It has become a familiar but frustrating drill for farmers in the Northeast. With temperatures expected to fall below freezing, they spent Friday preparing orchard heaters, irrigation systems and frost alarms to protect fruit crops that blossomed early and are particularly vulnerable to the elements.

While the predicted low early Saturday is not unusual for late April, farmers say fruit crops are as many as three weeks ahead of schedule due to a March warm spell. That has led to some sleepless April nights.

Don Preli, the owner of Belltown Hill Orchards in Glastonbury, Conn., said he rolled out anti-frost measures about eight times in the last month to protect his strawberries, blueberries and other fruit that typically do not blossom until early May. It costs him about $1,000 an hour to operate the equipment, including wind machines designed to keep cold air from settling around crops.

“We’ve had to do it so often this year it’s getting to be a really big burden,’’ Preli said.

Some farmers said the forecast of low temperatures in the mid-20s for Friday night posed the biggest threat yet for crop damage. The National Weather Service issued a freeze warning for parts of several New England states, much of New York and Pennsylvania and sections of Ohio, New Jersey, Maryland, Virginia and West Virginia. Forecasters urged measures to protect tender vegetation.

“This is by far going to be the scariest night of the year,’’ said Rick Macsuga, a marketing representative for the Connecticut Department of Agriculture. “There are going to be a lot farmers up late or all night tonight.’’

In Vermont, warnings were issued for areas where vegetation is ahead of schedule, said Andy Nash, a meteorologist at the National Weather Service office in Burlington.