Craig Mitchelldyer/Portland Timbers

On Sunday afternoon, Providence Park will play host to its biggest game since the first leg of the 2015 Western Conference championship: a Decision Day clash between the Portland Timbers and Vancouver Whitecaps, with number one seed in the Western Conference playoffs and the Cascadia Cup on the line (1:00 p.m., TV on ROOT Sports).

The scenarios are simple: with a win, the Timbers will win the West. With a draw or loss, they could finish anywhere from second to fourth. The top two seeds get a bye through the Wild Card round next week. After a long season, it all comes down to this.

The History

One year ago Monday, the Timbers went to BC Place on the final day of the MLS regular season needing only a win against Vancouver to clinch a postseason birth.

It didn't happen. The Whitecaps — long gone from playoff contention and playing for nothing but pride — walloped Portland 4-1. They led 4-0 after 55 minutes. It was a spectacular meltdown from the Timbers, who ended the 2016 season without a single road win.

It wasn't the first time that one of these clubs ended the other's season. In 2015, en route to their MLS Cup championship, the Timbers upended the Whitecaps in the Western Conference Semifinal — winning a dominant 2-0 victory in the decisive second leg in British Columbia.

The Timbers have taken the previous two meetings between the teams this season, with the 2-1 win in Vancouver in late July snapping a six-game winless run. It was a turning point. Since then, the Timbers have gone 5-2-2.

The Tactics

Caleb Porter's big call last weekend was to reinsert Alvas Powell, after two months out of the lineup, at right back. Powell, to say the least, took his chance — scoring a goal and playing his best game of the season in the Timbers' 4-0 win over DC United. Powell is expected to start again on Sunday, as is Vytas at left back.

Outside of the fullback spots, the Portland lineup is fairly settled. Fanendo Adi is getting closer to returning up top, and the Timbers would love to get him some time before the playoffs start, but Darren Mattocks has been effective in October and he scored against the Whitecaps earlier this year. Even if Adi is available off the bench, it'll be Mattocks to start.

Vancouver, last in the league in possession, will be to let the Timbers have the ball. They'll do their damage on the counter and from set pieces, where they have been dynamite this year.

The question for Porter, then, is how much he wants to risk his fullbacks. Last weekend against DC, both Vytas and especially Alvas Powell had no restraints whatsoever on their ability to get forward. But with Vancouver's countering threat in mind, they might be deployed more cautiously on Sunday.

Whitecaps coach Carl Robinson has a decision to make in goal, where longtime starter David Ousted — who appears set to leave the club in the offseason — has been replaced in recent weeks by Stefan Marinovic. The other big question for Vancouver is the fitness of Costa Rican winger Christian Bolaños, who hasn't played since the end of September.

Robsinon told the media this week that "Portland are clearly favorites" in this game. It was a telling comment. Despite the fact that they lead the conference, the 'Caps still feel most comfortable as underdogs. For a club that has never won a playoff game, it's not an entirely comforting stance.

The Lineup

1 - Attinella
5 - Vytas
24 - Ridgewell (C)
33 - Mabiala
2 - Powell
20 - Guzman
21 - Chara
6 - Nagbe
8 - Valeri
10 - Blanco
11 - Mattocks

The Pick

The Timbers are 5-0-0 in their last five home games, having outscored opponents by a combined 16-3. They're playing very, very good soccer — and they owe the Whitecaps after what happened at BC Place last year. 3-1 Portland.