PHILADELPHIA (AP) — Weather officials say Pennsylvania has already recorded the average number of tornados for a year.

Meteorologist Sarah Johnson of the National Weather Service office in Mount Holly, New Jersey says 16 tornadoes that have had preliminary confirmation in the commonwealth so far this year.

Severe weather is more common in June and into July, but Johnson cautions that that is by no means a hard and fast rule. For example, a large outbreak of severe weather was recorded Oct. 2 of last year across the commonwealth, she said.

"The high plains and mid-Atlantic have been getting the brunt of low-pressure systems that have been bringing precipitation," she said. The southeastern United States, by contrast, is "getting hot and humid weather but not much in the way of thunderstorms."

On Thursday, six tornado warnings were issued across the commonwealth in a span of just two hours.

Data on storms goes back to 1950, but Johnson said many storms may not have been recorded before the availability of Doppler radar systems.

"Back before the time of Doppler radar, a lot of times a tornado may have occurred but we wouldn't be aware of it, and depending on population density it may not have done damage that anyone would have noticed," she said.

Eric Horst, director of the Weather Information Center at Millersville University, said a Pacific El Nino is still enhancing the jet stream, so the atmosphere is highly dynamic with all of the fronts moving through, the York Daily Record reported.

"Some years are more active than others — it's a cycle we go through every few years," he wrote.