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Deadly Tornado Outbreak

Twister Outbreak Unusual? No, Just Seems That Way

Image: People walk between two destroyed houses in Mayflower, Ark.

People walk between two destroyed houses in Mayflower, Ark., Monday, April 28, 2014, after a tornado struck the town late Sunday. A tornado system ripped through several states in the central U.S. and left at least 17 dead in a violent start to this year's storm season, officials said. (AP Photo/Danny Johnston) Danny Johnston / AP

If you think Sunday’s deadly tornado outbreak was unusual, think again. Turns out it was pretty typical -- the big difference with most previous years is that it took so long in the season to develop.

“I'm not sure we've had a big day for tornadoes yet,” Harold Brooks, a senior researcher at the National Severe Storms Laboratory, told NBC News. Sunday’s outbreak might end up having produced around 15 twisters, he added, “which we see about 10 times per year” on average.

While tragic, even the death toll -– at least 17 fatalities -– was not out of the norm. It was deadlier than any outbreak up to this point last year, but in 2012 twisters had already killed 65 people through mid-April.

The long wait for twisters this year –- 2014 saw the slowest start to the season in possibly a century -- fed the perception that Sunday’s outbreak was unusual.

Deadly tornadoes rip through several states 9:28

But, as Brooks noted, “we see outbreaks of this size earlier in the year, about this time, and later in the year. Nothing special about the timing.”

The year got off to a slow start due to unusually cold air coming in via the jet stream last winter and into early spring. That, in turn, blocked warm Gulf of Mexico air from coming in over the central U.S.

In recent days, that warm air has begun to come inland, clashing with relatively cooler jet stream air and creating conditions favorable for twisters.

After the slow start, the season did get its first outbreak, albeit a small one, on Friday in North Carolina. That mini-outbreak was notable on two fronts:

  • One of those twisters killed a person –- the first twister fatality of 2014, capping the longest period without a death since at least 1950.
  • It produced the first major twister of the year, an EF3 on the EF0-5 Enhanced Fujita Scale.

The Weather Channel noted that Friday’s EF3 “marked the latest wait by far for the first such tornado in any year in modern records dating to 1950.”

Watch Tornado Rip Through Missouri 0:40

Those stronger twisters are also the deadliest: Some 83 percent of deaths were from EF3-5 twisters, according to Greg Forbes, the severe weather guru at The Weather Channel. So what to expect ahead?

“I have no expectations for May or June,” Brooks said. “What's happened earlier in the year tells us almost nothing about what will happen later.”

But Brooks also noted that April is typically more active than earlier months and that May and June are “the most active."

“I expect April to come in below average simply because we're so far below average that even a big day today won't catch us up,” he added. “We're probably almost 100 (tornadoes) below normal right now for April. That would require one of the biggest days in history to catch up.”

Forbes, for his part, noted that the record shows major tornadoes, EF3 or greater, are more likely in the coming months. While January to March averaged just over eight major tornadoes between 1950-2012, he said, that jumped to nearly 11 major tornadoes a month in April and May.