What is the key ingredient for Midwest killer tornadoes in the U.S.?

This winter's record warmth provided the key ingredient for a Midwest outbreak of deadly tornadoes and damaging gorilla hail that hit parts of the Midwest in the U.S. Wednesday and Thursday, tornado experts said.

At least three people were killed in Thursday's tornado outbreak in the states of Ohio, Kentucky, Indiana and Arkansas, which came a day after large hail struck Kansas. It's a bit early, but not unprecedented, for such a tornado outbreak usually associated with May or April, but that's also because of the hottest winter in both U.S. and global records, meteorologists said.

"In order to get severe storms this far north this time of year, it's got to be warm," said Northern Illinois University meteorology professor Victor Gensini.

Tornado formation

For tornadoes and storms with large hail to form, two key ingredients are needed: wind shear and instability, said Gensini and National Severe Storms Laboratory scientist Harold Brooks.

Wind shear, which is when winds whip around at differing directions and speeds as they rise in altitude, is usually around all winter and much of spring because it's a function of the normal temperature difference we see across the country, Gensini said.

But instability, which is that juicy warm humid air close to the ground that is the signature of summer, is usually missing this time of year, Gensini and Brooks said.

That's because normally in the winter and into early spring, Arctic air plunges south, pushing the warm moist air south into the Gulf of Mexico, leaving dry stable cool air in its place, said Matt Elliott, the warning coordination meteorologist for the U.S. Oceanic and Atmospheric Administration. And that cool stable air keeps tornadoes and large hail from forming.

But not this year. There was only one real Arctic blast this year and that was two months ago, the meteorologists said.

Tornado activity this time of year is much more common in the South, with what's happening "much further north than we normally expect," Gensini said.

NOAA's Elliott said it may be a tad early, but this is about the time of year that severe storms start to ramp up in the Midwest, but they do not usually peak until May.

El Nino and climate change

What also makes the Midwest outbreaks unusual is that there's an El Nino, though it is starting to fade. The natural El Nino, which is a warming of the central Pacific that changes weather worldwide, often leads to fewer severe storms in the Midwest especially in the spring, studies show.

Gensini said the El Nino factor is just one of several variables and only tilts the odds slightly.

As for climate change, no one has done the traditional scientific studies that link specific tornado outbreaks to that. There are so many issues that make that difficult, including poor tornado records in the past and tornadoes being small weather events for global climate models.

But given how off-the-charts temperatures and other climate variables have been, Gensini said, "if there ever was a fingerprint of climate change on severe weather it would be this year."

Because of other natural climate factors, Gensini said there's a strong chance for another Midwest outbreak of tornadoes in the end of March or early April.

(With input from AP)