CSI Tornado: Decoding – and chasing – supercells with the experts

The NWS is midway through a major upgrade to its network of weather radar that, by next spring, is expected to give forecasters a more detailed look at the inner workings of a storm and possible tornado triggers. It will also be able to spot tornado debris clouds, providing ready confirmation that a funnel cloud actually has reached the ground.

Researchers are working on another radar system that can scan the skies faster than current radar, with electronically steered beams that will allow forecasters to spot a twister radar signature several minutes earlier than mechanically rotating antenna now do. That means more lead time for people to reach shelter.

Also in the works is a warn-on-forecast system that will allow forecasters to routinely give at least 40 minutes' warning along a far more specific path than tornado warnings now cover.

US is the world capital of tornadoes

For all their tightly wound fury, tornadoes are very hard to produce and require specific geography. And nowhere else in the world hosts such optimal conditions as the Great Plains for building the towering thunderstorms that spawn twisters. So that's where scientists focus their inquiry on the forces that cause twisters.

Most significant tornadoes appear below so-called supercell thunderstorms – powerful, usually isolated storms. Only about 10 to 20 percent of these roiling behemoths form twisters, says Donald Burgess, a researcher with the Cooperative Institute for Mesoscale Meteorological Studies at the University of Oklahoma. And when they do form twisters, 80 percent of the tornadoes fall into the two weakest categories of the six-level tornado intensity scale.

Supercells rely on four basic ingredients to form: a source of warm, moisture-laden air near the ground and colder air at higher altitudes; a shift in wind speed or direction with altitude – known as wind shear – within a few thousand feet of the ground; something to trigger the rise of that low-level warm air; and a landmass that is closer to its hemisphere's pole than the source of the warm, moist air.

These ingredients are present elsewhere, such as South America, southeastern China, Bangladesh, and on the Tibetan Plateau, notes Paul Markowski, an associate professor of meteorology at Pennsylvania State University in State College. "But no place do these conditions occur on as vast a scale as the Great Plains of the United States," he says.

The Gulf of Mexico supplies warm, moisture-laden air that moves north near ground level. Winds flowing eastward over the Rocky Mountains provide an overlying layer of cold air. The initial lift can come from daytime heating, from wind deflected up as it blows across uneven terrain, or from a cold front moving east.

As the warm air rises and cools, water vapor in the air condenses, releasing back the heat that turned Gulf of Mexico seawater into a gas in the first place. This newly introduced heat source warms the surrounding air, giving the air parcel additional lift. If the surrounding air continues to get colder with altitude, the relentless flow of moisture-laden air into the updraft forms a sprawling, anvil-shaped thunderhead.

Meanwhile, wind shear at mid-levels sets up a rolling pin-like circulation of air drawn into the updraft, imparting rotation to the core of the storm.

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