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Posted at 11:35 AM ET, 10/26/2012

Hurricane Sandy on collision course with mid-Atlantic and Northeast

Track models for Hurricane Sandy overlaid on satellite image
As of this morning, hurricane Sandy’s peak winds have dropped to 80 mph (Category 1 hurricane), but that absolutely does not mean the threat to the eastern U.S. has decreased. Quite the opposite, in fact. It is forecast to re-organize and strengthen on its inevitable approach to the East Coast.

All forecast models are now clustering on a landfall between the Delmarva peninsula and Cape Cod. The official forecast from the National Hurricane Center forecasts the center to move over the Delaware Bay with the cone of uncertainty spanning from Cape Hatteras up to eastern Long Island.

Sandy is now responsible for at least 21 deaths across Haiti (9), Jamaica (1), and Cuba (11), as well as severe damage to homes and infrastructure due to flash flooding and mudslides.

At 11 a.m. EDT, the center was located just north of the western Bahamas heading north at 6 mph. Tropical storm watches have been posted for the Carolinas and will likely be extended northward later today.

Sandy is much less organized than it has been the past few days, with all of the deep “convection” (fancy word for thunderstorms) displaced to the north due to strong vertical wind shear (changing winds with height which disrupt the storm). Dry air is also wrapping around the system and feeding into the circulation - which removes energy from the storm. Given these factors, it should weaken some more in terms of wind speed in the near future.

But once the “baroclinic enhancement” phase - driven by contrast between cold air from the continental U.S. and Sandy’s tropical humidity - kicks into gear, the storm can strengthen even in what would normally be very hostile conditions. Sandy and the mid-latitude cold front (trough) that’s presently heading into the eastern U.S. will interact, and should result in something stronger than the sum of the original parts.

The storm has grown in size every day, with tropical storm force winds now extending 275 miles away from the center, and that is expected to increase dramatically once it begins interacting with the mid-latitude trough. It is the large wind field that will be such a problem along the entire coast.

Several days of onshore flow and strong winds blowing over the ocean for a long time from the same general direction generates huge swells. The storm surge is measured on top of the normal lunar tides, and by the worst possible timing, there is a full moon on Monday which results in higher (and lower) than normal lunar tides. During high tides on Sunday, Monday, and Tuesday, coastal flooding could be very serious.
Wave height forecast valid on Monday evening. The bright green contour is 16-20’, and the highest value is 45’ offshore. (NOAA)

Fri-Tue rainfall forecast. (NOAA)
In addition to the coastal flooding and beach erosion caused by the storm surge and large waves, inland flooding due to heavy rainfall is an extremely high threat. Day after day of heavy rain will not only cause streams and rivers to top their banks, but also soften up the ground allowing the strong winds to uproot trees and bring down power lines.

Already, the Hydrometeorological Prediction Center (HPC) is forecasting 3-9” of rain over many states, with the largest amounts in coastal North Carolina and then into Maryland, Delaware, Pennsylvania, New Jersey, New York, etc. This graphic (to the right) only includes rainfall out to Tuesday evening, and there will be more after that in many of these same areas.

As far as wind goes, the VA-NJ area could start seeing winds pick up by late Saturday night, then NJ-NY by Sunday afternoon, and RI-MA by Monday morning. Wind speeds will gradually ramp up and the strongest winds may not occur for 24-36 hours after they start increasing. The wind field may not entirely diminish until NEXT weekend, so that’s about a full week of windy conditions.

Wind speed forecast from the GFS model valid on Tuesday afternoon. Severe conditions will arrive long before the center does. (
Higher elevations in PA, WV, etc are expected to get more than a foot of very wet snow.

The most recent run of the GFS model places the center of the low directly over New York City on Tuesday afternoon while the European models favors the southern Delmarva, but refer to the first figure with all of the models to get an idea of the possibilities. If either of these scenarios verify, this will be an amazingly destructive storm with damages over a billion dollars and vast numbers of people without electricity and possibly with water in their houses.

* Brian McNoldy is a senior researcher at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.

By Brian McNoldy  |  11:35 AM ET, 10/26/2012

Categories:  Latest, Tropical Weather

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