The 'banding' behind snowstorms' surge

Posted: January 24, 2014

Not long after the first gentle flakes appeared like so many white fireflies Tuesday morning, the storm all but erupted, shutting down Philadelphia and dooming a 53-year-old record.

Suddenly, buildings, cars, and human beings were obscured behind dense white veils of dendrites - those snowflakes whose elegant design is so favored by holiday decorators and connoisseurs of accumulating snow.

The attack of the dendrites - dumping snow at rates of two inches or more an hour, according to the National Weather Service - was the result of what meteorologists call "banding," in which a geographically narrow area gets caught under an ideal "snow growth" region.

Translation: Banding is the perfect storm of storm conditions. It's been a major factor in our unusually snowy month - and the reason behind those seemingly elastic forecasts that suggest everything from inconvenience to immobility.

Bands set up where temperatures in the clouds are just right - about 5 degrees Fahrenheit - and strong winds are blowing upward. Upward motion is critical, because snow falls when vapor-filled warmer air is forced to rise above cooler air and then condenses.

Those conditions are perfect for producing those fluffy dendrites, which pile up better than their crystalline relatives, in part because they contain more air, said Kenneth Libbrecht, a flake expert at the California Institute of Technology.

"All of a sudden, the whole band enhanced from northern Delaware to Long Island," said Robert Oravec, a National Weather Service lead forecaster in College Park, Md.

While bands are common in snowstorms and summer thunderstorms, for reasons that meteorologists can't explain they have particularly enjoyed spending time above the Philadelphia area recently.

One of them generated the near-whiteout conditions during the nationally televised Eagles game on Dec. 8. Banding also bumped up totals in the Jan. 2-3 storm.

Tuesday's version had a particular ferocity.

The upward motion of the air was so strong and focused that for a while, it hurt snowfall rates to the east and west, said Brian Wimmer, a meteorologist with AccuWeather Inc. Only so much air can go up at any one time.

("If the air was rising everywhere, you couldn't breathe," said AccuWeather meteorologist Dave Dombek.)

Accumulating snow didn't make it to the Shore until after dark. Meanwhile, around Philadelphia, the heavy snows were creating near-paralysis.

Banding is one of the puzzles that bedevil accumulation forecasts. Computer models can hint at the most favorable conditions over a particular area, including strong temperature contrasts, but they don't have handles on the timing or precise locations, said Tony Gigi, a weather service lead forecaster at Mount Holly.

Although the forecast details were wanting, the potential for banding around Philadelphia was one reason the weather service accumulation projections jumped from very little Sunday to 10 to 14 inches at the last minute Tuesday morning.

The phenomenon helps explain why forecast wordings usually include several inches of wiggle room, as in four to eight inches with locally heavier amounts possible.

Another complication is trying to infer how much snow will pop out of a given amount of precipitation. Models can estimate precipitation in a storm, but guessing how much snow the liquid will generate is another matter. The longtime standard that an inch of rain equated to 10 inches of snow - a 1-10 ratio - turned out to be inaccurate.

Lower temperatures tend to produce higher snow ratios, but that's not always true. Not all flakes are dendrites, however, and some don't accumulate well. It so happened that ratios Tuesday were unusually high in much of the region, and were about 18-1 in Philadelphia, Gigi said.

The net result for Philadelphia was a run at weather history. The 13.5 inches lifted the seasonal total to 33.5, and made this month the fifth-snowiest January.

Already, this has become one of the 25 snowiest winters on record - and February on average is Philadelphia's snowiest month.

Tuesday's became the season's third six-inch-plus snowfall, the earliest that has occurred in a winter in the period of record, dating to 1884. The former standard was held by the winter of 1960-61, which did it by Feb. 3.

Had enough? Sorry.

Forecasts call for the wintry pattern to persist for perhaps three more weeks. Touches of snow are possible Thursday and Saturday, with a storm threat Sunday night into Monday.

That threat might turn out to be nothing more than a gleam in the eye of a computer model, but before season's end, the winter of 2013-14 seems sure to notch another precedent.

Philadelphia has never had four snows of six inches or more in one season, Gigi said. This could well be the first one.


twood@phillynews.com

610-313-8210

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