May is one of the more enjoyable months to visit Canyonlands National Park in southern Utah thanks to moderate temperatures and available water, but for a college professor from Minnesota a backcountry trek apparently has gone amiss.

Professor Jerry Wolff, who teaches biology at St. Cloud State University, was scheduled to return last Friday from a four-day solo excursion into the backcountry of the park's Needles District. When he didn't return on schedule, the park launched a search.

"He's in an at-large backpacking zone, which makes it a little bit more problematic for us," park spokesman Paul Henderson said Wednesday, explaining that since the professor had no designated campsites searchers weren't sure exactly where to look. "We could have a giant search party in one slot canyon and you could be in another slot canyon a quarter mile away and we wouldn't know it."

During the weekend the search involved aerial spotters and dog teams in the Horse Canyon area of the Needles District. Now the search has been scaled back.

"We have not found a single clue. But again, that's not that unheard of just because of the terrain we're in," Ranger Henderson said.

Professor Wolff, 65, was not conducting research in Canyonlands, but rather simply looking for a backpacking trip, according to the ranger.

"We know he flew into Moab, he hired one of the local shuttle companies to take him to the trailhead, we know when he picked up his permit at the visitor center," said Ranger Henderson. "We knew he was due to be out on Friday....We just have not seen a sign of him since they issued him a permit and the shuttle driver dropped him off at Peekaboo."

Earlier in his career the professor did some graduate work at Northern Arizona University, so he had spent some time previously on the Colorado Plateau.

While temperatures in Moab approached 100 degrees on Tuesday, by Wednesday a cold front had knocked down the heat. Water should be relatively plentiful at this time of year in Canyonlands' backcountry thanks to ephemeral streams and water pockets.

"He's had pretty decent weather," said Ranger Henderson. "May is a lot safer month to be there than say July."