(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.

Several major airlines have preemptively canceled hundreds of flights scheduled for Friday. The cancellations will impact airlines like a busy weather day, one airline official told CNN. Unlike a storm, however, they will be spread across multiple cities as opposed to a geographic region.

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, Houston, Los Angeles, Las Vegas, Miami, Phoenix, Seattle and Washington, DC, the source said.

Another 10 airports across the country are expected to be impacted, including ones in Anchorage, Alaska; Indianapolis; Portland, Oregon; and Ontario, California. Some smaller airports in the “Core 30” cities are also on the list.

“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.”

Flight volume will be reduced by 10%, Secretary of Transportation Sean Duffy said Thursday. But he did not say whether that’s 10% of the flight volume at the 40 impacted airports, or 10% of all US flights – which would be significantly more.

Either way, a 10% reduction could result in thousands of canceled flights a day – an unprecedented cut.