|Dr. Greg Forbes of the Weather Channel|
reviewing large tornado outbreaks
Now, meteorologists are warning of a severe weather outbreak, including tornados, for the midwest next week. What do meteorologists look for to predict a tornado outburst days or even a week into the future? First, they look for the jet stream to plunge to the south, bringing strong winds westerly or southwesterly winds and cold air aloft. Second, they look for warm and humid air flowing up from the Gulf of Mexico at lower levels, pushed by southerly winds. Being less dense than the cold dry air, the warm humid air is unstable. The required four ingredients are: warm air, cold air, moisture and winds. Low-level winds blowing with different strength at different elevations set up shearing that produces a horizontal spinning vortex, and regions where winds rise, producing updrafts, draw air flowing along the surface and its vortices in and up. A third layer of air, hot and dry, develops between the lower warm moist air and the colder upper air. This layer acts as a cap that prevents the warm moist air from rising, allowing it to warm even further, creating a positive feedback that makes the instability even greater. As the system moves from west to east across the U.S., the lift increases, the capping dry air is removed and explosive thunderstorms can develop.
The winds circulating around a low pressure center provide a mechanisms that can spin up a normal thunderstorm into a huge rotating vortex known as a "supercell." Supercells contain strong, rotating updrafts. Because they are so big, they are usually isolated from other thunderstorms in the area, sucking up energy and moisture from miles around. Tornados seem to develop within a supercell several thousand feet above the ground. Tornados begin in the supercell as a rotating, funnel-shaped cloud extending from the base of the supercell. When the funnel cloud is half-way between the cloud base and the ground, it formally becomes a "tornado."
BTW, last night was a short total lunar eclipse, and here's a link to a time-lapse of it from USAToday.