For at least two matches, Seattle Sounders fans will get to see what a grass pitch would look like at CenturyLink Field. This photo of the field was provided by JB Instant Lawn, who apparently finished installing the pitch on Friday. The grass pitch will be used for the July 16 match against the Colorado Rapids as well as the July 20 friendly against Manchester United.
As good as the field may look, though, I wouldn't get my hopes up about the quality of play being significantly improved over the current FieldTurf. Temporary grass fields are notorious for giving at inopportune times and are rarely adequate substitutes for the real thing. The Sounders also imported grass for a couple games in 2009.
Just in case you're wondering, this is unlikely to be a preview for next season. Although the Sounders have said there should be a new playing surface next season, it seems doubtful that it will be grass as its installation would be significantly more involved than just laying down sod and would have to include such technology as grow-lights.