Editor's choice

In weather business, snow hard to predict

Joel Achenbach / The Washington Post /

WASHINGTON — Snow is hard. This is a fact of meteorological life.

A forecaster has to keep track of many variables: the amount of precipitation, the intensity of precipitation, the temperature, the atmospheric structure, the snow/rain line, etc.

Any mistake in those calculations will be exaggerated — and made glaringly obvious to anyone who looks out a window — by the very nature of snow, the way it multiplies an inch of rain into 10 inches of white stuff (or 6 heavy inches, or 15 or 20 or even 30 powdery inches, depending on the snow’s wetness — another variable).

So, with a major storm just hours away from the mid-Atlantic, residents of the region don’t know if the precipitation event will be mostly solid or mostly liquid, whether it will be an epic snowstorm or just a sloppy, wet, puddle-making mess.

“You have to get not only the temperature right, but the temperature structure in the atmosphere — how the temperature varies with height,” says Cliff Mass, a professor of atmospheric sciences at the University of Washington.

For example, if there’s a very shallow warm area at the surface — where humans walk around and wonder how it could possibly snow — the flakes, formed higher in the atmosphere, can still make it to the ground, because they need about 1,000 feet of warm air to melt, Mass said.

The professional weather forecasters have had some recent triumphs, showing their prowess at predicting major storms or tornado outbreaks.

Fully a week in advance, for example, they warned last fall that a storm named Sandy could come up from the Caribbean and wallop the Northeast.

But just hours from impact, this storm remains enigmatic. About the only thing certain about it is that the forecasters from the Post’s Capital Weather Gang had no choice but to name it Snowquester. Some readers of the Gang’s blog posted skeptical comments Tuesday, predicting a bust — another snowstorm that mysteriously fails to launch.

Chris Strong, a meteorologist with the National Weather Service in Sterling, Va., said at midday Tuesday, “There are events where it’s a lock, and we’re going to get clobbered, and there are storms like this one, where the rain-snow line is close to the Washington area.”

Blame geography. And the Founders. In the debate over the siting of the national capital, back when this was a young republic, Virginia’s James Madison pointed out that a capital along the Potomac River would require all the northern members of Congress to travel 12,422 miles, collectively, while the southern members would collectively need to journey 12,782 miles. His point was that, contrary to northern fears of a “southern” capital, the Potomac site wasn’t truly in the South after all, but pretty much precisely in between.

It’s also between the Atlantic Ocean (warm) and the mountains (cold.)

“We straddle multiple boundaries,” said Chris Vaccaro, spokesman for the National Weather Service. “We straddle the ocean and the Chesapeake Bay. We straddle the Appalachians. We straddle the south and the north. We’re somewhat in the climatological crosshairs, in various dimensions. At some points, the cold and snow win out, and sometimes the warmer air and rain win out.”

We’re also straddling seasons. The calendar says we’re still in winter, and will be for another couple of weeks, but the weather experts have this notion they call “meteorological spring,” which begins March 1.

This is mud season, in short, and in a matter of days it’ll be time for sitting on the porch. Bulbs are sending up green shoots. Buds are swelling, weeds have awakened, the forsythia is about to explode.

And so it will be a typical spring-like snow — wet and heavy.

“This is not January or February. There isn’t a wealth of cold air for this storm to tap. The cold air is marginal,” Vaccaro said. “We are dealing with a heavy, wet, gloppy snow. It has the consistency of wallpaper paste, as opposed to the light, fluffy, powdery snow that you would typically admire on the ski slopes.”

And it’ll be ephemeral. Warmer weather is coming hard on the heels of this storm. The snow will soon be gone, as if it never actually happened.

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