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(CNN) -- When a powerful jet stream bringing in cold northern air collided with a large mass of warm, moist air from the south, the conditions were right for the tornadoes that have left at least 45 dead this week.
Two powerful storm systems in the Midwest and South spawned the tornadoes that damaged and destroyed homes and businesses from Kansas to Ohio.
There is not a defined tornado season like there is for hurricanes, but this year the first major tornadoes came earlier than usual. Last year, the most powerful tornadoes were not seen until April.
"The conditions simply have come earlier this year," said Harold Brooks, a research meteorologist at the National Severe Storms Laboratory in Norman, Oklahoma.
From a meteorological perspective, there was nothing unusual about the storms that produced so many deadly twisters this week. The ingredients just coalesced at a unique time.
However, the contrasting air masses -- cold, dry air from over the Rockies and warm, moist air from the Gulf of Mexico -- that created the conditions for tornadoes covered a particularly large area, Brooks said.
The week's severe weather could constitute one of the six largest outbreaks in the past 60 years, he said.
Although the first tornadoes of the season were deadly and powerful, that doesn't give any clues about the rest of the season, he said.
Warren Faidley, a professional storm chaser and severe weather survival expert, agreed, but said a mild winter has set the stage for what may be an unusually active tornado season.
"When you have mild winters you have more of a contrast between cold air and warm air to the south," he said.
Tornadoes thrive on these contrasts. They form at the boundaries of cold and warm air, and dry and moist air.
The current wild winter results in an abundance of moist warm air that will be available to clash with dry cold air from the north, he said.
"It looks like this year there is going to be more days with the possibility (of tornadoes) with these contrasting masses," he said.
But a variety of conditions must combine at just the right time to create a tornado, he said. So even though there may be a higher probability of the ingredients being found in the atmosphere this year, that cannot predict if these conditions will spawn more powerful tornadoes, he said.
The takeaway is that tornadoes are unpredictable, said Greg Carbin, warning coordination meteorologist for the National Oceanic and Atmospheric Administration and National Weather Service.
Not only do the ingredients have to be present, but they also have to be in the right pattern and the right proportion.
"It's actually quite easy to ruin the recipe most of the time," Carbin said, a fortunate fact for those living in areas affected by tornadoes.
"These storms are highly variable," he said.
Even a mild winter is not a good predictor of future tornadoes, because while a warmer Gulf of Mexico may mean warmer air, the correlation with twisters is tenuous, he said.
"We could go into a very active period, or we could stay relatively quiet for the coming weeks," he said.