A late try from flanker Tom Rooke and an impressive touchline conversion from flyhalf James Bailes put Cal up 14-13 in the 74th minute of the first match of their annual two-game series with Univrsity of British Columbia, and the Bears eventually pulled away 21-13 in windy, wet conditions at Treasure Island in the San Francisco.
UBC led 13-0 at halftime and it wasn't until six minutes to go that Cal took the lead.
Reserve winger James McTurk added a late try to end the Bears' scoring, which began with co-captain Blaine Scully's try to end the first half.
Bailes converted all three as Cal (15-0) exited the tense match in muddy conditions with an eight-point advantage going into the rematch in Vancouver, Canada, on March 24.
The 91st edition of the World Cup series, decided by total points over two matches, got underway with the Bears finding numerous ways to put themselves on the back foot, committing handling errors, dropping kicks and committing penalties deep in their own territory.
Thunderbirds fullback Dave Hilborn connected on two penalty kicks in the first half to give his team a 6-0 lead, followed by a try from UBC flyhalf Charlie Jones to make the score 13-0.
Scully's try and Bailes's conversion allowed the Bears to exit the first half down by less than a converted try at 13-7, and Cal came out of the locker room back into the pouring rain determined not to make so many mistakes in the final 40 minutes.
The T-Birds failed to widen the gap as Hilborn missed two penalty kicks from 40 and 25 meters, keeping the UBC lead at six. Meanwhile, Cal kept the ball in its opposition's side of the pitch and capitalized when it won a lineout from 10 meters and hooker Neill Barrett fed Rooke coming to the short side, scoring the try but leaving a difficult conversion from the corner that Bailes lined up and made in front of a standing-room crowd.
"I tried to block everything out and give it everything I had," Bailes said. "I'm just lucky that the hard work of the team paid off. Going into future matches, we know that we can do that now."
Cal finished off its better half by causing another UBC turnover, regaining possession and putting McTurk into the right corner with three minutes to play. Bailes's final conversion sealed the win for the Bears.
"It went back and forth and we snatched victory from the jaws of defeat, really," said Rooke. "We've got a lot to work on." Looking ahead to the rematch, he said UBC "is going to be up for it again and we're going to have to be ready."
"The weather was a big factor but it goes for both teams, and both teams adjusted pretty well," said UBC head coach Spence McTavish. "In the second half we had lost opportunities and came away with nothing, and when that happens, that's kind of a sign."
"It's going to be close when you play UBC," said Bears head coach Jack Clark. "We have a lot of respect for them as a team. They're always going to have the run of play for large parts of the game and that was difficult for us. We had to play more defense today than in the sum total of our games previously. We put some fixes in that I was really happy with and I was pleased that we looked after the ball better. I think the guys came out of the locker room [at halftime] with something to prove." California 21
Tries: Scully, Rooke, McTurk
Convs: Bailes 3
Pens: Hilborn 2