MESA, Ariz. -- As the A’s prepare for Thursday’s Cactus League opener against the Mariners, Mother Nature is readying to wash it away.

Heavy rain is in the forecast, though the A’s hold out hope that it will dissipate by first pitch, which is scheduled for 12:05 p.m. PT.

Should all go according to plan, rotation candidate Paul Blackburn will toe the rubber against Seattle lefty Marco Gonzales at Hohokam Stadium; otherwise, the right-hander may be forced into simulated action if the A’s want to stay on schedule.

Right-hander Daniel Mengden, who is also vying for a rotation spot, is penciled in for Friday’s rematch with the Mariners on the road.

The A’s tentative lineup for Thursday’s game features newcomer Robbie Grossman in the leadoff spot and playing left field. The switch-hitting Grossman, who sports a career .355 on-base percentage, will “probably get some time there” during the regular season, according to Athletics manager Bob Melvin, “so we’ll break him in quickly.”

“One of the reasons we picked him up,” Melvin said.

The two-hole has been reserved for Marcus Semien, but it will be Matt Chapman who sees the bulk of time there this year. The third baseman was held out of workouts because of an illness Wednesday and, as expected, won’t be seeing in-game action for at least a week while he completes his rehab from left shoulder surgery.

Melvin will begin experimenting with the No. 3 spot in the lineup as he seeks to replace Jed Lowrie. Stephen Piscotty and Matt Olson are the leading candidates to bat in front of Khris Davis, with the former drawing the start there Thursday. Melvin also said he could occasionally consider Chapman for the job, too.

Thursday’s starting nine also features new A’s second baseman Jurickson Profar and center fielder Dustin Fowler. The full lineup behind Blackburn:

Grossman, LF

Semien, SS

Piscotty, RF

Matt Olson, 1B

Mark Canha, DH

Profar, 2B

Chad Pinder, 3B

Josh Phegley, C

Fowler, CF

Worth noting

Right-hander Joakim Soria, slowed by right hip tendinitis, made an “amazing recovery yesterday,” Melvin said Wednesday.

Soria is expected to resume throwing off the mound this week.