.

The thought behind this change came from the idea that not all thunderstorms are created equal and that there needs to be a better way to differentiate from a thunderstorm that is capable of producing wind near 60 mph versus 80+ mph.

This is where a tagging system will come into play. While the appearance of the warning will not change, the way the language is shown inside the warning text will. This should help highlight more easily why the warning was issued and for what nature.

In addition to this new tagging feature, something else that will be instrumental will be that the highest tag of 80+ mph wind and or hail 2.75″ or greater will be sent out to all cell phones through the Wireless Emergency Alert (WEA) through FEMA‘s Integrated Public Alert And Warning System (IPAWS) to notify people of a dangerous/life-threatening situation if they reside in the warning. Currently, only tornado and high-level flash flood warnings are sent out over the cellular network to smartphones.

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 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.

MICROBURSTS:

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.

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