Another in a series of low pressure systems is moving from west to east across the U.S. and is expected to drive Hurricane Igor away from the U.S. mainland, according to Jeff Masters, meteorological director for the Weather Underground website. In low-pressure systems, air tends to rise, cooling and forming clouds in the process.

"That's been the steering pattern this year," Masters said. "It's been pretty constant since June—a steady progression of low-pressure systems moving off the East Coast."

Low-pressure systems, he added, "are just a normal part of the weather. ...