How can Portland play Seattle in the play-in round?

Thanks to Drew's work this week on playoff scenarios, enumerating many of the scenarios that would lead to a potential repeat of this is now a simpler task. One interesting note I discovered working through these scenarios: there is only one scenario related to Seattle and Portland in which today's goal differential might matter. If San Jose ties, and Seattle loses by at least three goals, then San Jose could take Seattle's seed. Otherwise, no goal differential today, no matter how lopsided, can determine the fate of Seattle or Portland.*

Here are the results that would lead to a Timbers-Sounders one-game playoff.

Portland hosts Seattle: 20% Probability

Portland is currently sitting fourth and Seattle is in fifth, which would, of course, lead to Portland hosting a play-in game midweek. There is no way that Sporting and Vancouver or Sporting and LA somehow switch places and leave Portland and Seattle in fourth and fifth, so this first-round scenario only holds if everyone just stays where they are (relative to the Timbers and Sounders, that is). The probability of this is 13%, and an example from Drew's spreadsheet is below. 

If Portland got up to third, and Seattle dropped to sixth, Portland would again host a play-in game between the two. This requires Portland leapfrogging just one of Vancouver and LA, while only one of Sporting and San Jose takes fifth from Seattle. The probability of this is 7%, and an example from Drew's spreadsheet is below. 

Seattle hosts Portland: 13%

Scenarios where Seattle finishes ahead of Portland first and foremost require that Seattle beats RSL, and Portland musters no more than a point at home against Colorado. In the case of a Portland draw, Seattle would win the tie-breaker on season wins 15 - 14, so again, goal differential just doesn't matter. Specifically, for Seattle to finish fourth and Portland fifth, Sporting can't beat LA because that would bump Portland down, and Vancouver would have to beat Houston so that Seattle didn't jump into third. The probability of this is 7%, and an example from Drew's spreadsheet is below. 

Finally, the least probable of any of these scenarios is Seattle finishing third with Portland in sixth. This requires not only a Seattle win and Portland draw/loss, but Seattle would also have to jump just one of Vancouver and LA while Portland would have to be taken over by only one of Sporting and San Jose, meaning that this scenario additionally hinges on the Sporting-LA result. The probability of this scenario is 6%, and two examples from Drew's spreadsheet are below.

If LA beats SKC, then Vancouver must draw/lose for a SEA3 - POR6 matchup.

If SKC beats LA, then Vancouver must win for a       SEA3 - POR6 matchup.

Notes:

*Some tricky seeding scenarios involving Portland and Seattle include:

If Vancouver ties and Seattle wins. They'd be tied on points, tied on wins, so it would seem as though goal differential today would matter. However, because Seattle and Vancouver are currently tied on season goal differential 6 - 6, any win by Seattle and tie by Vancouver results in Seattle taking Vancouver's seed because the Sounders would have to have a higher season goal differential at that point. A Sporting KC win and Portland draw leads to similar goal differential logic, as would a San Jose win and Portland loss. All three of these result in ties in the standings at the points and wins level, but logically there is no variance in the outcome due to specific goal differentials.

**The margin of error on these probabilities due to simulation is about half a percent.