In January, youngsters in Leesburg, Va., got the day off from school because of snow — and headed to a hill for sledding. (Michael S. Williamson/The Washington Post)

Announcing a snow day on Twitter is enough to catapult local government officials to the stuff of legend — by teenagers’ standards, anyway.

A few years back, students superimposed the face of a Fairfax County, Va., School Board member on images of the Sistine Chapel and Mount Rushmore and cast him as a light-saber-wielding Jedi knight after he began tweeting school cancellations. Similar tweets from Superintendent Steven L. Walts in Prince William County, Va., easily generated a couple thousand “likes” and comments that anointed him a “man of the people” and the “greatest superintendent in the country.”

Whether a school system decides to cancel or delay classes because of inclement weather — or remains open — opinions and irritation inevitably abound, especially when systems close and little snow or rain actually falls. Some parents and students appreciate the caution; others decry what they view as an unnecessary hassle.

With a winter storm warning covering much of the D.C. region for Wednesday amid predictions of a perilous brew of snow and ice, the decision to cancel classes was proving straightforward for school leaders.

The storm warning was set to start at 1 a.m. Wednesday and continue till 7 p.m. for much of the region. Crews had taken to the roads to prepare for the onslaught, and utilities remained on alert, concerned about the potential for heavy, wet snow and ice.

By early Tuesday evening, most school systems had decided to close Wednesday — including Arlington, Fairfax, Loudoun and Prince William counties in Virginia, and Montgomery, Prince George’s, Howard and Anne Arundel counties in Maryland. D.C. schools are on February recess this week.

But an elaborate calculus lies behind each call to close schools in large systems with sprawling geographies, such as in Northern Virginia. It involves monitoring weather forecasts, testing road conditions — and very early mornings.

“Second-guessing weather decisions is Washington’s favorite sport,” said Matthew Guilfoyle, an associate superintendent for Prince William County Public Schools. “It’s just really important folks understand that the goal is student safety.”

It’s easy to call off classes when six to 12 inches of snow is expected to fall, he said. It becomes far dicier when temperatures hover above freezing but could drop and turn rain-slickened roads icy.

Elevation in different parts of Prince William County can differ by hundreds of feet, Guilfoyle said, so poor conditions in one neighborhood may not be evident in another corner of the county.

When bad weather is forecast in Loudoun County, Va., a team of at least 10 people starts assessing roads at 3 a.m. for the school system, spanning the mountains in the west to the suburbs in the east, said Kevin L. Lewis, assistant superintendent for support services.

Canceling classes at one school in a county isn’t an option — transportation and after-school activities require traveling across neighborhoods, creating logistical challenges.

Virginia law requires 990 hours of instruction each academic year, and school systems generally build in more hours than required, providing a cushion for days when classes are canceled or delayed.

During a recent two-hour delay to the start of classes in Fairfax, which has the largest school system in Virginia, parts of the county experienced temperatures in the mid-30s, while patches of ice coated other neighborhoods, said Jeffrey Platenberg, assistant superintendent for facilities and transportation services.

Before closing the school system, Platenberg consults local television meteorologists, the National Weather Service and agencies charged with salting and clearing roads.

A few dozen drivers across the county check roads overnight. And on days schools are closed, employees shovel parking lots and prepare campuses for the next day.

“When people have a day off,” he said, “we don’t.”