Frost advisories and freeze warnings were issued for the entire Washington region Thursday afternoon, hoisted in advance of a chilly night with temperatures dropping into the 30s. It comes one day after a dramatic cold front sliced through D.C. and surrounding areas, plunging temperatures from near 70 at noon into the 40s by sunset Wednesday.

The chilly weather could prove problematic for plants and tender vegetation outside, while also heightening risk for homeless and vulnerable populations who may be more exposed to the cold.

Tonight marks the coldest night since the start of April, with the potential for frosty temperatures extending all the way east to the Chesapeake Bay.

Temperatures in D.C. are likely to drop into the mid- or upper 30s downtown, with lower 30s west. Even if downtown Washington remains in the upper 30s, due in part to warmth trapped by asphalt, buildings and the nearby Potomac River, as well as areas north and west of town could approach freezing.

The Northern Virginia towns of Culpeper, Warrenton and Leesburg could all get down into the 30-to-32 degree range, while Harrisonburg may fall into the upper 20s.

St. Mary’s and Calvert counties in Maryland should remain in the mid-to-upper 30s.

Why the temperatures matter

Temperatures in the 30s aren’t terribly unusual for the D.C. area; on average, Washington sees lows at or below 36 degrees about 85 times a year. Thanks to climate change, nights that chilly are about 20 percent less common than they used to be. But no matter how you slice it, temperatures in the mid-30s or colder are still pretty common.

For Dulles International Airport and much of Washington’s western suburbs, the average last freeze occurs around April 19, so if the mercury settles to 32 degrees or lower tonight, it will be pretty close to average.

What sets nights like tonight apart is that, since it’s mid-April and we’ve gone several weeks without a freeze, the growing season has already started.

That means vegetation outdoors that may already be budding or sprouting could be injured or killed by subfreezing temperatures. Unprotected outdoor plumbing could also be damaged.

What the start of growing season means

Nights this chilly are usually only observed once or twice each April. That’s why the National Weather Service issues special alerts targeting those with plants outside.

“Most of the area that we cover starts on April 11,” said Dan Hofmann, a meteorologist at the Weather Service in Sterling, Va., who explained what the start of “growing season” was defined as.

“That includes the greater D.C. area. Once you get well north and west, that’s May first. Once you get out toward the Shenandoah Mountain area and Garret County [in the Panhandle of Maryland], May 21.”

That’s why those areas aren’t under frost advisories or freeze warnings — even though they’ll be colder. A gap in the alerts occupies much of the Blue Ridge and Appalachian mountains.

His office uses the average date of an area’s last spring freeze as a proxy for when the growing season starts. Any near-freezing temperatures after that date will trigger a frost advisory of freeze warning.

However, that date can be moved up or postponed depending on the weather of a particular year.

“There are some years where it’s running much colder than normal and we haven’t started growing anything yet … then we’ll go a little later,” Hofmann said.

The same alerts are used if freezing conditions are expected before the end of the growing season — defined as Nov. 1 locally, and Oct. 1 well to the west.

How temperatures above freezing can still be problematic

Freeze warnings are usually issued when a hard freeze is expected — with temperatures of 32 or below. The expansion of water trapped in plant cells when it freezes can cause the cells to burst, killing the plant.

Frost advisories are issued when temperatures will be more marginal. That’s because of radiational cooling.

“The temperatures are officially measured at two meters [above the ground],” Hofmann said. “But at leaf level, on a night with good radiational cooling, that could be several degrees [lower].”

Radiational cooling requires clear, dry nights. Clouds giving way to clearing skies are in the cards for the Mid-Atlantic tonight.

Lows in the 40s are expected tomorrow night before a warmer pattern arrives into next week.