News•October 25, 2012

East Coast Facing Major Threat From Hurricane Sandy

After undergoing a period of rapid intensification on Wednesday, Hurricane Sandy is now approaching the Bahamas as a Category 2 storm, with tropical storm force winds and battering surf spreading to the east coast of Florida. While there remains a lot of uncertainty about its path, computer-model projections continue to show that the storm will likely make a feared turn into the densely populated Northeast coastline early next week as a hurricane, or hurricane-like storm system. That encompasses nearly 50 million people from the Washington, D.C.-Boston corridor, and a roughly 600-mile stretch from Norfolk to Boston.

Projection from the European model showing a worst-case scenario storm track for parts of the Mid-Atlantic.
Click to enlarge the image. Credit: Weatherbell Analytics.

Since Wednesday, the threat of Sandy making landfall along the Delaware, Maryland, and Virginia coast has heightened, as more computer-model simulations have been projecting that scenario. If that happens, it could bring major impacts to vulnerable low lying areas, including Norfolk, Va., Rehobeth, Del., and even Washington, D.C. Those impacts could include major coastal flooding associated with onshore winds driving a storm surge on top of astronomical high tides, prolonged periods of damaging winds, heavy rains that would lead to inland flooding, and even heavy snowfall in higher elevations. Widespread power outages would also be likely, and the storm would bring an abrupt end to the fall leaf-peeping season, not to mention ruin Halloween for thousands.

Additionally, if damage is severe enough, it could even affect Election Day. Other models take Sandy further north, making landfall near New York City, which would put Long Island, Connecticut, Rhode Island, and Massachusetts in the area of strongest winds and most significant flooding.

However, there is still considerable uncertainty about the storm’s path, strength, and even its structure and as it moves northward near the East Coast. The bottom line is that people in areas from the Carolinas northward to Maine, but most especially from Norfolk, Va., to Boston need to begin making preparations as directed by local weather and emergency management officials. In a briefing for emergency managers, the National Weather Service forecast office in Philadelphia warned of the potential for “record coastal flooding,” “record river flooding,” and winds of 70 mph or higher in the event that the storm storm makes landfall near southern New Jersey and Delaware.

The coastal flooding threat is perhaps the biggest concern, since the storm will have a large wind field and will affect the East Coast during a full moon, when tides are at their highest for the month. In addition, the storm will be a slow-mover, meaning that battering waves and storm surge may coincide with multiple high-tide cycles.

Notably, this storm is coming almost exactly a year after the “Snowtober” storm that struck the Northeast in 2011, and 21 years after the “Perfect Storm” sunk ships and flooded coastal communities.

A “spaghetti plot” of the range of computer model projections for Hurricane Sandy. Many of the projections show the storm striking the East Coast.
Click to enlarge the image. Credit: Weatherbell Analytics.

The forecast track for Sandy that the models are showing would be extremely rare, if not unprecedented. As The Weather Channel reported on Thursday, only six hurricanes with surface pressures as low as Sandy is projected to have (the lower the pressure, the stronger the storm) have moved within 200 nautical miles of the East Coast north of Virginia Beach and then made landfall. The most recent of those storms were Irene in 2011, Hurricane Bob in 1991, and Hurricane Gloria in 1985.

None of those storms struck during late October, however, and the Perfect Storm was not quite as strong, judging by its lowest surface pressure, as Sandy is projected to be when it nears the Northeast.

Forecasting the intensity of Sandy as it moves northward is a particular challenge, because the storm will be interacting with powerful jet stream winds aloft that are associated with an outbreak of Arctic air that has brought snow to many Western states, resulting in Denver’s first snowfall of the season on Wednesday.

Typically, such a jet stream setup would help sweep a tropical storm or hurricane out to sea before it could threaten the U.S. However, in this case, the weather pattern across the Atlantic is boxing Sandy in as if it were facing a giant sea of bumper cars. A massive dome of high pressure near the Canadian Maritimes and Greenland, along with a storm in the Central Atlantic, are likely to combine to deflect Sandy back toward the U.S., according to most computer-model simulations run during the past 24 hours.

Recent studies have shown that blocking patterns have appeared with greater frequency and intensity in recent years, which some scientists think may be related to the loss of Arctic sea ice as a result of global warming. The 2012 sea ice melt season, which just ended one month ago, was extreme, with sea ice extent, volume, and other measures all hitting record lows. The loss of sea ice opens large expanses of open water, which absorbs more of the incoming solar radiation and adds heat and moisture to the atmosphere, thereby helping to alter weather patterns. Exactly how weather patterns are changing as a result, however, is a subject of active research.

The official “cone of uncertainty” track forecast from the National Hurricane Center.
Click to enlarge the image. Credit: NOAA/NHC.

While it is not unusual to have a high pressure area near Greenland, its intensity is striking for this time of year. As Jason Samenow of the Capital Weather Gang wrote on Wednesday, the North Atlantic Oscillation, which helps measure this blocking flow, “is forecast to be three standard deviations from the average — meaning this is an exceptional situation.”

Hurricane Sandy may be an example of what can happen when a blocking pattern that may have been supercharged by sea ice loss occurs at just the wrong time — when a hurricane happens to be moving up the Eastern Seaboard.

A Hurricane or Hurricane-like Hybrid?

One of the many forecasting challenges is the likelihood that as Sandy moves northward, it will interact with strong jet stream winds that will add energy to the storm, but could also disrupt some of the thunderstorms near its core. The National Hurricane Center is predicting that Sandy will undergo a transition to a post-tropical cyclone, as it draws more of its energy from the dip in the jet stream rather than from the warm ocean waters. However, computer models tend to do a poor job of simulating such transition phases, which adds more wrinkles to the forecast.

It is a safe bet, though, that Sandy’s wind field will spread significantly as it nears the Mid-Atlantic and Northeast, which increases the threat of coastal flooding. Because of the combination of factors that will make Sandy a hybrid storm, the National Weather Service referred to the upcoming event as “Frankenstorm” in an online discussion on Thursday morning.

Weather Channel hurricane expert Bryan Norcross said the interaction between Sandy and the jet stream is a major wild card in the forecast. “. . . the energy from the jet-stream can begin the transition of Sandy to a hybrid type of storm with a large center and strong winds spread out over a much larger area,” he wrote in a blog post on Thursday. “These large-diameter storms, whether they are tropical like Ike or Irene, or nor'easters like a big northeast blizzard, produce storm surge and other effects a long way from the center. In fact, the weather near the center is often not significant at all.”

In addition, computer models are much less skillful at predicting hurricane intensity changes compared to making track forecasts.