JACKSON -- Sylvan Pass will remain open when avalanche conditions permit safe travel, the National Park Service announced Tuesday.

And Park Service officials will meet with state and area representatives in the next six months to determine how and if more expansive avalanche control can be conducted to keep the pass open in winter.

Under a final decision for winter use for Yellowstone and Grand Teton national parks signed Tuesday -- one day after the Park Service's own deadline -- Sylvan Pass will remain open this winter with the use of howitzers and helicopters for avalanche control.

But between now and June 1, the agency will meet with state, Park County and Cody officials to see if there are any "long-term solutions we haven't thought about," according to Mike Snyder, Intermountain regional director for the Park Service, who signed the decision Tuesday.

The decision represented a major shift from Yellowstone officials' desire to close the park's East Entrance to oversnow traffic, a move that drew the ire of Wyoming elected officials and people in Park County. The change was hailed by state elected officials as a victory for those who fought the planned closure.

"This about-face by the Park Service is a reflection of what can happen when a community unites on an issue so important to its economic health and heritage," said U.S. Rep. Barbara Cubin, R-Wyo.

Although the Park Service's final plan called for a closure of Sylvan Pass in winter because of safety and cost concerns, Snyder said the final decision leaves the option open.

Snyder said he didn't think the agency had an "effective understanding" of the tools available for avalanche control, and the cost. He said he thought it was best to join in a mediation process to help make a decision rather than make one himself.

Sylvan Pass has been a major sticking point of the most recent study -- now the fourth -- of winter use of Yellowstone and Grand Teton national parks.

A mediator was hired, and the Park Service and community members met in September to work on solutions. Out of that process came Tuesday's decision to continue negotiations until June.

If no agreement is struck by June, the Park Service will leave the pass open when avalanche forecasting dictates the area is safe for travel. Otherwise, the pass will be closed.

Snyder said the negotiations will also involve issues of cost, and who might help pay for avalanche control.

Although the final decision on winter use in the parks was signed one day late based on the Park Service's deadline of Nov. 19, Yellowstone and Grand Teton national parks will open on schedule Dec. 19 for the winter season.

The signing of the final winter-use decision for both parks comes after more than a decade of back-and-forth between those wanting snowmobile use to remain and those arguing the agency's own studies have shown the best way to protect the natural resources is to shift to snowcoach access. A snowcoach is similar to a van mounted on tracks.

The final decision, as expected, allows for a maximum of 540 snowmobiles per day in Yellowstone, and those riders must be guided. That number was scaled down from 720 sleds allowed in the parks during the last three years under a temporary rule.

This winter will operate with numbers outlined in the temporary rule, meaning the 540 limit will be enacted beginning next year.

All machines must be "best available technology," meaning the cleanest and quietest machines available.

There will be 83 snowcoaches allowed per day in Yellowstone. Those machines must meet best available technology requirements in 2011.

Snyder said the number of sleds was scaled down from 720 to 540 because 720 was a number used for modeling, and 540 is a number from which the agency actually has data.

Still, even with an average of 280 snowmobiles per day in recent winters, sound and pollution levels have been exceeded at various points in the park.

Snyder said he is confident those problems will be corrected once all machines use best available technology.

The Park Service was expected to sign a decision for winter use Monday, but officials cited "ongoing discussions" between the regional office and officials in Washington, D.C., as the reason for the delay. Policy dictates a final decision must be signed 30 days before a decision becomes law to allow for public comment, but the Park Service has said it can expedite that deadline to allow the parks to open as schedule Dec. 19.

Park Service officials stopped short of saying this final decision represents an end to the back-and-forth about snowmobiles in Yellowstone and Grand Teton national parks.

"I hope we're able to move forward and we're able to implement it," Snyder said.

Environmental reporter Whitney Royster can be reached at (307) 734-0260 or at royster@tribcsp.com.