(CNN) — Millions of Americans expecting to fly this month could have their plans grounded if the government shutdown drags on.

Starting Friday, the Trump administration will reduce flights at 40 airports nationwide if the shutdown continues.

The Federal Aviation Administration’s “Core 30” airports are among those on a preliminary list of sites expected to be impacted, an FAA source with direct knowledge of the situation told CNN.

Here’s what to know if the travel nightmare turns into reality Friday:

The “Core 30” high-traffic airports include New York City’s three major hubs and large airports in Atlanta, Boston, Chicago, Dallas, Denver, Honolulu, Los Angeles, Las Vegas, Miami, Phoenix, Seattle and Washington, DC, the source said.

Another 10 other airports across the country are expected to be impacted.

“As we start to implement this drawdown in service, it will be restricted to these 40 high volume traffic markets,” FAA Administrator Bryan Bedford said. “We’re going to ask the airlines to work with us collaboratively to reduce their schedules.”

The affected airports’ traffic will be reduced by about 10%.

The airlines were notified of the plan only about an hour before it was announced, an airline source told CNN.

“We are working with the federal government to understand all details of the new reduction mandate and will strive to mitigate impacts to passengers and shippers,” Airlines for America, the industry trade group, said in a statement.