The former Scottish Amateur champion is used to the more pleasant climes of California, but the San Diego University student acclimatised to the miserable conditions in his homeland and carved out a battling four-under 68 over the Old Course for a nine-under aggregate of 134 to finish two shots clear of New Zealand prospect Tae Koh.
"That was brutal," said Forrest. "As we got to the turn, we were all dreading having to play the back nine."
Forrest made an early surge on the front nine and reeled off a trio of birdies at the fourth, fifth and six. In the worsening weather, the Craigielaw golfer held on and, despite leaked shots at the 11th and 17th, a brace of birdies, including a raking putt of 50 feet at the 13th, helped to limit the damage.
It was a day where potential calamity lurked at every turn and overnight leader Gregoire Schoeb suffered more than most. The Frenchman had opened with a stunning 11-under 60 on the New Course on Friday but took 19 shots more on the Old Course and trudged home in 43 en route to a damaging 79.
In the professional ranks, at the Lyoness Open in Austria, Scott Henry enjoyed a better day following on from his 75 on Friday. He goes into today's final round in joint 10th following a third-round 68, but he is still seven shots off the lead, held by defending champion Joost Luiten.