Union manager Peter Nowak expressed disappointment that his team couldn't finish off the win.
"At some point, we need to not be afraid to score a second goal," Nowak sad. "With the wind and rain we didn't expect to play a beautiful game, but still, simple passes from A to B need to be delivered."
Seattle (1-2-3, 6 points) had most of the possession early in the game, but as the weather got worse, the Union (3-1-1, 10 points) saw more of the ball.
In the 31st minute, a good buildup through the middle of the field led to Kyle Nakazawa being fouled about 25 yards from goal by Seattle's Osvaldo Alonso, who was given a yellow card for the tackle.
Carlos Ruiz stepped up to take the free kick. He spun the ball to the right of a diving Keller - perhaps with the help of a deflection along the way, though there wasn't much of one - and put the Union up, 1-0.
After the game, Ruiz admitted that he had asked the permission of Sebastien Le Toux, the Union's usual free kick-taker, to take the shot.
"I asked for the ball, everybody gave me confidence, and I tried to shoot," Ruiz said.
The Union had a great chance to double their lead early in the 51st minute. Ruiz deftly trapped a long ball that was caught in the wind, then played a square pass to an onrushing Le Toux. The French forward was wide open as he ran into the box, but he shot low and straight at Keller.
Seattle had a number of opportunities to tie the game late in the second half, but only broke through on its very last chance.
In the 91st minute, the Sounders won a corner kick. Mauro Rosales served the ball up, and it was brought down by Fredy Montero. He passed the ball to Steve Zakuani, then got it back at the top left side of the 18-yard box.
Montero then sent the ball into the a scrum of players in front of the net. With Mondragón stuck in traffic, Fernandez got open and headed the ball into the net off the right post.
"Today was one of those things that can happen in soccer," Mondragón said. "A last-minute goal with a lot of rain and a slippery ball, and we just found ourselves in an unfortunate situation."