SEATTLE, Wash. — The first bit of celebrating by the Portland Timbers was a bit premature. The confusing MLS playoff rules created a moment of misunderstanding at the end of extra time and a wild 120 minutes.

The second celebration, after Dairon Asprilla deposited his winning penalty past Seattle Sounders goalkeeper Stefan Frei, was pure elation for the Timbers as they advanced to the Western Conference finals and ended the season for their Cascadia rivals to the north.

"I thought it didn't disappoint. Seattle and Portland brought the rivalry to this match and it was incredible," Timbers coach Giovanni Savarese said.

Asprilla's kick sent the Timbers past Seattle 4-2 in a penalty shootout Thursday night after an emotionally exhausting second half and extra time that saw the teams combine for five goals and wild swings of elation and desperation.

Seattle won the second leg of the semifinal matchup 3-2, scoring late in regulation to force extra time before both teams scored early in the extra session. Eventually, the sides decided the winner via penalties, although Portland initially believed it had won at the end of extra time after scoring a second road goal in the match.

The Timbers claimed the celebration at the end of extra time was due to pride and exhaustion for taking the match to penalties. It ultimately didn't matter because the Timbers finished the job in the shootout.

Asprilla, Lucas Melano, Diego Valeri and Sebastian Blanco scored penalties for Portland. Seattle's goals came from Raul Ruidiaz and Handwalla Bwana, but Will Bruin hit the post and Osvaldo Alonso's shot was saved by Portland goalkeeper Jeff Attinella, atoning for an earlier mistake that led to Seattle's first goal.

"Being a goalkeeper, you have to shake that off because you always know another one is coming," Attinella said. "All the credit to the guys — they responded so well for me. I'm just very grateful for the way it turned out."

Portland will face Sporting Kansas City or Real Salt Lake in the West finals. It's the Timbers' first trip to the conference finals since winning the MLS Cup in 2015.

"The end result was not good. But certainly, there were moments of joy and euphoria, happiness. Thinking you had the game in control," Seattle coach Brian Schmetzer said. "And then that gets snatched out away from you."

Seattle played without standout midfielder Cristian Roldan and center back Chad Marshall. Roldan strained a hip flexor in the first leg and was considered a game-time decision but was not included among Seattle's starters or subs. Marshall tore the meniscus in his right knee and underwent surgery earlier this week.

"We showed a lot of heart; just that we care. We didn't give up and that's been a trademark of this team," Frei said. "Somebody has to win. We would have liked it to be us."