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Sandy slams Cuba, intensifies over the Bahamas

By: Dr. Jeff Masters, 2:39 PM GMT on October 25, 2012

Hurricane Sandy shrugged off wind shear of 20 knots and passage over the southeastern tip of Jamaica yesterday afternoon, explosively deepening into a top-end Category 2 hurricane with 110 mph winds. Sandy made landfall in Southeastern Cuba around 1 am EDT this morning near Santiago de Cuba, which experienced sustained winds of 78 mph, gusting to 114 mph. Winds at Guantanamo Bay, Cuba peaked at 58 mph, gusting to 72 mph, at 3 am local time this morning, and the base received 3.51" of rain from Sandy as of 8 am EDT this morning. Punta Lucrecia, Cuba on the north coast of Cuba received 8.42" of rain from Sandy as of 8 am EDT. Cuban state media is reporting that one person was killed on Cuba, and damage was heavy, with thousands of homes damaged or destroyed. Damage was also substantial on Jamaica, where one person was killed, and power was knocked out for 70% of the island's residents. One death has been reported in Haiti due to flooding.

Sandy survived the crossing of Cuba's high mountains with its inner core relatively intact, and is now re-intensifying over the warm waters of the Central Bahama Islands. The latest 9:30 am center fix from the Hurricane Hunters found a central pressure of 965 mb, down 3 mb in 1.5 hours. The eye is intermittently visible on satellite loops, and Sandy appears to be holding its own against the high 30 knots of wind shear affecting it.

Figure 1. Hurricane Sandy approaching landfall in Southeast Cuba as seen by Cuban radar at 10:15 pm EDT Wednesday, October 24, 2012. Image credit: Cuban Institute of Meteorology.

Figure 2. Morning satellite image of Sandy.

Forecast for Sandy
Wind shear is expected to rise to 40 - 55 knots by Friday, as Sandy interacts with a trough of low pressure to its west. The high shear should disrupt Sandy's inner core and reduce the maximum winds. However, the trough will also inject energy into Sandy, and the hurricane's winds will spread out over a wider area of ocean, keeping the storm surge threat high. This large wind field will likely drive a storm surge of 5 - 8 feet in the Bahamas. Sandy will make its closest pass by Nassau around 8 am EDT Friday.

The latest set of 00Z (8 pm EDT) and 06Z (2 am EDT) computer model runs are in substantial agreement for the next 3 days, but Sandy's future is as clear as mud after that. Sandy will continue to punish the Bahamas today and Friday, as it tracks north to north-northwest. Sandy will probably come close enough to the Southeast U.S. on Saturday afternoon to spread heavy rains to the coasts of South Carolina and North Carolina. However, the 4 - 6 day computer model forecasts for Sunday - Tuesday diverge widely. The GFS model, which has been one of our two top models for predicting hurricane tracks the past two years, has been very inconsistent with its handling of Sandy. Runs of the GFS model done 6 hours apart, at 8 pm last night and 2 am EDT this morning, were 300 miles apart in their position for Sandy on Tuesday, with the latest run predicting a landfall in Maine on Wednesday morning. On the other hand, the ECMWF model, our other top model for predicting hurricane tracks, has been very consistent in its handling of Sandy. The ECMWF model has Sandy hitting Delaware on Monday afternoon, the same forecast it has had for three consecutive runs. The other models tend to follow one extreme or the other, and NHC is picking a solution somewhere in the middle of these two extremes. An extra set of balloon-borne radiosondes is going to be launched at 2 pm EDT Thursday all across the U.S., which should help this evening's model runs. Extra radiosondes will be launched every 6 hours through Saturday afternoon.

Figure 3. This Maximum Water Depth storm surge image for the Bahamas shows the worst-case inundation scenarios for a Category 2 hurricane with 105 mph winds, as predicted using dozens of runs of NOAA's SLOSH model. For example, if you are inland at an elevation of ten feet above mean sea level, and the combined storm surge and tide (the "storm tide") is fifteen feet at your location, the water depth image will show five feet of inundation. No single storm will be able to cause the level of flooding depicted in this image. Sandy's maximum storm surge may reach levels portrayed in this image for some islands in the Bahamas. See wunderground's storm surge pages for more storm surge info.

The Northeast U.S. scenario
If Sandy makes landfall farther to the north near Maine and Nova Scotia, heavy rains will be the main threat, since the cold waters will weaken the storm significantly before landfall. The trees have fewer leaves farther to the north, which will reduce the amount of tree damage and power failures compared to a more southerly track. However, given that ocean temperatures along the Northeast U.S. coast are about 5°F above average, there will be an unusually large amount of water vapor available to make heavy rain. If the trough of low pressure approaching the East Coast taps into the large reservoir of cold air over Canada and pulls down a significant amount of Arctic air, the potential exists for the unusually moist air from Sandy to collide with this cold air from Canada and unleash the heaviest October rains ever recorded in the Northeast U.S., Nova Scotia, and New Brunswick. This Northeast U.S. scenario would probably cause damages near $100 million dollars.

The mid-Atlantic U.S. scenario
Landfall Monday along the mid-Atlantic coast on Monday, as predicted by the ECMWF and NOGAPS models, would likely be a billion-dollar disaster. In this scenario, Sandy would be able to bring sustained winds near hurricane force over a wide stretch of heavily populated coast, causing massive power outages, as trees still in leaf fall and take out power lines. Sandy is expected to have tropical storm-force winds that extend out more than 300 miles from the center, which will drive a much larger storm surge than its winds would ordinarily suggest. The full moon is on Monday, which means astronomical tides will be at their peak for the month, increasing potential storm surge flooding. Fresh water flooding from heavy rains would also be a huge concern. Given the ECMWF's consistent handling of Sandy, I believe this mid-Atlantic scenario has a higher probability of occurring than the Northeast U.S. scenario. However, it is likely that the models are overdoing the strength of Sandy at landfall. The models have trouble handling the transition from tropical storm to extratropical storm in these type of situations, and I expect that the 940 mb central pressure of Sandy predicted at landfall Monday in Delaware by the ECMWF model is substantially overdone.

Jeff Masters


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.