ATLANTA — A franchise known for its postseason flops switched to fast-forward this time.

The Atlanta Braves went ahead and blew it before the playoffs.

Joining the Boston Red Sox in a pair of unprecedented September swoons, the Braves spent Thursday cleaning out their lockers rather than packing for an NL division series that seemed all but certain just a few weeks ago.

For reliever Jonny Venters — and, undoubtedly, many of his teammates — the end-of-the-season ritual followed a sleepless night.

“I was just sitting in bed,” he said. “I couldn’t really believe what happened.”

He’s not alone.

Atlanta had a 10 1/2-game lead on St. Louis in late August. The margin was still 8 1/2 games the first week of September. But the pitching faded, and the offense that had been a problem all year was finally exposed.

A dismal 9-18 record in the final month capped by a five-game losing streak finished off the Braves. The Cardinals claimed the wild card by a single game when Atlanta closed the season with another excruciating defeat, 4-3 to Philadelphia in 13 innings after closer Craig Kimbrel — who set a rookie record with 46 saves — squandered a lead in the ninth.

“Our goal was to make playoffs,” first-year manager Fredi Gonzalez said. “We wanted to keep putting pennants up there (on the stadium facade). It didn’t happen.”

The Braves have plenty of pennants above the left-field stands at Turner Field, most of them commemorating an unprecedented 14 straight division titles from 1991-2005. While impressive, most people remember all the championships they failed to win.

Only once during that streak did Atlanta win it all, and that was 16 long years ago. Four other seasons ended with World Series defeats. Nine more times, the Braves didn’t even make it that far.

After a four-year lull without a playoff appearance, Atlanta was back as a wild card in 2010. Again, there was postseason misery. Brooks Conrad made three errors to give one game away, and Bobby Cox’s managing career ended with the last of three one-run defeats to the eventual World Series champion San Francisco Giants.

This wasn’t even the playoffs, but it didn’t lessen the pain.

Heck, it might’ve hurt even worse.

“You can’t dictate when you’re going to go into a slide like that as a team,” said 39-year-old Chipper Jones, the only holdover from Atlanta’s lone World Series champion. “We started the season kind of sliding like that, turned it around, played good ball through the middle and really had ourselves in prime position to go to October.”

Then, it all fell apart.

In fairness to the Braves, they were short-handed down the stretch. Two key starting pitchers, All-Star Jair Jurrjens (13-6) and Tommy Hanson (11-7), were sidelined by injuries. The bullpen, led by the dominant threesome of Kimbrel, Venters and Eric O’Flaherty, lost some of its swagger. And the hitting totally shut down, averaging just over three runs a game the final month.

“Everybody started struggling at the same time,” Venters said. “Everything just kind of snowballed.”

Looking ahead to 2012 while the Cardinals get ready for the playoffs, Atlanta will undoubtedly need to address a lineup that seemed to have all the necessary parts — especially after acquiring leadoff hitter Michael Bourn at the trade deadline — but never found its groove.

The Braves ranked 13th in the NL with a .243 average and 10th in runs (3.96 a game). They were impatient, rarely working deep into counts, and hit under .200 with runners in scoring position over that devastating final month.

“If you look at our offense for the whole year, it never clicked for whatever reason,” Gonzalez said. “It never hit on all cylinders. The pitching covered it up for five months.”

Indeed, this was a pitching-dominated team that, even with the injuries, still ranked fourth in the NL with a 3.48 ERA.

In the end, that wasn’t enough to make up for a dismal season by 22-year-old Jason Heyward (.227, 14 homers, 42 RBIs), a hideous finish by All-Star catcher Brian McCann (.180 over the last 37 games) and a mediocre showing by 2010 All-Star Martin Prado (whose average dropped from .307 the previous season to .260). Dan Uggla got off to a miserable start, bounced back with a 33-game hitting streak and finished with a career-best 36 homers and 82 RBIs. But he still hit only .233.

The Braves are especially concerned by Heyward’s slide. After a breakout rookie season that earned him a trip to the All-Star game, he was slowed early by a shoulder injury and all year by his attempts to make adjustments to his swing that the Braves feel are necessary for long-term success.

“He’s too good of a baseball player, too good of a hitter to write that off as a sophomore jinx,” Gonzalez said. “But it’s not like he’s over the hill and can never get it fixed. This guy is still young. He’s just a puppy.”

The Braves have plenty of pitching depth, with rookies Brandon Beachy, Mike Minor and Randall Delgado all making solid contributions in a rotation led by Tim Hudson (16-10). The only flop was Derek Lowe, who battled mechanical issues, lost all five of his starts in September and finished 9-17 with a 5.05 ERA. The 38-year-old will likely return for one more season, since he has no trade value with $15 million left on the final year of his contract.

Atlanta has enough pitching to make a deal to bolster the offense, and there should be some wiggle room in the budget with Nate McLouth ($7 million, with a $1.25 club buyout that will surely be exercised) and Kenshin Kawakami ($7.3 million) coming off the payroll.

One thing that won’t be changing: the coaching staff. Gonzalez said everyone will return in 2012 — even hitting coach Larry Parrish, who took plenty of heat for the offensive struggles.

“It’s easy to fire someone and throw them to wolves,” Gonzalez said. “I don’t work that way.”

Driving to Turner Field on Thursday, Gonzalez spoke with his predecessor. Cox wanted to make sure his friend wasn’t taking the loss too hard.

Indeed, it was a tough start to Gonzalez’s tenure considering the guy he replaced — one of the winningest managers in baseball history.

“There’s probably nobody in Atlanta sports under as much pressure as he is filling Bobby Cox’s shoes. To have it slip away in late September, it’s cruel,” Jones said. “It’s not indicative of the way this team played, or the way he managed and what we deserve in this situation. But baseball’s like that. It’s kind to you one day and fickle the next.”