PITTSBURGH, SEPT. 6 -- Three Rivers Stadium had a kind of apocalyptic feel to it tonight. Forbidding storm clouds left the field in near-complete darkness a few hours before game time and an alarmingly intense deluge of rain and lightning followed. It proved an appropriate setting for the at least temporary demise of the reeling New York Mets in the National League East.

"Maybe someone's trying to tell us something," Mets Manager Bud Harrelson said as he received pregame reports of the downpour. His words proved excruciatingly prophetic.

Harrelson's club proceeded to dig itself a six-run hole in the first four innings en route to a 7-1 loss to the Pittsburgh Pirates before 32,528 that changed the complexion of this two-team sprint at the division title.

The Pirates got a three-hitter from rookie Randy Tomlin to win their sixth straight game, matching their season's-best streak, and gain their first three-game sweep of New York here since 1978. Pittsburgh is a season-high 25 games above .500 at 81-56, its lead over the Mets (77-59) up to 3 1/2 games.

The Pirates put this victory to rest quickly, making Mets starter Julio Valera's second major league outing a disastrous one. The rookie lasted but two innings in an eight-hit, five-run shellacking, and New York never threatened that early deficit.

Andy Van Slyke, Bobby Bonilla, Barry Bonds and Wally Backman -- the former Met -- each produced two of Pittsburgh's 11 hits. Bonilla had two RBI to establish a career high with 101 this season. Bonds got his 101st too. And the Mets were stumbling and bumbling afield and struggling at the plate again.

New York's vulnerability to left-handed pitching was exposed once more, with Tomlin -- a 24-year-old who began the season at Class AA Harrisburg (Pa.) -- the culprit. In his seventh big league start, Tomlin (2-2) struck out six, walked none, and held New York hitless the initial 4 1/3 innings. He baffled the free-swinging Mets with a crafty style that prompts comparisons from Manager Jim Leyland to former Baltimore Orioles mound technician Scott McGregor.

"It's the most incredible feeling I've ever had," said Tomlin, who lowered his ERA to 2.48. "This is the absolute thrill of a lifetime. . . . I was so nervous, but I just threw strikes and everything else fell into place."

New York has scored two runs in its last four games, and its underwhelming totals for this series were two runs and 11 hits. That's for a team that leads the NL in homers and came here two runs behind the Detroit Tigers as the majors' top-scoring offense.

But the Mets succumbed to three lefties in this series, leaving them 21-26 against left-handed pitching and prompting infielder Tim Teufel to grouse: "We'll probably see those wrong-handed guys 80 percent of the time from here on."

Said Mets shortstop Howard Johnson: "We didn't perform, and they socked it to us. We've got to be better if we hope to stay in this thing."

The momentum certainly has shifted. New York seemed primed to steamroll the division with its three-month binge that followed Harrelson's hiring. The Mets overtook the Pirates five days ago and appeared capable of winning going away. Pittsburgh began the week on the heels of a six-losses-in-seven-games drought.

The script seemed similar to 1988, when the Pirates battled the Mets through the the season's first half before dropping three of four games during a high-intensity, late-July series. New York pulled away to win the East by a comfortable 15-game margin.

Pittsburgh players have devoted much time and energy these past few days to analyzing that '88 debacle. They promise it can't be repeated, and they backed those words with a show of fortitude and big-game proficiency -- starting with Wednesday's doubleheader sweep -- that comes with maturity.

"We're two years older and two years smarter than 1988," Bonilla said. "We won't collapse again. The Mets may beat us yet, but it'll be because they beat us. It won't be because we gave it to them. We're going to get after people and show them who we are."

The Pirates went to work immediately tonight, grabbing a 2-0 lead in the first inning without the aid of a hit that reached the outfield. Backman led off with a chopper up the middle that he turned into an infield single, and Jay Bell followed by drawing a walk off Valera (1-1).

Van Slyke then sent a potential double-play grounder toward Tom Herr, but the Mets' second baseman botched the play for an error that loaded the bases. Backman scored when Bonilla hit into a double play, and Bonds made it 2-0 with an infield hit that brought Bell home.

Pittsburgh finished Valera and increased its advantage to 5-0 in the third. The first five batters in the inning produced hits -- doubles by Van Slyke and Bonilla and singles by Bonds, Sid Bream and Jeff King. A walk to Bell, a single by Van Slyke and Bonilla's sacrifice fly made it 6-0 an inning later.

Meanwhile, Tomlin was breezing. He retired the first 11 batters before shortstop Bell's error in the fourth, and he set down 13 of the final 14 New York hitters after Johnson's RBI single in the fifth.

"We've gotten a lot of great pitching performances lately, and this was another one," Leyland said. "For a young kid to step in and do what he did in this situation is just tremendous. It's a big boost."

The Pirates -- who last captured the division in their World Series-winning year of 1979 -- are quick to point out that although the Mets have finished first or second each year since 1984, New York hasn't won a tight race during that span.

But these teams still have five meetings remaining -- including three matchups here Oct. 1-3 to close the season -- and the Mets pledge they'll find a way to regroup and make those games meaningful.

"Whatever happens, I don't think we'll fold," outfielder Kevin McReynolds said. "They're not intimidated by us anymore, but we were never afraid of anyone to start with. We'll find a way to keep this thing interesting until the last moment."