A powerful swell out of the northwest will slam San Diego County beaches this weekend, producing 4- to 8-foot waves, with sets to 10 feet, at favored breaks, particularly south of Torrey Pines, says the National Weather Service.

The conditions are likely to lead lifeguards to close the Ocean Beach Pier for part of the weekend, for the second weekend in a row.

The greatest threat people face is being caught unaware by a breaking wave and getting knocked down, or even washed out to sea, said Mark Moede, a weather service forecaster. You should not turn your back on the ocean.

The threat is heightened at this time of year. There are fewer lifeguards at the coast. And the ocean is cold. Local sea surface temperatures are in the 56 to 59 degree range.

Lifeguards say you shouldnt turn your back to the ocean during a big swell

(Gary Robbins /The San Diego Union-Tribune )

Moede said the swell will arrive on Friday and will peak on Saturday. The waves will moderate on Sunday, then become dangerously big again on Monday. A high surf advisory will be in effect through 2 p.m. on Tuesday.

A large swell hit greater San Diego last Sunday and hit and, in some cases, knocked over beach-goers in Sunset Cliffs who got too close to the ocean.

Temperatures will be in the seasonal range, topping out at about 65 degrees in San Diego on Friday and Saturday. It could be even chillier at local beaches due to the sea breeze.

San Diego will reach 70 on Sunday when weak Santa Ana winds flow from the mountains to the sea.

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Dangerous waves to slam San Diego beaches this weekend - The San Diego Union-Tribune