Sounders FC (9-4-8, 35 points) extends their club-record unbeaten streak to eight straight matches (5-0-3) and improve their record to 4-2-5 away from CenturyLink Field this season, as they surpass FC Dallas and move into sole possession of second place in the Western Conference standings.
The Timbers (5-9-3, 16 points) are still searching for their first win since May 21, falling to 0-6-1 since that time.
Alonso's game-winning penalty kick came after Seattle's Lamar Neagle was taken down in the box by Portland's Eric Brunner. Brunner was issued a red card on the play. Alonso put away the ensuing penalty kick low and to his right, past Portland goalkeeper Troy Perkins, giving Sounders FC their first lead of the day, 3-2.
The Timbers held a 1-0 and 2-1 advantage in the second half, but Montero's second-half heroics kept Sounders FC in contention.
The first of Montero's equalizers came in the 57th minute when he cracked a free kick over the wall of Portland defenders and into the corner of the net from 27 yards out for his fourth goal of the season.
Then, in the 74th minute after the Timbers had taken a 2-1 lead, Neagle collected the ball on the left side of the box and crossed it to Mauro Rosales on the right side of the box. Rosales volleyed the ball back to Montero who was alone in front of the net and Montero knocked it home for his second goal of the match and fifth of the season, equalizing the score once again, 2-2.
Montero's five goals are a club-high. Rosales leads the team with five assists.
Portland's first goal came in the opening minute of the second half when Kalif Alhassan took the ball up the right side and sent a cross into the box toward Jorge Perlaza. The ball found the feet of Seattle defender Jeff Parke at the near post, who was marking Perlaza on the play. Parke redirected the cross through the legs of Seattle goalkeeper Kasey Keller for an own goal that put the Timbers up 1-0.
It was the first road goal allowed by Sounders FC in their last 319 minutes of play.
Portland regained a 2-1 advantage in the 70th minute, when Perlaza got out on a run up the left side and sent a cross into the box that was deflected by Seattle defender Tyson Wahl and into the back of the net.
At halftime, Seattle's Keller and Portland's Jack Jewsbury were named to the MLS All-Star First XI. The 2011 AT&T MLS All-Star Game takes place on July 27 at Red Bull Arena.
Sunday afternoon's match marked the third of four Cascadia Cup fixtures for Sounders FC and the second of four for the Portland Timbers, in a competition that also includes Vancouver Whitecaps FC and is organized by the clubs' supporter groups. Sounders FC improves to 1-0-2 and Portland falls to 0-1-1.