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Hot summer leads to record low number of tornadoes

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CHICAGO (FOX 32 News) -

The massive drought that has taken over the lower 48 states is on pace for one of the worst ever. It also is having an effect on the number of tornadoes. In fact, a near-record low number of tornadoes.

Compared to last year, when tornadoes killed hundreds during the spring and early summer, this year has seen a huge drop off in the number of tornadoes since June. Through July 23, there had been just 12 tornadoes recorded in the U.S. this month.

In fact, the U.S. may even break its record for the least tornadoes in any summer month.

The key ingredients for forming tornadoes just have not come together very often this summer, and the weather pattern that has spawned the devastating drought is a key reason why.

Most of the cold fronts have been producing hail and damaging straight line winds. Keep in mind that tornadoes depend on wind shear which has been very weak this month.

A huge dome of high pressure has been nearly fixed over the Central United States. This has brought brutal heat waves and very little rainfall. Little Rock, AR, hit 110 degrees on Monday July 30th. St. Louis, Mo., has reached or exceeded 105°F a record 11 times so far this year, which is more than occurred during the Dust Bowl in 1934. This July has produced 3,908 daily high temperature records have been broken or tied in the United States and 169 of those records have been all-time high temperature records.

Even though we need heat for thunderstorm formation, there are other ingredients that have been almost absent. For tornadoes to form, there needs to be high dew points, strong jet stream winds, and wind shear, which is winds that change direction or speed with height. In July, those ingredients have not come together in the right amounts, at the right time.

The heat dome acts like a lid on the atmosphere and causes the air to sink. Sinking air will compress against the surface of the earth. When you compress air, it heats up. Remember the atmosphere needs to get "vertical" meaning rising air for thunderstorms to fire up. The dry ground also releases less moisture into the air, cutting down on the instability available for storms to form. The main energy and jet stream has been well to the north, across the U.S.-Canadian border, depriving thunderstorms of the wind shear needed to form large hail and tornadoes.

The main severe weather events in June and July have mainly been straight-line wind producers, like this "derecho" storm complex on June 29, as seen in this radar time series courtesy of NOAA.

The least amount of tornadoes in any year since records began in 1950 occurred in 1988, which was also a severe drought year.

The quietest July on record for the least number of tornadoes is 1960, which saw 42 tornadoes. When using data that has been adjusted to account for an artificial increase in tornado counts with time, due to population growth and better storm-detection technology, the least active July is 2007, when there were 73 tornadoes.

 

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