MOREHEAD CITY, N.C. (AP) — Five people have been killed in rip currents this summer at North Carolina beaches in what is being partly attributed to a rare meteorological event.

The News & Observer of Raleigh reports a long swell period produces bigger, rougher surf and a greater chance of rip currents. Swell periods refer to the distances between developing waves.

Shane Kearns, a National Weather Service meteorologist in Morehead City, says long swell periods are a rare occurrence on the state's central and southern coast, where all the deaths occurred. Kearns says the wave patterns are not predictable more than a week in advance.

Local officials say waters have calmed, with emergency calls having regressed to the average after the mid-June spike.