Snowquester imminent, but snow is the hard stuff for forecasters

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.

Graphic

How can it snow so much when it's not freezing?

A gentle winter

Snowfall this winter is below normal in many regions of the country, including Washington, D.C.

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.

The snowfall prediction maps for this storm, as of Tuesday afternoon, showed dramatic differences in accumulations across relatively short distances. The Capital Weather Gang’s forecast showed roughly one to four inches on the Eastern Shore, three to eight inches (a rather large margin of error) for the capital proper, six to 12 (!) inches in the western and northern suburbs and up to 15 inches along the Blue Ridge Mountains.

But confidence, like snowfall predictions, was also stratified by geography. Out toward the mountains, the forecast Tuesday morning showed a big snowstorm as a certainty. Confidence in a big snow event shifted eastward, and the National Weather Service on Tuesday afternoon extended a winter storm warning (meaning five inches of snow or more, no doubt) to the closer suburbs and the District.