News•October 26, 2012

Historic Sandy Poised to Blast Mid-Atlantic, Northeast

Hurricane Sandy is evolving into a potentially unprecedented storm, nicknamed “Frankenstorm,” and is headed for the Mid-Atlantic and Northeast, where it threatens to cause widespread damage that will rival some of the biggest storms on record in the region. The storm, which has already killed at least 29 people in the Caribbean and Bahamas, will be part hurricane, part fearsome nor’easter, and is expected to bring tropical storm force winds to a wide swath of the coastline, from North Carolina northward to Maine. Although the exact point of landfall is still in question, computer models are converging on a most likely target from Norfolk, Va., to Atlantic City, N.J. on Tuesday into Wednesday, although the impacts from the storm may be felt in those areas as soon as Sunday.

Track guidance from computer model runs on Friday morning, showing the expected turn into the Mid-Atlantic region.
Click to enlarge the image. Credit: NCAR.

That track would bring the threat of major coastal flooding and very strong winds to the Boston to Washington corridor, home to about 50 million people, along with billions in expensive coastal property and infrastructure. About 66 million people live in the larger “Cone of Uncertainty” from the National Hurricane Center, which illustrates the storm's projected path. Hurricane Irene, which is not an ideal comparison because it affected a smaller area and was weakening as it approached New York City, caused $15.8 billion in insured losses alone, making it the costliest Category 1 hurricane on record. The biggest threat right now appears directed at the Delaware, Maryland, and New Jersey coastlines, especially in terms of coastal flooding from the combination of astronomical high tides and wind-driven storm surge. Those threats also exist for New York City, and all the way up to Boston.

The Weather Service is warning that winds of tropical storm force (sustained winds greater than 39 mph) to hurricane force (sustained winds greater than 74 mph) will lash the Mid-Atlantic and Northeast states starting Sunday and lasting through Wednesday, bringing “potentially serious coastal erosion and coastal flooding.” The strong winds could spread as far inland as Ohio, depending on the exact storm track.

“THE TIMING OF THE FULL MOON AND THE BUILD-UP OF TIDES OVER MULTIPLE TIDAL CYCLES SHOULD EXACERBATE THE SITUATION ALONG THE COAST, PARTICULARLY IN CORNERS SUCH AS THE NEW YORK BIGHT,” the Weather Service said. In fact, this storm has the potential to exceed the flooding seen in New York City from Hurricane Irene in 2011, when a storm surge came within mere inches of flooding the subway system in parts of Brooklyn and lower Manhattan. That flooding could have rendered some subway lines unusable for months.

The Weather Service forecast office in New York City is warning of the potential for sustained winds of 45-to-50 mph, with gusts as high as 80 mph, which could for last as long as 30 consecutive hours.

The large-scale weather pattern responsible for pinning Sandy along the East Coast.
Click to enlarge the image.
Credit: Remik Ziemlinski, Climate Central

Heavy rains on the order of at least 5 to 10 inches are expected, particularly along the north of the storm’s track. Inland snow is also possible, with current projections calling for accumulating snow in western Pennsylvania and Southeastern Ohio.

The storm is the result of a bizarre confluence of events, including a strong push of Arctic air moving southeastward from Canada and into the Midwest and Southeast, a powerful area of high pressure near Greenland, and storm systems out in the Atlantic. All of these factors are combining to pin the storm close to the East Coast, like a hapless football quarterback surrounded by onrushing defensive linemen.

Atlantic Weather Pattern Is Stuck

The upper air weather pattern that is steering Sandy, and helping it transition into a hybrid Frankenstorm, is relatively unusual, but not unheard of. The problem is that the pattern is occurring at precisely the wrong time — when a hurricane is moving out of the Caribbean.

Weather Channel hurricane expert Bryan Norcross wrote about this on Friday. “The freak part is that a hurricane happens to be in the right place in the world to get sucked into this doubled-back channel of air and pulled inland from the coast,” he said. “And the double-freak part is that the upper-level wind, instead of weakening the storm and simply absorbing the moisture — which would be annoying enough — is merging with the tropical system to create a monstrous hybrid vortex. A combination of a hurricane and a nor’easter.”

Recent studies have shown that blocking patterns such as the one that is currently over the Atlantic have appeared with greater frequency and intensity in recent years. Some scientists think that may be related to the loss of Arctic sea ice, which is one of the most visible consequences of manmade global warming. The 2012 sea ice melt season, which ended just 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 then 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.

Global warming is also expected to alter hurricane frequency and strength, making North Atlantic hurricanes slightly more powerful, while reducing the overall number of storms during the coming decades. Detecting such changes in the observational record is difficult, considering the varying ways people have kept tabs on hurricanes prior to the era of hurricane hunter aircraft flights. A recent study published in the Proceedings of the National Academy of Sciences found that warmer sea surface temperatures are tied to an increase in stronger Atlantic hurricanes.

Projected sea level pressure from the European model, showing a direct hit on Washington, D.C.
Click to enlarge the image. Credit: WeatherBell Analytics.

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.”

As Sandy transitions to a hybrid storm — a transition that appears to have already begun, based on the current satellite imagery, its wind field will expand, making the exact track of the storm’s center less relevant for the public to focus on. The storm may hold onto tropical characteristics near its core, since it will be traveling over the warm waters of the Gulf Stream. Sea surface temperatures off the East Coast are currently much warmer than average, as they were during the summer, when records were set off the New Jersey and New England coastlines.

In official and unofficial forums, meteorologists are discussing the rare nature of this storm, given that computer model simulations show that it could intensify to a strength that has never been observed before in Southern New England or the Mid-Atlantic. In fact, experts at the National Weather Service’s Hydrometeorological Prediction Center in College Park, Md., who are responsible for forecasting the storm if and when it is no longer classified as a tropical storm or hurricane, are adjusting the computer model projections because they think they may be overdoing the strength of the storm. “[Computer model simulations] SHOW PRESSURE SOLUTIONS WELL BEYOND WHAT HAS EVER BEEN OBSERVED NEAR THE NEW JERSEY/NEW YORK COAST (EVEN EXCEEDING THE 1938 LONG ISLAND EXPRESS HURRICANE) EARLY IN THE MEDIUM RANGE PERIOD . . . EVEN THE PERFECT/HALLOWEEN STORM OF 1991 “ONLY” BOTTOMED OUT AT 972 HPA, AND THIS FORECAST WAS ALLOWED TO EXCEED THAT SYSTEM WITHOUT GOING OVERBOARD,” the center’s meteorologists said in a discussion posted online on Friday morning.

In other words, this storm may still be of unprecedented intensity as judged by its minimum central air pressure, but it may not be the absolute horror show that is being projected by some of the computer models, which tend to overdo storm strength when hurricanes are transitioning into hybrid storms.

Bob Henson of the University Center for Atmospheric Research wrote that the Northeast and Mid-Atlantic region’s lowest measured surface air pressure on record occurred during the 1938 hurricane on Long Island, which was 946 hPa. Computer models have consistently been projected pressures down to between 935 to 950 hPa during this event. The minimum central pressure in a storm, be it a hurricane or a nor'easter, signifies how intense it is, and since wind results from differences in air pressure, it has a major impact on the strength of the winds around the storm center.