Officials with a prominent regional conservation group continue to call for tweaks to the proposed Yellowstone National Park winter-use plan but are hopeful a lawsuit can be avoided.

The Greater Yellowstone Coalition is dissatisfied with the park’s justification for allowing more snowmobiles than are currently permitted, for proposing to allow unguided riders and for its plans to keep Sylvan Pass open to over-snow traffic. But the group, which has sued over Yellowstone’s winter use time and time again during the past 15 years, has landed on a “begrudging acceptance” of the plan, said Mark Pearson, the coalition’s conservation program director.

Pearson said he thought the plan could be workable as a long-term solution.

“We’ve always wanted to see a plan that protects the park’s air quality and soundscape and wildlife,” he said. “This latest iteration, with stronger [best-available technology] standards for snowmobiles and first-time-ever [best-available technology] standards for snow coaches is a good step toward doing that.”

Yellowstone’s plan, a supplemental environmental impact statement, is expected to be finalized into a rule “in coming weeks,” Yellowstone spokesman Al Nash said Tuesday.

Once the rule is issued, the park will open up a 60-day public comment period.

Under Yellowstone’s preferred alternative, interim regulations allowing 318 commercially guided snowmobiles and 78 snow coaches per day will continue for one more winter. In the winter of 2014-15, officials will shift to managing with “transportation events” — defined, initially, as a single snow coach or a group of seven to 10 snowmobiles.

Up to 110 transportation events would be allowed each day, but no more than 50 transportation events could be snowmobiles.

If snowmobile groups fill all their allotted slots, the limits translate into 32 more snowmobiles and 18 fewer snow coaches allowed per day.

The transportation event approach is designed to have less impact on park resources, Yellowstone officials say, while allowing for an overall increase in over-snow traffic.

While Yellowstone was drafting the plan, the Greater Yellowstone Coalition, Coalition of National Park Service Retirees and other groups criticized the Park Service for not including an explanation for the 7-to-1 ratio of snowmobiles to snow coaches. The final plan includes a 30-page section that justifies the formula.

“The [National Park Service] does not state the two types of transportation are equivalent,” the park’s analysis reads.

However, the document does not conclude that one vehicle is “cleaner, quieter or less harmful to wildlife” than the other.

“At the levels prescribed under the preferred alternative,” the document reads, “neither form of oversnow transportation will result in a level of adverse impacts on park resources that would necessitate an outright ban on that type of transportation.”

The Greater Yellowstone Coalition, Pearson said, still is “trying to wade through and come to a conclusion about” the 7-to-1 analysis.

In a late February interview with the News&Guide, Yellowstone superintendent Dan Wenk lauded his plan for beefing up air-quality and sound requirements for both forms of technology.

“More robust standards” for snowmobiles would require the machines be 25 percent cleaner and 50 percent quieter, Wenk said. There also would be incentives for making snow coaches cleaner and quieter, he said.

Pearson criticized some of the particulars of the best-available technology standards.

“They’re proposing a [best-available technology] standard for a snowmobile that doesn’t yet exist,” he said. “And they’re proposing a [best-available technology] standard for snow coaches that’s worse than most of the existing equipment on the road right now.”

“That’s a peculiar approach,” Pearson said.

Mark Menlove, executive director of the Winter Wildlands Alliance, worries about the park reverting back to allowing noncommercially guided riders.

Under the rules currently in place, commercial guides are required for all over-snow travelers.

Yellowstone’s preferred plan allows one transportation event at each of Yellowstone’s five gates to be set aside for noncommercial “guides.” Private snowmobilers would be required to undergo online training if selected for a permit.

“I think the commercial-guiding aspect — more than any other component — is responsible for really alleviating a lot of the problems,” Menlove said. “It’s brought snowmobiles into staying within speed limits, not going off trail.”

Yellowstone law enforcement, Pearson said, will be pressured to make sure the unguided riders follow the rules.

“I think those folks are going to be so heavily scrutinized that if they make a mistake, the Park Service will be all over them,” he said.

The winter-use plan also proposes continued access — three transportation events per day — from Yellowstone’s east gate, which requires using artillery to clear avalanche paths on Sylvan Pass.

“The Sylvan Pass situation is a complete head-scratcher,” Pearson said.

In past winters, maintaining the pass has cost an average of $350,000 a year, according to the National Parks Conservation Association. The expense under the proposed plan averages out to $500 to $3,600 per snowmobiler, depending on the level of use.

Despite the perceived shortcomings, Pearson said he thinks the coalition will be able to “thread the needle” on the plan and avoid a return trip to the courtroom.

“There’s only so far that litigation is going to get you,” he said. “There’s not really much left to be arguing about in court at the end of the day.”