The call to close school on Thursday, as a blizzard warning loomed, was easy.

Usually, it’s not.

Normally, school superintendents said, deliberations on “snow days” start several days before a storm hits. They browse weather apps and worry about scheduled events, from school musicals to out-of-state field trips. The discussions often culminate with middle-of-the night messages — and then, the decision.

“Most of the time, we’re making the call between 3 and 5 in the morning,” said Katy Graves, superintendent of the Sag Harbor school district. “We say we’re going to get up at 4 and start talking and sharing information.”

Graves and schools chiefs in about 20 East End systems have taken to using their own “Group Me” message thread, firing off rapid responses and suggestions to one another.

It’s no small thing to cancel a school day on short notice: Students and their families, teachers and staff, transportation chiefs and bus drivers all must be notified. Districts seek to stay within a limited number of snow days each winter. And scheduled activities must be considered.

advertisement | advertise on newsday

For example, earlier in the day Wednesday, Graves had prepared to postpone the middle school’s production of “The Little Mermaid” and had written a “script” announcing the postponement in advance of the snow-day closure publicized on the district’s website.

On the eve of a major snowfall, superintendents of districts clustered near one another agree to wake up early and talk it through. They don’t always agree after a restless night’s sleep.

“You’re kind of sleeping with one ear and one eye open, understanding you’re going to have to monitor this through the night,” said Henry Grishman, Jericho’s superintendent.

As this storm approached, many districts made the call at 5 p.m. Wednesday after the National Weather Service issued a blizzard warning for Suffolk County. That warning was expanded to include Nassau later Wednesday night.

But on other occasions, when forecasts have been murkier, conversations have dragged into the next morning, when superintendents drive the streets and talk with police officials and transportation experts.

There is limited time for debate and second-guessing, with bus operators warming up their vehicles at 6 a.m., about the time teachers leave for work.

“You’re up against a time clock,” Grishman said.

Joseph S. Famularo, Bellmore superintendent and president of the Nassau County Council of School Superintendents, said, “We wait as long as we can.”

advertisement | advertise on newsday

There is a need “to inform our parents as soon as possible — they may need to make arrangements for home care,” he said.

Charles Russo, president of the Suffolk County School Superintendents Association and superintendent of the East Moriches district, pointed out the gamble posed by fickle weather conditions.

“We’ve seen that where the storm has literally been a rain-snow mix on the South Shore, and the North Shore’s hit with 5 to 6 inches” of snow, he said.

Lars Clemensen, superintendent in the Hampton Bays district, said “there’s nothing regional to consider.”

“As you think about the hills of Port Jefferson, to the farms of Southampton, to a storm that may linger in Montauk, all of these decisions become really local,” he said.

advertisement | advertise on newsday

The Hampton Bays district is scrambling to arrange a new flight for 105 band and music students to Disney World in Orlando, where they were scheduled to perform this weekend. Their Friday flight was canceled.

Superintendents acknowledged that calling a snow day was not covered in their training as administrators — and that they sometimes take heat for it.

“You can’t take anything personally,” Graves said. “As a superintendent, you have to realize you’ll have critiques on either side.”

Some superintendents keep score. A good call at 5:30 a.m. may seem like a blunder two hours later, Jericho’s Grishman said, remembering one decision he made about a decade ago.

“I heard about ‘Grishman’s rain day’ for at least a year after that,” he said.