Will Henri Be Another Superstorm Sandy? Here's What the Forecast Says. - The New York Times

Adam Sobel is a professor and director of the Initiative on Extreme Weather and Climate at Columbia University. He is an atmospheric scientist and host of the “Deep Convection” podcast.

There are some striking similarities between Tropical Storm Henri, which is forecast to become a hurricane before making landfall along the Northeast coast this weekend, and Hurricane Sandy, which devastated parts of New York and New Jersey in 2012.

At the same time, there are some very important differences that will probably affect the track and impact of Henri. New York City, in particular, is not at great risk this time, though some forecast models still show Henri turning west and making landfall there.

There’s a reason that hurricanes rarely reach New York or New England, where none has made landfall in the 30 years since Hurricane Bob in 1991. As storms drift north, they get caught up in the prevailing winds at higher latitudes. These winds generally blow from west to east (unlike tropical winds, which generally blow the opposite way), and push hurricanes out to sea, away from the Eastern Seaboard.

Something has to break that pattern before the Northeast can get a direct hit.

What can do that? Either a high-pressure system offshore to the east of the storm, or a low-pressure system approaching from the land to the west, or both, can drive a hurricane northward rather than eastward. When those conditions occur, the south-facing parts of the coast — from Long Island to Cape Cod — become the most likely landfall area, as it is for Henri.

Similar meteorological situations have been responsible for most, if not all, of the hurricane landfalls in the area, like the 1938 “Long Island Express” storm and several hurricanes in the 1950s. Those events prompted the building of storm surge barriers in Stamford, Conn., Providence, R.I., and New Bedford, Mass.

Sandy was an extreme case. An approaching low-pressure system was strong enough to cause Sandy to revolve around it (and vice versa) as the two systems merged in what is called the Fujiwhara effect. This process strengthened Sandy and slung it westward, resulting in the “left hook” that brought the storm into the New Jersey shore at nearly a right angle. No other storm is known to have done that.

A similar configuration is developing now: An approaching upper-level low-pressure system is predicted to do a Sandy-like dance with Henri. But it doesn’t look as though the Fujiwhara effect will be powerful enough this time to sling Henri as far west as Sandy turned, nor is it likely to give Henri the strength of Sandy, which reached Category 3 at one point. (By the time Sandy came ashore, it was back down to Category 1, which Henri is predicted to be at landfall.)

Beyond that, Sandy was an extremely large storm. Its size and westward track conspired to drive a catastrophic surge of seawater into New York Harbor. With Henri looking less extreme in both respects, a major disaster for New York City and New Jersey is unlikely this time.

There are reasons to hope that Henri won’t actually be disastrous anywhere. It is forecast to slow down and weaken before landfall. But it is too early to say that with confidence.

At this point, preparation and vigilance are very much in order, especially on Long Island and across southeastern New England.

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Will Henri Be Another Superstorm Sandy? Here's What the Forecast Says. - The New York Times
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