By Kevin Gray
MIAMI (Reuters) - A rare and potent combination of cool air clashing with warm, humid weather and extreme winds at varying altitudes ignited the deadly tornadoes that ravaged the U.S. South this week, meteorologists said on Thursday.
"Everything you need for a tornado outbreak was in place," said David Imy, a meteorologist from the Oklahoma-based U.S. government Storm Prediction Center.
April is normally an active month for tornadoes across the region as spring sets in. But weather watchers said this month was on track to be record-setting.
AccuWeather.com said more than 900 tornadoes had been reported since April 1 in an onslaught of severe thunderstorms that has spawned flooding and deadly twisters in parts of the Midwest and Southeast.
On Wednesday, at least 164 tornadoes tore across a region that stretched from Mississippi to Virginia, flattening homes, flipping over cars and leaving a trail of destruction. The tornadoes and storms killed at least 284 people.
"This could turn out to be one of the worst years for severe weather and tornadoes in history," said Dan Kottlowski, expert senior meteorologist for AccuWeather.com.
He said the weather phenomenon known as La Nina was making it easier for cold air that normally sits over the Northern United States to move deeper into the warmer South, a clash that frequently triggers more storm activity.
La Nina is an abnormal cooling of waters in the Pacific Ocean that can wreak havoc with weather patterns.
Parts of the United States have also been grappling with extremely dry conditions.
Texas is facing its worst drought in more than 40 years, with high winds and heat causing crop losses topping $3 billion.
But large parts of the South have seen warm and moist conditions for an extended time, making it potentially ripe for tornado activity, Imy said.
"It's very hard to get all the conditions together for a tornado outbreak," he said. "But we had warm, moist conditions, a jet stream going right across that warm air mass -- everything was in place."
Residents in states like Mississippi, Alabama and Georgia are accustomed to tornado activity. Seven of the deadliest tornado days in U.S. history have involved at least one of the states.
But Kottlowski said extreme wind activity on Wednesday also helped stoke the deadly series of tornadoes.
"What makes this dramatic is you had extreme winds aloft coming over this area," he said. "What those extreme winds did was they helped to bring all the ingredients together."
(Editing by Pascal Fletcher and Peter Cooney)