Gas prices are about to soar dramatically, possibly increasing by 30 cents a gallon by the weekend, consumer advocates and an industry expert said Thursday.

The increases are tied to the forthcoming switch to summer-blend gasoline, which is designed to reduce pollution in warmer weather and costs more to produce. Wholesale prices jumped 51 or 52 cents a gallon this week because of the summer-blend issue, they said.

Consumer Watchdog, a consumer advocacy organization, scheduled a news conference at its Santa Monica headquarters this morning to warn motorists of the impending price spike.

“Consumer advocates will advise California drivers to take advantage of Thursday’s gasoline prices because they will be gone by Friday and keep climbing for the foreseeable future,” a Consumer Watchdog statement said before the news conference, forecasting a 30-cent-per-gallon increase.

“It’s true,” said Marie Montgomery of the Automobile Club of Southern California, adding that a distribution problem is also a factor.

The summer blend is to be in gas stations by April 1. Consumer Watchdog said “fire-sale prices on remaining winter stocks” are now at an end.

The average price of a gallon of self-serve regular gasoline in Los Angeles County dropped today to its lowest amount since May 6, 2009, decreasing eight-tenths of a cent to $2.361. The Orange County average price dropped one- tenth of a cent today to $2.346, its lowest amount since May 5, 2009.

According to Thursday’s warning, an imminent return to the $3-range is imminent.