BOISE, Idaho -- A driving rainstorm kept Boise State's high-scoring offense grounded for two quarters Saturday night. But once the clouds parted, the floodgates opened as the No. 15-ranked Broncos pulled away and then held on for a 28-23 home victory over Colorado State.

Star tailback Jeremy McNichols ran for a career-high 217 yards and two touchdowns and the Broncos shook off a slow start by scoring touchdowns on their first three possessions of the second half.

"It's a W," Boise State coach Bryan Harsin said. "There's nothing weird about that, it's a win. Watch college football. Every single week, it's hard to win every single week. There's no apologies for a W."

Brett Rypien threw for 207 yards and tossed a scoring pass to Thomas Sperbeck, who also threw a TD pass out of a wildcat formation for the Broncos (6-0, 3-0 Mountain West Conference). But Boise State had to weather some nervous moments down the stretch, as Colorado State (3-4, 1-2) recovered back-to-back onside kicks and scored three touchdowns in a 1:25 stretch late in the fourth quarter.

"We dug ourselves a hole, but I'm proud of our guys and how we fought back," Colorado State coach Mike Bobo said. "It wasn't the fourth quarter we envisioned, but we put ourselves back in position to get a stop and have a chance to win the ball game."

With the Broncos clinging to a 28-23 lead, receiver Cedrick Wilson recovered the Rams' third onside kick attempt. The Broncos couldn't quite kill the rest of the clock, but a desperation pitch on Colorado State's last-ditch play was ruled a forward lateral with four seconds left, resulting in a game-ending penalty runoff.

Boise State held a slim 7-3 lead at halftime, but with more favorable second-half weather conditions, the Broncos' offense came alive. McNichols capped a 10-play, 79-yard drive with a 5-yard TD run to make it 14-3.

On the next Boise State possession, McNichols burst through the line and sprinted 80 yards down the sideline to make it 21-3.

"We called that play earlier in the game and I felt like I almost broke that one, I just got tripped up," said McNichols, who had a school-record 40 carries. "(Harsin) asked me what play I thought would be good ... we just called it again and broke it for 80."

The Broncos appeared to put the game out of reach early in the fourth quarter, marching 90 yards in 16 plays. On the touchdown, Sperbeck took the snap and ran an option to the right before pulling up and firing an 11-yard strike to A.J. Richardson.

But Colorado State nearly mounted an incredible comeback. It started with a long drive capped by tailback Izzy Matthews, who scored on a 2-yard run to cut the lead to 28-10 with 5:53remaining.

After a successful onside kick, Nick Stevens fired a strike to Michael Gallup, who broke a tackle and raced 53 yards for a one-play scoring drive.

Colorado State used the same double-kicker, misdirection setup on its second onside kick and recovered again. Tailback Dalyn Dawkins scored 1:13 later on an 11-yard catch-and-run to make it a five-point game, with the Rams missing the two-point conversion.

"We just came out and made plays," Stevens said of the fourth-quarter rally. "I think we were more relaxed, and when we had to pass the ball more there towards the end, our offense got a little more into rhythm."

McNichols and the Broncos used all but 15 seconds of the clock before the Rams' multiple lateral play came up short.

"We really wanted it," said Gallup, who finished with 91 yards receiving. "The ball just went the wrong way. We needed it, but we didn't get it."

Sperbeck scored the game's first touchdown just 27 seconds before halftime. His 12-yard scoring catch from Rypien capped a well-executed Boise State 4-minute drill that covered 75 yards in 10 plays.

With the rain coming down in sheets, offense was hard to come by early. Boise State turned the ball over on downs to open the game when Sperbeck dropped a pass in the end zone on fourth-and-13 from the 38.

Colorado State responded with a 10-play drive, resulting in a 39-yard field goal by Wyatt Bryan. The kick gave the Rams a 3-0 lead -- and put the Broncos behind on the scoreboard for the first time this season.

That deficit nearly lasted the rest of the half, with eight consecutive possessions ending in punts. Boise State's Sean Wale was particularly effective, averaging 52.5 yards on four kicks.

Boise State outgained Colorado State 171-141 in first-half offense. Neither team committed a turnover.

NOTES: Colorado State played without true freshman QB Collin Hill, who is out for the season with a torn ACL. Redshirt junior Nick Stevens started in Hill's place. ... Boise State starting WR Cedrick Wilson did not play in the first half, though he was in uniform. He played a few snaps in the second half, including the onside kicks, but wasn't targeted. Wilson has 432 yards receiving and five touchdowns this season.