SAN FRANCISCO (CBS SF) — With gas soaring near $4 a gallon in San Francisco, a petroleum industry groups says it’s best to avoid the weekend if you are looking for a deal on a fill-up.

According to California AAA, the average price for a gallon of unleaded this week in San Francisco was $3.98 while pumps in San Luis Obispo and Mono counties have crested above $4 a gallon. The statewide average in California is up 20 cents a gallon, or 5%, to $3.83 in just the last week.

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“Motorists in California have been hit hard with surging gas prices in the last month, but the situation is about to get worse,” said Patrick DeHaan, head of petroleum analysis at GasBuddy.com. “GasBuddy is predicting that the average price for gasoline will soon rise to the highest level since 2015.”

DeHaan’s industry group published its latest gas price statistics on Wednesday, showing that if you are looking for a deal avoid the pumps on the weekend.

“Finally consumers have reason to be motivated about Monday since it offers the biggest savings on gasoline and little wait, if any, to fill up,” DeHaan said. “As the week progresses and our excitement builds for the weekend, gas prices also have a tendency to rise.”

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Friday and the weekends are to be avoided, DeHann said.

“The most expensive day to fill-up barely remains Friday when looking at averages, while 16 states saw Saturday have the highest average price,” he said. “Sundays represented the third-worst day to fill up, bringing our study to a conclusion that weekends are the worst time to fill up. You might as well flush money down the toilet.”

GasBuddy cited a combination of problems causing the surge in prices.

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“Refinery problems have plagued the state and the entire West Coast as gasoline supply remains tight due to the change-over to summer gasoline,” he wrote in an email last week. ” That has meant refineries must start building inventories from scratch to meet seasonally stringent specifications for gasoline. In addition, several California refineries are also in the midst of planned maintenance, further tightening supply.”