Thunderstorms happen in two ways, either air masses collide or the surface airgets too hot and humid for its own good. Both situations destabilize theatmosphere, and huge pillars of air boil upward. The rising air cools andcondenses forming towering thunderstorm clouds.The severity of a thunderstorm is determined by the level of instability in theatmosphere and by how the jet stream interacts with the surface flow (windshear). If the wind at all levels is pretty much in alignment, a severethunderstorm is more likely to produce powerful straight-line winds than atornado. Tornadoes are more likely to occur when the overall environment has arotational component. The rotation comes from a marked change in wind directionwith altitude.
Hail, on the other hand, largely has to do with the coldness of the upperatmosphere. It forms when droplets of water caught in the updraft of athunderstorm are thrust high into the clouds. Here, the drops freeze and growin size. Once free of the storm's updraft, gravity takes over, and thehailstones fall to earth. The largest hailstones occur in the most severestorms with the strongest updrafts.
Another round of severe weather is expected to slam the Great Plains andMidwest Tuesday night and Wednesday. The maps above and below show theprogression of things and where we expect the worst storms to occur.
Remember that the big tornadoes (EF3 or stronger) not only cause a huge amountof damage, theycan kill. Improved warnings over the years have no doubt saved many lives, buteven one death is one too many. Please come to grips with what these storms cando and by all means stay alert.
Story by AccuWeather.com Senior Meteorologist John Kocet.
