What is 'speed difluence' and how do non-frontal thunderstorms work?

Thunderstorms happen all the time in Texas. Every season has them and some seasons see them more often than others. And there are tons of ways for them to form.

The most common cause for thunderstorms is what is called "frontal thunderstorms" which form along cold fronts. This is why normally the weather gets much sunnier when the thunderstorm departs and often the humidity goes way down as well.

Cold fronts are the most common cause of thunderstorms because they’re a great forcing mechanism. They push the warm, humid air up into the atmosphere and allow it to create the cumulonimbus clouds the storms need. But cold fronts aren’t the only way thunderstorms can form.

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In other cases, the forcing can come from the upper part of the atmosphere. This is what happened Friday morning with the weak storms that moved through central Texas. In this case, the vertical motion comes from the jet stream and a phenomenon called speed difluence.

Difluence is the process of wind moving in opposing directions. That movement apart causes a low-pressure center which crates upward motion in the atmosphere. In the right conditions that upward motion can form rain and thunderstorms if the upward moving air is going fast enough.

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Difluence can be caused in a couple of ways. The first is just wind turning in opposite directions from itself. Meaning a south to north wind splits with half turning westward and half turning eastward. Along the axis of that split, there will be a low-pressure center which will enhance upward motion.

The other is called speed difluence which is when wind speeds suddenly increase, like when upper-level air enters the jet stream. The acceleration "leaves" the neighboring air molecules behind causing a localized low-pressure area. Both these forms of difluence can cause rain showers or even thunderstorms.

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That’s what happened Friday! A strong part of the jet stream was following behind the cold front across central Texas. As the westward part of the jet moved across central Texas, eventually exiting the state, the wind speeds at high levels of the atmosphere suddenly increased causing a localized low-pressure area at the upper levels of the atmosphere. This low caused vertical motion which, in turn, caused the rain showers and thunderstorms many saw Friday morning.

Speed difluence is also an important part of severe weather as the faster-moving air can help to "suck up" more warm, humid air from the surface. The stronger the "suck" the stronger the thunderstorms will be and the greater chance for severe weather.

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