The Coalition of National Park Service Retirees warns of specific closures at 11 national park sites. That includes delayed spring snow plowing in Yellowstone, Yosemite, Glacier and Alaska's Denali National Park. Seasonal road openings also would be delayed in Grand Canyon.
Visitor centers could close in at least three parks: Grand Teton and Mount Rainier national parks and Cape Cod National Seashore.
"The ripple effect will be felt across the entire economy. It's not just a state. It's not just a community," said Joan Anzelmo, a former Colorado National Monument superintendent who is spokeswoman for the retirees' group.
"We haven't seen anything like this before."
The Park Service didn't deny any of it was possible.
"The public should be prepared for reduced hours and services provided by National Park Service employees at 398 national parks, historic sites, monuments and memorials, parkways, trails, preserves and reserves, seashores and lakeshores, recreation areas and national battlefields," Park Service spokesman Jeffrey Olson said in a statement.
Wyoming is set to be especially hard-hit while many Americans begin to think about summer vacation plans.
Staffing cuts could delay plowing anywhere from two to four weeks, affecting more than 78,000 visitors and hurting tourism in nearby Jackson and Cody, the group said.
Other routes that wouldn't open on time in late May include Going to the Sun Road, the only road across Glacier National Park. Plowing delays also would postpone opening Yosemite's Tioga and Glacier Point roads up to four weeks.
In Denali, delayed plowing would postpone opening of a road and the park's Eielson Visitor Center.
Meanwhile, he East Rim and West Rim drives in Grand Canyon would open later than usual in May and hours would be cut back at the Grand Canyon Visitor Center, according to the coalition.
Other cuts predicted by the group would include:
-- Grand Teton would close its Jenny Lake Visitor Center, Laurence S. Rockefeller Preserve and Flagg Ranch Visitor Contact Station.
-- Great Smoky Mountains National Park would close five campgrounds and picnic areas while cutting back on road maintenance and emergency response capabilities.
-- Cape Cod National Seashore's Province Lands Visitor Center could close.
-- The Natchez Trace Parkway in Mississippi, Alabama and Tennessee would close 25 comfort stations one day a week.
-- Mount Rainier National Park would close the Ohanapecosh Visitor Center.
The park service cuts are just a small portion of the federal budget "sequestration" set to take effect as a result of the latest episode of Congress and the White House not agreeing to more modest cuts. Congress could vote in the months ahead to prevent any specific portion of sequestration from taking effect.
"We're really trying to take this theoretical sequestration—which a lot of people really don't know what it means—and put a face on it," Anzelmo said.