National Park Service employee puts up a sign announcing closures due to a government shutdown on Oct. 1, 2013. (Matt McClain/ The Washington Post)

Note: This story has been updated. It originally ran on Sept. 23, 2015, before Congress approved a temporary funding measure to avert a government shutdown. The legislation expires this week, leaving Congress faced with the same spending issues that went unresolved earlier this fall.

Old Faithful would erupt with no one watching, because Yellowstone National Park and 407 others would close to the public.

Homebuyers would find their mortgage applications stalled because the offices they need to approve paperwork for loans would be shuttered. And the Centers for Disease Control and Prevention would have to stop monitoring the spread of the flu to direct vaccines where they are needed.

[A government shutdown: What federal employees need to know]

This is what a shutdown of the federal government will look like starting this weekend if Democrats and Republicans in Congress cannot break the ideological stalemate preventing them from passing a budget for the current fiscal year.

The Obama administration has already asked agencies across the government to determine which operations are essential — or paid for by fees or funding that isn’t dependent on the budget passing — and which are not.

[White House starts shutdown ball rolling]

While the action — or lack of it — that will determine whether the government stops running is playing out in Washington, 85 percent of the federal workforce operates outside the capital city. So Americans in every state would be affected, whether they are late filers hoping to get a speedy tax refund from the Internal Revenue Service or businesses applying for small loans through the Small Business Administration.

In a shutdown, the government does not stop operating or serving the public completely. Air traffic controllers stay on the job to keep flights going, and federal prisons stay staffed. The meteorologists at the National Weather Service and the cyber security experts at the Department of Homeland Security come to work. Meat and poultry inspectors at the Agriculture Department stay on the job. Veterans’ hospitals stay open and Social Security checks still go out.

[Who gets sent home if the government shuts down]

Mail still gets delivered, since the U.S. Postal Service gets only a sliver of its funding from Congress. And the active-duty military  — and many of the Defense Department civilians who support it —keeps working, at home and abroad.

These operations are considered essential to national security and safety. But hundreds of other services the public depends on from the government would not be available. And whenever Congress agrees to pass a budget that allows federal offices to reopen, the work that got backed up during a shutdown could cause big delays to a range of services.

[Federal workers: Are you prepared for a government shutdown?]

The head of the National Institutes of  Health  and Rep. Chris Van Hollen (D-Md.), the top Democrat on the House Budget Committee, warned in September that patients with critical illnesses will be turned away and research disrupted during a shutdown. NIH’s clinical facilities at its Bethesda, Md., campus will not admit new patients, they said.

At Yellowstone, Park Superintendent Dan Wenk and his staff have worked to determine what new wildlife management programs and services are in place today that weren’t in 2013, when the government closed for 16 days in the most potent shutdown in modern history. The park would close to the public. But there’s a native fish restoration program, for example. Contractors are fixing and upgrading roads in and out of the park.

[Van Hollen on government shutdown: Patients will be turned away from NIH]

“We are going through the ‘what’s different this year?’ questions about activities in the park that might be affected,” Wenk said in an interview two months ago.

Two years ago, about 850,000 of 2.1 million civilian federal workers were sent home. About 15 percent of these employees are paid through fees, budgets that span several years or other appropriations, said John Palguta, a former federal manager who is now a vice president at the nonprofit Partnership for Public Service.

But don’t count on them to fill in the huge gaps in customer service a shutdown would leave. Most phone calls, to the IRS, for example, will go unanswered. And if you try e-mailing a federal employee, you won’t get an answer. They must turn off their BlackBerrys and i-Phones during a shutdown, or they could be in trouble for working.

The Department of Veterans Affairs has warned that overtime for employees who are whittling down the agency’s large backlog of applications for disability payments would cease, creating further delays for veterans.

Head Start, the education, health and nutrition program for low-income children and their families, would close its local programs slowly as their federal grant money dries up. Businesses would not be able to use E-Verify, the Homeland Security system that allows them to check the immigration status of potential new hires.

The Obama administration and Congress have discretion about whether to declare their employees essential to government operations. In two shutdowns in the 1990s, many agencies worked with thin crews. Social Security payments were delayed, and passports were not issued. By 2013, “it was clear that these services needed to continue,” Palguta said.

Yet the State Department’s shutdown plan from 2013 notes that any passport offices that operate out of government buildings, as opposed to U.S. consulates, which have a different funding source and would stay open, would close.

Some other effects of a shutdown would take a few days or weeks to be felt. At the federal courts, officials have said they could continue operations for 10 days to two weeks, using money from court fees and budgets that carry over from the previous fiscal year, said David Sellers, spokesman for the Administrative Office of the United States Courts.

“Then we have to look at where to turn off the spigot,” he said.