Atmospheric conditions cause Hurricane Sandy to intensify

October 29, 2012

Hurricane Sandy, what a beast: The flooding rains we’ll see, the tree-toppling winds we’ll feel, the hours or days we might endure without power — we’ll blame all our weather-related upheaval this week on Sandy. And probably long afterward, we’ll remember her name.

In fairness, though, what’s happening here isn’t entirely her fault.

The true culprits are a couple of other atmospheric conditions — a high-pressure system centered west of Greenland and a huge wave of cold air moving eastward across the United States. If not for those two enormous bundles of air, which pushed the hurricane toward land and beefed up its intensity, Sandy likely would have stayed offshore, causing relatively minimal damage as it wobbled past the Mid-Atlantic region and finally died far out to sea, meteorologists say.

Like a drunken tourist, she might have just made some noise and broken some glass, then staggered off, soon to be forgotten.

Born in the Caribbean, fueled by the warmth of the sea there, Sandy was a fairly ordinary hurricane, if somewhat stronger than most, as it stormed across Haiti and the Dominican Republic, Cuba, Jamaica, Puerto Rico and the Bahamas, causing at least 65 deaths.

Under different circumstances, Sandy “probably would have stayed out to sea” as it skirted the East Coast, tracking northeastward and losing power along the way, said meteorologist Wes Junker, a contributor to The Washington Post’s Capital Weather Gang blog. The region would have experienced some heavy rain and wind, some coastal flooding and beach erosion, but probably no widespread destruction, said another contributor, meteorologist Brian McNoldy.

Eventually, over the colder water of the North Atlantic, Sandy “would have run out of fuel and rather quickly dissipated,” McNoldy said.

But no such luck. That high-pressure system near Greenland has reached out, grabbed Sandy and pulled her toward us.

High-pressure systems feature clockwise winds. Imagine a giant pinwheel. Sandy was plowing northeastward, not much bothering the East Coast, when those clockwise winds swept down and hit her from the right, causing her to turn left — smack into the Mid-Atlantic states.

Meanwhile, that wave of cold air moving eastward across the United States “actually captured Sandy,” also drawing it toward land, Junker said. The confluence of these three mammoth atmospheric systems — the hurricane, the high-pressure system, the mass of cold air — was a complex and unusual meteorological event. The effect is hard to explain in simple English, but it’s easy to see: Just look out your window. The cold air mass “is fueling Sandy and intensifying her,” Junker said.

The storm, born in tropics, began to look less and less like a hurricane over the colder Mid-Atlantic, McNoldy said. He said it lost the characteristics of a tropical storm, such as its warm core and its symmetrical cloud field, eye wall and eye. Sandy slowly became what’s called “extratropical storm,” featuring “a cold front and a warm front at the ground level,” all of which is of great interest to meteorologists. But what does that mean to the layperson?

“The average person really doesn’t care,” McNoldy said. “When it becomes an extratropical storm, that doesn’t mean it’s any less of a storm than it was as a hurricane. It’ll cause a storm surge and rip your roof off just the same.”

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