Where do bomb cyclones occur most frequently?

News topic of the day

Yesterday, Thursday, the National Storm Prediction Center (NSPC) issued a so-called "High Risk" for the third time this year. This is the highest warning level for thunderstorms in the USA. It affected parts of the Midwest where 20 tornadoes were observed. One hears again and again in the news of tornado outbreaks in the Midwest. It is not unusual for the largest outbreaks to be accompanied by well over 100 tornadoes. But what makes the area around the Great Plains so ideal for you?

Tornadoes need thunderstorms to develop, preferably the so-called super cells. These are thunderstorms that rotate for long periods of time. The conditions in the Great Plains are particularly favorable for a number of reasons:

One ingredient for severe thunderstorms is moisture. From the beginning of April to the beginning of June, this is often brought up to the front of low pressure areas. With a south-easterly current, warm golf air in the lower air layers can then penetrate far north to the Midwest.

Furthermore, you need a strong temperature decrease with altitude, so that for hydrostatic reasons, the lighter warm air can rise in the colder air. This occurs more often in the vicinity of air mass boundaries. Air mass limits are particularly pronounced in the USA. Because in the USA, unlike in Central Europe, there are no blocking mountains that stop the air on your north-south movement. This means that polar cold air can penetrate far to the south and warm, moist golf air can penetrate far to the north. As a result, stronger temperature differences can build up.

A third favorable air layer arises in the high regions of the Rocky Mountains, which act as heating surfaces and create a warm and dry air layer that is often transported eastwards. The boundary to this dry air layer is also known as the dry line.

If the cooler layer of air is pushed above the warm, humid golf air with a westerly wind close to air mass limits and then the dry layer of air from the Rockys is pushed in between, the result is an explosive mixture in which particularly violent thunderstorms form.

One property of air mass boundaries is that near them the wind increases with altitude and changes direction. With south-east winds close to the ground, which become stronger with increasing altitude and turn to the west, one can easily imagine that the strong updrafts of the thunderstorms are set in rotation and the above-mentioned supercells are formed. These, in turn, are a prerequisite for most tornadoes.

The area of ​​the Great Plains, from northern Texas, Oklahoma, Kansas to Nebraska, where these conditions frequently come together, is also known as Tornado Alley.

Dipl.-Met. Christian Herold
German Weather Service
Prediction and advice center
Offenbach, May 19, 2017

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