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Sunday, October 28, 2012

Jeff Masters: Massive Hurricane Sandy building a huge and destructive storm surge

by Jeff Masters, wunderground, October 28, 2012 (2:34 p.m., GMT)
Massive and dangerous Hurricane Sandy has grown to record size as it barrels northeastwards along the North Carolina coast at 10 mph. At 8 a.m. EDT, Sandy's tropical storm-force winds extended northeastwards 520 miles from the center, and 12-foot-high seas covered a diameter of ocean 1,030 miles across. Since records of storm size began in 1988, no tropical storm or hurricane has been larger (though Hurricane Olga of 2001 had a larger 690-mile radius of tropical storm-force winds when it was a subtropical storm near Bermuda).

Sandy has put an colossal volume of ocean water in motion with its widespread and powerful winds, and the hurricane's massive storm surge is already impacting the coast. A 2' storm surge has been recorded at numerous locations this morning from Virginia to Connecticut, including a 3' surge at Virginia's Chesapeake Bay Bridge Tunnel and Sewells Point at 9 a.m. EDT. Huge, 10-15 foot-high battering waves on top of the storm surge have washed over Highway 12 connecting North Carolina's Outer Banks to the mainland at South Nags Head this morning. The highway is now impassable, and has been closed. The coast guard station on Cape Hatteras, NC, recorded sustained winds of 50 mph, gusting to 61 mph, at 5:53 a.m. EDT this morning. In Delaware, the coastal highway Route 1 between Dewey Beach and Bethany Beach has been closed due to high water. 

Even though Sandy is a minimal Category 1 hurricane, its storm surge is extremely dangerous, and if you are in a low-lying area that is asked to evacuate, I strongly recommend that you leave.

Figure 1. A fright to behold: morning satellite image of massive Hurricane Sandy.

Sandy's death toll now at 65
Sandy was a brutal storm for the Caribbean, the storm's death toll now stands at 65. The death toll is highest in Haiti, with 51 deaths. Prime Minister Laurent Lamothe told the Associated Press that "This is a disaster of major proportions. The whole south is under water." Approximately 8-10" of rain (200-250 mm) fell in the capital of Port-au-Prince. Eleven people were killed in Cuba, where 35,000 homes were damaged or destroyed. Sandy is also being blamed for 1 death in Jamaica, 1 in Puerto Rico, and 1 in the Bahamas. 

Figure 2. A resident carries a metal sheet from a house after heavy rains damaged by Hurricane Sandy in Port-au-Prince, Haiti, Thursday, Oct. 25, 2012. Sandy is being blamed for 51 deaths in Haiti. (AP Photo/Dieu Nalio Chery)

Figure 3. Satellite-estimated rainfall amounts from NASA's TRMM satellite show that portions of Haiti received over 12.75" (325 mm) of rain (pink colors) from Hurricane Sandy. The capital of Port-au-Prince received 8-10" (200-250 mm.) Image credit: NASA.

Intensity and Track Forecast for Sandy
Sandy has a rather unusual structure, with the strongest winds on the southwest side of the center, but a larger area of tropical storm-force winds to the northeast of the center. Most of the storm's heavy thunderstorm activity is on the storm's west side, in a thick band several hundred miles removed from the center, giving Sandy more the appearance of a subtropical storm rather than a hurricane. 

Satellite loops show that the low-level center of Sandy is no longer exposed to view, and heavy thunderstorms are increasing in areal extent near the center, due to a reduction in wind shear from 35-40 knots last night to 25-30 knots this morning. Wind shear is expected to drop another 5 knots today, which may allow the storm to build an increased amount of heavy thunderstorms near its center and intensify by 5-10 mph over the next 24 hours. 

The NOAA Hurricane Hunters noted this morning that Sandy had a partial eyewall on the west through SE sides of the center, and the storm may be able to build a nearly complete eyewall by Monday morning. By Monday afternoon, though, Sandy will be moving over cool 25°C waters, which should slow down this intensification process. However, the trough of low pressure that will be pulling Sandy to the northwest towards landfall on Monday will strengthen the storm by injecting "baroclinic" energy (the energy one can derive from the atmosphere when warm and cold air masses lie in close proximity to each other). Sandy should have sustained winds at hurricane force, 75-80 mph, at landfall. 

Sandy's central pressure is expected to drop from its current 951 mb to 945-950 mb at landfall Monday night. A pressure this low is extremely rare; according to wunderground weather historian Christopher C. Burt, the lowest pressure ever measured anywhere in the U.S. north of Cape Hatteras, NC, is 946 mb (27.94") measured at the Bellport Coast Guard Station on Long Island, NY, on September 21, 1938, during the great "Long Island Express" hurricane. 

The latest set of 00Z (8 p.m. EDT) and 06Z (2 a.m. EDT) computer model runs are in agreement that Sandy will make landfall between 10 p.m. Monday night and 4 a.m. Tuesday morning in New Jersey.

Figure 4. Predicted maximum storm surge from Hurricane Irene. There is a 10% chance that the storm surge could exceed the heights given here, so most regions will receive a surge lower than this forecast. The greatest surge is expected in the waters surrounding New York City, since the shape of the bays will act to funnel the water to higher levels.

Sandy's storm surge a huge threat
Last night's 9:30 p.m. EDT H*Wind analysis from NOAA's Hurricane Research Division put the destructive potential of Sandy's winds at a modest 2.6 on a scale of 0 to 6. However, the destructive potential of the storm surge was exceptionally high: 5.7 on a scale of 0 to 6. This is a higher destructive potential than any hurricane observed between 1969 and 2005, including Category 5 storms like Katrina, Rita, Wilma, Camille, and Andrew. The previous highest destructive potential for storm surge was 5.6 on a scale of 0 to 6, set during Hurricane Isabel of 2003. Sandy is now forecast to bring a near-record storm surge of 6-11 feet to Northern New Jersey and Long Island Sound, including the New York City Harbor. 

While Sandy's storm surge will be nowhere near as destructive as Katrina's, the storm surge does have the potential to cause many billions of dollars in damage if it hits near high tide at 9 p.m. EDT on Monday. The full moon is on Monday, which means astronomical high tide will be about 5% higher than the average high tide for the month. This will add another 2-3" to water levels. Fortunately, Sandy is now predicted to make a fairly rapid approach to the coast, meaning that the peak storm surge will not affect the coast for multiple high-tide cycles. 

Sandy's storm surge will be capable of overtopping the flood walls in Manhattan, which are only 5 feet above mean sea level. On August 28, 2011, Tropical Storm Irene brought a storm surge of 4.13' to Battery Park on the south side of Manhattan. The waters poured over the flood walls into Lower Manhattan, but were 8-12" shy of being able to flood the New York City subway system. According to the latest storm surge forecast for NYC from NHC, Sandy's storm surge is expected to be several feet higher than Irene's. If the peak surge arrives near Monday evening's high tide at 9 p.m. EDT, a portion of New York City's subway system could flood, resulting in billions of dollars in damage. I give a 50% chance that Sandy's storm surge will end up flooding a portion of the New York City subway system.

An excellent September 2012 article in the New York Times titled, "New York Is Lagging as Seas and Risks Rise, Critics Warn," quoted Dr. Klaus H. Jacob, a research scientist at Columbia University’s Earth Institute, on how lucky New York City was with Hurricane Irene. If the storm surge from Irene had been just one foot higher, "...subway tunnels would have flooded, segments of the Franklin D. Roosevelt Drive and roads along the Hudson River would have turned into rivers, and sections of the commuter rail system would have been impassable or bereft of power," he said, and the subway tunnels under the Harlem and East Rivers would have been unusable for nearly a month, or longer, at an economic loss of about $55 billion. Dr. Jacob is an adviser to the city on climate change, and an author of the 2011 state study that laid out the flooding prospects. “We’ve been extremely lucky,” he said. “I’m disappointed that the political process hasn’t recognized that we’re playing Russian roulette.” 

Sandy's winds
Sandy will bring sustained winds of tropical storm-force to a 1000-mile swath of coast on Monday and Tuesday. Winds of 55-75 mph with gusts over hurricane force will occur along a 500-mile-wide section of coast. With most of the trees still in leaf, there will be widespread power outages due to downed trees, and the potential for several billion dollars in wind damage. A power outage computer model run by Johns Hopkins University predicts that 10 million people will lose power from the storm.

Sandy's rains
Sandy's heavy rains are going to cause major but probably not catastrophic river flooding. If we compare the predicted rainfall amounts for Sandy (Figure 5) with those from Hurricane Irene of 2011 (Figure 6), Sandy's are expected to be about 30% less. Hurricane Irene caused $15.8 billion in damage, most of it from river flooding due to heavy rains. However, the region most heavily impacted by Irene's heavy rains had very wet soils and very high river levels before Irene arrived, due to heavy rains that occurred in the weeks before the hurricane hit. That is not the case for Sandy; soil moisture is near average over most of the mid-Atlantic, and is in the lowest 30th percentile in recorded history over much of Delaware and Southeastern Maryland. 

One region of possible concern is the Susquehanna River Valley in Eastern Pennsylvania, where soil moisture is in the 70th percentile, and river levels are in the 76th-90th percentile. This area is currently expected to receive 3-6 inches of rain (Figure 4), which is probably not enough to cause catastrophic flooding like occurred for Hurricane Irene. I expect that river flooding from Sandy will cause less than $1 billion in damage.

Figure 5. Predicted 5-day rainfall for the period ending Friday morning, November 2, 2012, at 8 a.m. EDT. Image credit:NOAA/HPC.

Figure 6. Actual rainfall for 2011's Hurricane Irene, which caused $15.8 billion in damage, most of it from river flooding due to heavy rains. Sandy's rains are predicted to be about 30% less than Irene's. Image credit: NOAA/HPC.

Sandy's snows
You can add heavy snow to the list of weather frights coming for the Eastern U.S. from Sandy. A Winter Storm Watch is posted for much of southeastern West Virginia for Sunday night through Monday, when 2-6 inches of wet, heavy snow is expected to fall at elevations below 2000 feet. At higher elevation above 3,000 feet, 1-2 feet of snow is possible. With high wind gusts of 35-45 mph and many trees still in leaf, the affected area can expect plenty of tree damage and power outages. Lesser snows are expected in the mountains of Virginia, Tennessee, and North Carolina. 

Sandy's tornado threat is minimal
The severe thunderstorm and tornado threat from Sandy Sunday and Monday looks low, due to minimal instability.

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