Thursday, October 8, 2015

Snow: The upside-down storm

Yet again we learn that the atmosphere has less regard for computer models than meteorologists.

Snow: The upside-down storm


To their immense disappointment, hundreds of thousands of school children are being deprived of educational-enrichment opportunities today because of the weather.

But in some cases, we should point out, because of the forecast.

Snow continues in South Jersey and Delaware, and some places there could wind up with as much as half a foot.

North of Philadelphia, however, if we could choose a name for this one it be Kohoutek, for the famously hyped comet of the 1970s that ended up being barely a blur in the sky.

For the commercial and government forecasters who were predicting a widespread disruptive event with the potential for double-digit snows totals, nature has delivered yet another dose of humility.

“It’s nice to be wrong together,” said Dan DePodwin, a meteorologist with Accu-Weather Inc., “but it’s not good to be wrong at all.”

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, with the only issue left to resolve being temperature.

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

In fact, late Wednesday the respected European did say that no measureable snow would fall in Philly – but for the wrong reason; it was calling for a major rainstorm.

On Friday, the models went cold, and they started pushing the heavier precipitation toward the southeast -- the reverse of the pattern of the winter, which was to bring in farther northwest.

Still, the evidence for a big snow was compelling, and the National Weather Service called for 8-10 inches in and around Philly.

That was at the high end of the computer guidance, said Gary Szatkowski, the meteorologist in charge of the National Weather Service, and starting with Dec. 8, winter snowstorms had been showing up with more juice than expected.

As it turned out, this one showed up drier.

Why were the models having such a hard time?

 This one was different in several ways.

 “We haven’t seen a storm like this all winter,” said DePodwin. It raced across the country in a day and a half,  and did not spend much time under the surveillance of land-based observing systems.

The models rely on “initial conditions,” snapshots of the atmosphere at a given time, derived from millions of observations. Models then forecast what the atmosphere will look like in six hours, then six hours after that, on out in time.

Any gaps in initial observations will result in errors that will continue to grow out in time. That’s one reason why virtual snowstorms four and five days away should be viewed with skepticism.

The discussion about this one kicked off on Feb. 23 on the American Weather Forums chat board.

All along, the models 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.

As it turned out, 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 meantime, we can report that the models will continue to get back on that bicycle and spit out more storm threats during the next few weeks.

Be skeptical.


Inquirer Weather Columnist
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About this blog

Everyone talks about the weather, and here we write about it.

When we’re around and conditions warrant, we’ll keep you updated about what’s coming, but we will do our best always to discuss weather and climate developments in context and remind you that nothing in the atmosphere happens in a vacuum.

Tony Wood has been writing about the atmosphere for The Inquirer for 26 years.

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Tony Wood Inquirer Weather Columnist
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