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Miami Valley ripe for strong storms

Editor's note: The following story first appeared in the Dayton Daily News in 2003.

The heart of severe weather season has arrived in the Miami Valley. Three out of four tornadoes occur from April to July, when warm air and cold air are most likely to collide.

All weather occurs because of temperature and moisture differences in the atmosphere. The atmosphere wants to balance these differences, so it develops winds that mix the differing air masses together.

Sometimes they mix in a gentle fashion and the result is clouds, with perhaps a few showers. But if the differences are great, they can clash violently. The greater the difference, the greater the clash, and the more extreme the weather that results.

In the Miami Valley, clashes can be intense, spawning thunderstorms with damaging winds, hail and even tornadoes. Why? It's just like real estate: location, location, location. The central United States sits in the part of the world with the best and most frequent setup for severe weather, where very warm, very humid air masses meet very cool, very dry air masses.

Quite often, southerly winds bring us warm, humid air from the Gulf of Mexico. If northerly winds at the same time bring cooler, drier air from Canada, we have our setup in place.

That combination is often enough to produce thunderstorms, but to get severe weather you need a bit more. The jet stream, the fast-moving river of air tens of thousands of feet above our heads, adds energy to these storms. Often, the setup that brings about the clash of air masses also will bring the jet stream near the Miami Valley. Under the right conditions, thunderstorms can grow to 40,000 feet or higher because of very strong updrafts. Compensating downdrafts on the backside of storms can usher those jet stream winds, sometimes 80 to 100 mph, down to the ground. That's how we get damaging winds that can at times be stronger than a weak tornado.

The same updrafts can carry raindrops to the tops of the storms, where they freeze. The frozen raindrops then fall, only to get coated with rain again. The updrafts, sometimes 60 mph, can carry these water-coated pieces of ice to the top of the storm many times, creating a new layer of ice with each trip.

Hailstones are born, and when they grow large enough that the updrafts can't support their weight any more, they fall to the ground. Sometimes they can be softball- or grapefruit-sized and create significant damage.

Thunderstorms that produce tornadoes have something in addition to those conditions.

They also have "wind shear," a change in wind direction or speed above the ground. We can get this wind shear when a jet stream from the west rides over the top of southerly surface winds.

This causes the air to spin in a horizontal fashion at first, parallel to the ground. Then the strong updrafts come into play again. The updrafts tilt the horizontal spin to the vertical, and that leads to the development of a tornado.

Luckily, that setup isn't that common, but it is one we see at times in the Miami Valley.

• Jamie Simpson is chief meteorologist at Channel 7 (WHIO-TV). His weather column appears Wednesdays in the Dayton Daily News. Contact him by writing Jamie Simpson, WHIO-TV, 1414 Wilmington Ave., Dayton, OH 45420 or by e-mailing jamie.simpson@cox.com; your questions may appear in a future column.