Some Yoshino cherry trees are already blooming this week. (gypsybug via Flickr)

After an exceptionally early spring onset, winter is about to return to the Washington area. Temperatures are expected to drop to freezing this weekend, and snow is in the forecast for Sunday.

If frost forms or overnight lows drop far enough to develop a hard freeze, it could be bad news for the cherry blossoms, which are entering a vulnerable stage of development this weekend.

Michael Stachowicz, the turf management specialist at the National Park Service, is concerned about bud damage by early next week. "They're pretty hardy until they get to the peduncle elongation stage, which is what we're heading into this weekend," he said.

Stachowicz would know — he's the guy who creates the cherry blossom forecasts for the Park Service.

Its horticulturalists monitor a specific group of Yoshino cherry trees to track them through the six stages to peak bloom: green buds, florets visible, extension of florets, peduncle elongation, puffy white and peak bloom. The trees reached the extended-floret stage Tuesday. Peduncle elongation stage could come before temperatures begin to fall Friday night.

Stachowicz has been at the Park Service for three years, and he says that he hasn't seen cold weather damage the blossoms in his time there. But that certainly doesn't mean it can't happen.

"A frost on the elongated-peduncle stage could do damage," Stachowicz said, "and there's direct-freeze injury if we get down around 28 degrees. So we could end up with some damage on the buds that are farthest along."

Frost can be difficult to forecast. It forms when the temperature drops to the "frost point" — which is simply the dew point, if it happens to be below freezing. The dew point is a measure of how much moisture is in the air; a higher dew point means more moisture. When the temperature falls to the dew point, water will deposit onto surfaces. If the dew point is below freezing and the temperature falls to it, ice will deposit onto surfaces.

The trick with frost, though is that it doesn't necessarily take temperatures below freezing to trigger it. Overnight lows can be as warm as the upper 30s, and frost can still form, particularly on cloudless nights. This is due to radiative cooling — the grass and trees cool faster than the air. In other words, the air temperature might be 38 degrees, but the temperature of the blossoms could be below freezing.

Frost is possible this weekend, especially on Sunday and Monday night when overnight lows are forecast to bottom out around the freezing mark and when skies could be clear. There's even the risk of a hard freeze if the temperature drops to that magic 28-degree mark.


(Weather Underground)

Stachowicz is hopeful that the city's location, which tends to be warmer than surrounding areas, and the warming effects of the nearby water will help ward off the impact. "Things tend to be a little more moderated down by the Tidal Basin, and that might take the edge off."

The earlier that spring warmth arrives, the earlier the cherry blossoms will bloom — it's just a matter of getting a certain number of warm days. The cherry tree buds got a very early start this year after temperatures ran well above average in the second week of March.

"This is really strange, when you think about it," Stachowicz said.

Research suggests that early spring is happening more often in a warming world, but not without consequence. "As global climate warms, increasingly warmer springs may combine with the random climatological occurrence of advective freezes … to dramatically increase the future risk of false springs, with profound ecological and economic consequences," scientists reported in 2013.

In other words, it's getting warmer earlier, but there's always the risk of freezing cold snaps, especially in March — which is exactly what's happening this weekend.

As far as the snow itself goes, Stachowicz isn't as concerned. "If we have a bloom the snow would knock the petals off, if it were a heavy enough snow," he said. "But we're not going to have a significant bloom by then."