Snowquester imminent, but snow is the hard stuff for forecasters
By Joel Achenbach, Washington Post, March 5, 2013
Snow is hard. This is a fact of meteorological life.
A forecaster trying to predict snowfall has to track many variables: the amount of precipitation, the intensity of precipitation, the air temperature, the surface temperature, the atmospheric structure, the timing of everything, the migration of the rain/snow line, and so on.
Any mistake is exaggerated by the very nature of snow, the way an inch of rain gets multiplied into 10 inches of white stuff (or six heavy inches, or 15 or 20 or even 30 powdery inches, depending on the snow’s wetness—another variable!).
Snow exposes failure. If the weatherperson botches a rainfall prediction, no one notices, because it’s hard for an ordinary person to judge rainfall totals, and the storm sewers gobble up the excess. But someone can detect a bad snow forecast—too much snow, or too little—just by looking out the window.
“With winter storms, the devil’s always in the details,” said Christopher Vaccaro, spokesman for the National Weather Service.
So it was that in Washington on Tuesday the weather remained stubbornly uncertain. A big storm—Snowquester!—was erupting from all the computer models and was just hours from slamming into Washington, yet the weather in the real world remained strikingly pleasant for much of the day, with nothing even slightly ominous on the horizon. Snowstorm coming? Really? With thundersnow???
The experts said yes, but they hedged their language and dribbled caveats across the forecast. This was a slippery situation, typical for Washington in March. Particularly in the eastern parts of the region, it remained unclear who, exactly, would get hit with how much snow, and if the event would be mostly solid or mostly liquid, and whether, when it was all over, we would say it was an epic snowstorm or complain that it was just a soggy 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, he said, if there’s a very shallow warm area at the surface—where humans walk around and wonder how it could possibly snow, much less snow enough to shut down the government and parole the schoolkids—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.
Despite improvements in storm prediction, driven by better satellite data and advances in computer modeling, snowfall totals remain notoriously hard to nail down before the snow actually falls on top of the backyard grill.
About the only thing certain about this new storm was that the forecasters from the Capital Weather Gang had no choice but to name it Snowquester.
Chris Strong, a meteorologist with the National Weather Service in Sterling, said 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.) Washington sits approximately on the Fall Line, the boundary between the Piedmont and the coastal plain, a geological demarcation that roughly underlies the old post roads linking the cities of the mid-Atlantic and is now known as the I-95 Corridor. The divide between snow and rain often follows more or less the path of the big highway.
“We straddle multiple boundaries,” said Vaccaro. “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.
And it’ll be ephemeral. Warmer weather’s coming hard on the heels of this storm. The snow will soon be gone, as if it never really happened.