High winds gusted and rain greeted the players with early tee times Thursday before eventually dying down throughout the round.
Rocca was one of the early players out and managed a 1-under 36 on his first nine, the back nine at Ballyliffin. It was the start of the second nine that moved Rocca up the leaderboard.
The Italian birdied three holes in a row, starting at No. 2, to reach 4-under par and take sole possession of the lead. Rocca stumbled into the clubhouse with three bogeys over his last four holes but still had a piece of the first-round lead.
'Some holes were very hard,' said Rocca, who won the inaugural North West of Ireland Open at Galway Bay in 1999. 'When you play a 120-yard 5-iron and have to hit the ball very low, it is very difficult. You just have to try to keep your timing and keep the ball in the right place.'
Forsbrand, who owns two top-10s in 2002, mixed four birdies and three bogeys Thursday to join Rocca and others at minus-1. The 41-year-old from Sweden was not complaining because conditions coupled with the difficulty of the course made 71 a good start.
'It was tough,' said Forsbrand. 'When I hit bad shots I got decent lies and managed to hole a few putts. If you stay away from bogeys you can do well.
'It is a course where you will make at least a couple of birdies but when it is that windy it is hard. Every hole is hard out there. There is not one let up. It is a great golf course but it is hard. And when it blows like that it is tough. You need to play well and get lucky.'
Hogh, who won the last two Danish Amateur Stroke Play Championships, flew out of the gate Thursday with a 40-foot chip-in birdie at his second hole, the 11th. He birdied the next hole and collected two birdies and three bogeys the rest of the way for a share of the opening-round lead in only his second start on the European Tour.
Arruti recovered from a double-bogey at his sixth with five birdies over his next 10 holes. He held the outright lead but bogeyed his 18th, the ninth, after landing in a bunker.
Florioli also had first place to himself with one to play but dropped a shot at his final hole, also No. 9, and settled for a tie for first.
England's Mark Sanders, Sweden's Pehr Magnebrant, Ireland's Damien McGrane and Niels Kraaij of Holland share sixth place at even-par 72. Titch Moore is alone in 10th place at 1-over 73.
Full-field scores from the North West of Ireland Open