What conditions make a tornado likely to form??
For tornadoes to form you need instability, moisture, wind shear, high helicity, cold dry air aloft and warm moist air at the surface. (To make this less confusing I will use the Plains as my example). The reason why the mid-section of the US gets so many tornadoes is because the conditions that are needed usually all come together here. Warm moist air advects Northward from the Gulf of Mexico. Cool dry air advances Southward from Canada. The warm moist air sits at the surface and the lower levels of the atmosphere (especially if winds are from the South or the Southeast) and in the mid and upper levels of the atmosphere temperatures are cold and the air is dry (winds coming from the North, or Northwest from Canada). The warm and moist air at the surface helps to create instability (measured in Jules per kilogram, J/KG). The more unstable the atmosphere the higher the chance for severe weather and tornadoes. You can measure how unstable the atmosphere is by looking at Surfaced-Based Ca
