An index measuring San Diego’s economy increased 0.6 percent in September, the fourth consecutive time it’s risen and the largest monthly spike since February 2013.

Alan Gin, the University of San Diego professor who compiles the Burnham-Moores Center for Real Estate Index of Leading Economic Indicators for San Diego County, said in September five of the six components increased, led by a decline in the number of unemployment insurance claims, and the outlook for the national economy.

Other gains in the September index came from a rise in local stock prices, local consumer confidence and help wanted advertising. The only negative component was a sharp drop in building permits.

“The forecast remains for strong growth in the local economy, which is poised to add another 30,000 jobs in 2015,” Gin said.

Another factor that should boost the local economy in the short term is a recent plunge in gasoline prices (about 30 cents in the last month).

Each 1 cent of decrease in the gas price translates to about $1 million a month for local consumers, who can spend the money on a variety of other things, Gin said.

Other benefits of lower gas prices include lower prices for products shipped into the county and a boost in consumer confidence, he said.