...According to Harold Brooks, a tornado expert at the National Severe Storms Laboratory in Norman, Okla., 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.
The weather has been characterized by a sprawling area of High Pressure over the Central U.S., which has brought stifling heat and much below-average rainfall to a broad swath of the country.
St. Louis, Mo., for example, 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.
During July this year, 3,908 daily high temperature records have been broken or tied in the U.S., and 169 of those records have been all-time high temperature records.
One might think that all of this heat would provide ample fuel for severe thunderstorms. After all, a warm air mass is a main ingredient in severe weather.
However, for tornadoes to form, there also needs to be high humidity, 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 discourages storms by causing the air to sink, warming as it does so, and choking off any storms. The dry ground also discourages storms by releasing less moisture into the air, cutting down on the instability available for storms to form.
The jet stream has been shunted well to the north, across the U.S.-Canadian border, depriving thunderstorms of the wind shear needed to form large hail and tornadoes.