Of the ingredients for severe thunderstorms, what is most important to a chaser?
Without enough of any one of them (instability, lift, moisture and shear), storms may be weak or not form at all. But most chasers are in the hunt for supercells — the special breed of long-lived, rotating thunderstorms which tend to produce the most and strongest tornadoes. The moisture, instability and lift for thunderstorms is actually rather common in spring and summer; but to get supercells, there must be some vertical wind shear. This is defined as a change in wind direction and speed with height. A change in speed (like with light surface winds and a jet stream overhead) will support severe storms, but a change in direction, particularly in the lowest 6,000 feet of the atmosphere, is an additional element that helps to sustain supercells. An excellent scenario for tornadic storms is where winds are blowing at 15 knots out of the east at the surface, out of the southwest at 40 knots at 5,000 feet, and out of the west at 50 knots at 10,000 feet. Forecasters and chasers tend to me
