Ingredients for a severe thunderstorm and warning criteria

SYRACUSE, N.Y. – How does a thunderstorm form and what makes it considered “severe”? Plus, what is the criteria for warnings? Let’s dig in!

First, for a severe thunderstorm we need four ingredients. Using the acronym SLIM, shear (wind speed/direction change), lift (air pushed up by convection/front), instability (rapid cooling with height), and moisture (warm southern air/high dewpoints).

A cap or layer of warm air high up in the atmosphere can also help to create explosive severe thunderstorms. As the day heats up, warm updrafts form. As long as the temperature keeps falling with height, air will continue to rise. If the updraft hits a layer of warmer air, it is unable to continue upward to form larger clouds or storms.

However, if there is sufficient hearing or there is an area where the warm air “cap” is thin or weak the “cap” can be broken, allowing strong storms to form.

In order for a strong thunderstorm to be labeled as “severe” it needs to have wind gusts of 58mph or stronger or quarter sized hail. A severe thunderstorm warning is labeled “considerable” when wind gusts are 70mph or stronger or golf ball sized hail. The most dangerous severe thunderstorm is labeled as “destructive” if it reaches wind gusts of 80mph or higher or baseball sized hail. When this happens, it triggers and emergency alert on phones and devices in the warned area.

The severe weather threat today is low, but there is still the risk to see an isolated severe storm. Central New York is under the green shading, which is a marginal risk or level 1 threat out of 5. This means there is a small chance for a short lived, low end, isolated severe thunderstorm with wind gusts 40-60mph and hail up to 1 inch possible.

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