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Hurricane Sandy kills 21, heads towards the U.S.

By: Dr. Jeff Masters, 3:34 PM GMT on October 26, 2012

Hurricane Sandy plowed through the Bahama Islands as a powerful Category 2 hurricane with 105 mph winds yesterday, but high wind shear has destroyed the hurricane's eyewall and reduced Sandy to Category 1 strength, with 80 mph winds. Satellite loops show that the low-level center of Sandy is now exposed to view, with most of the storm's heavy thunderstorm activity pushed to the north side by strong upper level winds from the south-southwest. Cuban state media is reporting that eleven people were killed on Cuba, and damage was heavy, with thousands of homes damaged or destroyed. It was Cuba's deadliest hurricane since Category 5 Hurricane Dennis killed sixteen people in 2005. Damage was also substantial on Jamaica, where one person was killed, and power was knocked out for 70% of the island's residents. Nine deaths have been reported in Haiti due to flooding, and heavy rains from Sandy continue there this morning.

Figure 1. Morning satellite image of Sandy.

Forecast for Sandy
Wind shear is expected to remain a high 30 - 55 knots for the next four days, as Sandy interacts with a trough of low pressure to its west. The high shear should keep Sandy from intensifying the way most hurricanes do--by pulling heat energy out of the ocean. However, the trough approaching from the west will inject into Sandy what is called "baroclinic" energy--the energy one can derive from the atmosphere when warm and cold air masses lie in close proximity to each other. This transition will reduce the hurricane's peak winds, but strong winds will spread out over a wider area of ocean. This will increase the total amount of wind energy of the storm, keeping the storm surge threat high. This large wind field will likely drive a storm surge of 3 - 6 feet on Monday and Tuesday to the right of where the center makes landfall, on the mid-Atlantic or New York coasts. These storm surge heights will be among the highest ever recorded along the affected coasts, and will have the potential to cause hundreds of millions of dollars in damage.

The latest set of 00Z (8 pm EDT) and 06Z (2 am EDT) computer model runs still have wide differences in the timing and landfall location for Sandy. The ECMWF has been very consistent in its handling of Sandy, and continues to predict that Sandy will hit Delaware or Maryland on Monday afternoon--basically the same forecast it has had for three days. Our other top model for forecasting hurricane tracks, the GFS, has been more inconsistent, and predicts a landfall on Long Island, New York on Tuesday afternoon.

Figure 2. Predicted 5-day precipitation total for Hurricane Sandy as forecast by the 2 am EDT October 26, 2012 run of the HWRF model. A wide swath of 8 - 16" of rain (dark yellow colors) is predicted for North Carolina, Virginia, and Maryland. Image credit: Morris Bender, NOAA/GFDL.

Figure 3. A customized version of the Weather Research and Forecasting (WRF) Model run by Weather Decision Technologies (http://www.wdtinc.com/) computed the precipitation expected to fall from Sandy over the 5-day period ending at 5 am EDT Tuesday, October 30. Using the predicted precipitation, METSTAT, Inc. (http://www.metstat.com) computed the Extreme Precipitation Index (EPI) to represent how unusual the storms' precipitation is expected to be in terms of an average recurrence interval ("return period"). Several swaths of 24-hour rainfall amounts that one would expect to recur on average every 100 years (red through pink colors) are predicted. The recurrence interval statistics were computed based on precipitation frequency estimates from NOAA Atlas 14 Volume 2, published in 2004 (http://dipper.nws.noaa.gov/hdsc/pfds/.) Metstat does not supply the forecast EPIs for free, anyone can monitor the real-time EPI analysis (observed) at: http://metstat.com/solutions/extreme-precipitation-index-analysis/

Severe impacts likely in the U.S.
Sandy's expected landfall along the mid-Atlantic coast is likely to be a billion-dollar disaster. Sandy should bring sustained winds of 50 - 60 mph with gusts over hurricane force to a large section of coast, and the storm may be moving slowly enough that these conditions will persist for a full 24 hours. With most of the trees still in leaf, there will be widespread power outages due to downed trees. Sandy is expected to have tropical storm-force winds that extend out more than 400 miles from the center, which will drive a much larger storm surge than its winds would ordinarily suggest. The latest H*Wind analysis from NOAA's Hurricane Research Division put the destructive potential of Sandy's winds at 2.1 on a scale of 0 to 6, and the destructive potential of the storm surge much higher, at 4.2 on a scale of 0 to 6. The full moon is on Monday, which means astronomical tides will be at their peak for the month, increasing potential storm surge flooding. With Sandy's strongest winds expected to last at least 12 hours near the time of landfall, the peak storm surge will affect the coast for at least one high tide cycle, and possibly two. This will greatly increase the potential for storm surge damage and coastal erosion. If Sandy hits Long Island, as the GFS model predicts, the storm surge will be capable of over-topping the flood walls in Manhattan and flooding portions of the New York City subway system. Fresh water flooding from heavy rains is also a huge concern. Rainfall amounts of 5 - 10 inches will occur over several hundred mile-long swath of coast, with isolated amounts of 15 inches possible. Fortunately, soils are dry and river levels are low over most of the threatened region, which should keep Sandy's river flooding lower than that experienced last year during Hurricane Irene. Nevertheless, Sandy is shaping up to be a historic storm for the mid-Atlantic and Northeast U.S. that has few precedents.

Jeff Masters


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