Snow forecasts: A model of imperfection

Brandon Wong (left), 15, and Sophenny Thom, 15, take advantage of the cancellation of school by playing a little one-on-one at 15th and Morris Streets in South Phila.
Brandon Wong (left), 15, and Sophenny Thom, 15, take advantage of the cancellation of school by playing a little one-on-one at 15th and Morris Streets in South Phila. (MICHAEL BRYANT / Staff Photographer)
Posted: March 05, 2014

To their immense disappointment, no doubt, hundreds of thousands of the region's schoolchildren on Monday were forced to accept yet another extended weekend in the endless winter of 2013-14.

But this time, many of them could blame the weathermen as opposed to the actual weather.

Several inches of snow fell in parts of South Jersey, but after a weeklong drumbeat, a storm that threatened to bring the region to a standstill was largely a bust west of the Delaware and didn't quite meet expectations elsewhere.

What happened?

The short answer: Meteorologists learned once again that the atmosphere pays far less attention to computer models - which foresaw a widespread heavy-snow event - than they do.

"It's nice to be wrong together," said Dan DePodwin, a meteorologist with AccuWeather Inc., "but it's not good to be wrong at all."

"I don't see anyone bragging about this one," said Gary Szatkowski, meteorologist at the National Weather Service forecast office in Mount Holly. "I'm sure there's some teeth-grinding for school administrators."

The Philadelphia School District decided on Sunday night to close Monday, based on the forecast, said spokesman Fernando Gallard. Yes, had officials known that the snow amounts would be around one to three inches and would shut off before 10 a.m., they might have opened schools late or even on time.

Officially, 3.4 inches was reported at Philadelphia International Airport, enough to push the seasonal total to 62.9 inches, third most in the period of record. Because of the difficulties of measuring snow at the airport, the official ruler is wielded by an observer across the river in National Park.

In Northeast Philadelphia, only 1.3 inches was recorded. Throughout the region, accumulations generally were dependent on latitude, with less to the north, the reverse of usual case for a March storm.

But this one was atypical of March, and of this season.

"We haven't seen a storm like this all winter," DePodwin said.

It raced across the country in a day and a half. That meant it didn't spend much time under the surveillance of land-based observing systems.

Such observations are critical to fixing the "initial condition" of the atmosphere. Computers then tackle complex equations to predict how the atmosphere will change over time in six-hour increments. Gaps and errors in observations lead to errors in forecasts.

"I wonder if the models had trouble with the speed of the system," Szatkowski said.

Late last week, the major U.S. and European models were in accord that heavy snow was going to cover much of Pennsylvania and New Jersey.

That consensus, however, came after a bipolar week for the models. "There was a lot of flip-flopping around," DePodwin said.

By the weekend, the evidence for a big snow was compelling, and the National Weather Service called for eight to 10 inches in and around Philadelphia, and up to 14 to the east.

That was at the high end of the computer guidance, Szatkowski said, in keeping with seasonal trends. Starting with Dec. 8, just about every winter storm had come on whiter than advertised. As it turned out, this one showed up drier.

The models were on to the right idea as long as a week ago. All along they saw a storm interacting with the cold, upper-level "polar vortex" that was plunging southward, with heavy precipitation forecast to fall along the fringe of the cold air.

But when push came to shovel, areas just to the north of the city wound up in the heart of the cold, dry air rather than on that fringe, and that made all the difference.

In the end, none of the models deserved high grades, Szatkowski said.

"I'm not handing out any A's or B's on this," he said. "If I looked hard, I might hand out a gentleman's C."


610-313-8210 twood@phillynews.com

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