Initial jobless claims jumped last week, the Labor Department said, tempering Thursday morning's optimism on Wall Street.

Initial jobless claims rose by 7,000 last week to 428,000. Economists were expecting a rise of 1,000 claims for the week, according to a Dow Jones Newswires report.

A spokesman at the Labor Department attributed the increase to layoffs at Midwestern businesses that were hit by tornadoes that swept through the region.

This marks the 14th straight week that initial claims have been above the 400,000 mark -- a level that signals economic instability.

On the bright side, the four-week moving average fell 7,750 to 433,000, its lowest level since April 12. This indicates that businesses may be starting to dismiss workers at a slower rate.