Traveling to Frisco, Texas to face FC Dallas in the second leg of the series, the Seattle Sounders have a really simple advancement scenario - win and they move on, draw and they move on. There are certain losses where they advance and certain losses where they do not, and even a loss that forces extra-time and maybe even penalty kicks.
But here's the simplest scenario: If the Sounders win or tie, they advance, no matter the score.
It gets a little more complicated, otherwise. A 2-1 Dallas win is the only scenario that would lead to overtime. If the Sounders lose by one goal and score more than one, they advance to face the winner of Vancouver and Portland. If the Sounders lose 1-0, FC Dallas advances. If the Sounders lose by more than one goal, they fail to advance.
Steve Fenn has this handy chart of the various scenarios.
Fixed: MLS 2nd leg possibilities with proportions of historic results. (Earlier version driven by POR-VAN typo) pic.twitter.com/yqPDbm3O4b— Steve Fenn (@StatHunting) November 2, 2015
Earning a shutout is the easiest way to advance. Giving up just a goal, which is lower than their average during this 11-match unbeaten versus MLS streak, also has a strong chance of advancement since the Sounders are scoring two goals a game for two months now.
If Seattle scores that two goal average they are nearly certain to advance.
But again, the simplest thing is to draw or win as that forces MLS to watch an all-Cascadia Western Conference Final.