NEW YORK (AP) – Gasoline prices rose again at the pump Friday, extending a trend that analysts believe will continue until mid-September because of an end-of-season shortage of summer blend gasoline.

In the futures market, meanwhile, prices were mixed as investors kept an eye on tropical weather patterns.

Refiners are running low on summer-grade gas, a mixture designed to emit fewer pollutants, analysts said. Refiners try to run down their supplies of the more expensive summer grade gas just in time for Sept. 15, when stations can legally begin selling winter grade gas, he said. But this year, that run down came a little early.

Gas prices at the pump rose 0.8 cent overnight to a national average of $2.777 a gallon, according to AAA and the Oil Price Information Service. It was the third increase in as many days.