After completing the Eastern and Western Conference playoff scenarios yesterday, it only makes sense to move on to each team's chances at the Supporters' Shield and a potential home-field advantage in the MLS Cup Final. Only four teams could mathematically win the shield: New York, Sporting KC, Portland and Real Salt Lake.
New York has the best chance due its current lead in the tables and the fact that it's playing a home game. If New York wins at home against Chicago, then it will be the Shield winner, regardless of other outcomes, but that's not the only way the Red Bulls could hoist the trophy. A tie against Chicago would eliminate both RSL and Portland from contention---since New York holds the "wins" tie-breaker over Portland---and then an SKC tie or loss would leave the Red Bulls as Shield winners, as well. In fact, even a loss from New York could leave them in first overall if SKC, Portland and RSL all don't win. However, it's not probable that both Portland and RSL would each earn less than three points against Chivas USA. In the end, New York's Shield chances sit at 73.7 percent, with 61.8 percent of that coming from its probability of beating Chicago this weekend.
Sporting Kansas City has the next-best chance at the trophy at 15.5 percent. Obviously it needs New York to lose or tie and then---due to SKC losing the potential tie-breaker to New York,---SKC would need to win. The only scenario where SKC ties and still gets the Shield involves crazy scenarios like an 8-to-8 tie with Philly.
Like SKC, Portland and Real Salt Lake both need to win, and then things need to go their way. RSL would (likely) hold tie-breakers over both SKC and New York, so RSL would need SKC to lose or tie, Portland to lose or tie, and then New York has to lose. Portland loses tie-breakers to SKC and New York, so it needs to win, and then have SKC lose or tie and New York lose. In the end, Portland's probability at the Supporters' Shield is just 6.0 percent, while RSL's is 4.8 percent.