WASHINGTON (AP) — A winter storm packing heavy snow blew into the nation’s capital on Monday, closing government offices and schools and grounding the president’s helicopter. As much as 10 inches of snow was forecast for the District of Columbia, northern Virginia and central Maryland through the afternoon.

The National Weather Service issued a winter storm warning for the area until 4 p.m. EST Monday. Wind gusts of up to 35 mph were forecast, and travel was expected to be very difficult because of the hazardous conditions, the weather service said.

“The timing of this isn’t great,” said National Weather Service meteorologist David Roth. “For the D.C. area, it’s morning rush hour. At least for places to the northeast, it’ll be closer to midday.”

More than half the flights were delayed or canceled Monday morning at Ronald Reagan National Airport, Baltimore/Washington Thurgood Marshall International Airport and Washington Dulles International Airport, according to FlightAware.com’s misery map. A quarter of the flights at New York’s three major airports were delayed or canceled as well.

The snow also grounded President Joe Biden’s helicopter, so he was motorcading to the White House from Andrews Air Force Base in suburban Maryland after a weekend in Delaware. With snow blanketing the streets in and around the nation’s capital, the White House Press briefing was canceled, although Biden’s other public events were still on.

The Weather Prediction Center said 2 inches of snow per hour could fall in some areas, and thunder snow was possible.

More than 500,000 customers were without power Monday morning as the winter storm warning extended from northern Alabama and southern Tennessee through Georgia, Kentucky, North Carolina and West Virginia, Virginia and Maryland.

Snow began falling Sunday night in parts of Alabama, Kentucky and Tennessee. As much as 6 inches accumulated in north Alabama, where authorities reported multiple roads were blocked because of icy spots and wrecks, and businesses, schools and government offices delayed opening until mid-morning to allow time for temperatures to rise above freezing.

Authorities across Virginia and Maryland were reporting numerous crashes and treacherous roads. The Virginia State Police urged people to travel only if necessary after responding to 82 traffic crashes as of 8 a.m., as people drove too fast in slick conditions.

The largest snowfall total reported in the Baltimore-Washington region on Monday morning was in Virginia’s Augusta County, where a trained spotter in Greenville reported 4 inches, according to the weather service.

Maryland State Police said they responded to nine crashes, three disabled vehicles and 57 calls for service well before the heaviest snow fell. Flooding made some roads dangerous in North Carolina.

In Washington, the U.S. Office of Personnel Management announced that federal offices in the area would be closed on Monday. Emergency employees and telework employees were expected to keep working, the OPM said on its website.

Many COVID-19 testing and vaccination sites were closed in Virginia and in Maryland due to the weather.

Multiple school districts in the region said they would be closed, delayed or have virtual learning Monday. DC Public Schools said students and staff wouldn’t be returning to school until Thursday.

Associated Press contributors include Colleen Long in Washington, Julie Walker in New York and Jay Reeves in Birmingham.