Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather
By: Dr. Jeff Masters , 8:54 PM GMT on October 24, 2012
Hurricane Sandy hit the southeastern tip of Jamaica near 3:20 pm EDT this afternoon, as a Category 1 hurricane with 80 mph winds and a 973 mb pressure. According to NOAA's Historical Hurricane Tracks website, Sandy is the thirteenth hurricane to make a direct hit on the island, and the first since Hurricane Gilbert of 1988. Kingston, Jamaica recorded sustained winds of 44 mph and a pressure of 972 mb in the west eyewall of Sandy at 4 pm EDT. The eastern tip of Jamaica will see the strongest winds of the right-front quadrant and the heaviest damage, though. A distorted eye is apparent on visible satellite loops, but Sandy is showing only minor disruption to its inner core structure as a result of hitting Jamaica. According to the Jamaica Observer, "Alligator Pond [in St Elizabeth] was inundated with the high waves that came ashore. We are now getting reports of impacts out in St. Catherine, Portland and St. Thomas as the ground becomes saturated. We are now seeing where light poles are toppling and landslides being reported and roadway being flooded to the point where there is impeded access in east St. Thomas." Heavy rains from Sandy are falling in Haiti. A NOAA forecast based on microwave satellite data predicts 12 inches of rain for the tip of Haiti's southwestern Peninsula, which will likely cause life-threatening flash flooding. Fortunately, much lighter rainfall amounts are predicted for the capital of Port-au-Prince, where 350,000 people still live in the open under tarps in the wake of the January 2010 earthquake. In August, flooding from Hurricane Isaac killed at least 29 people in Haiti.
Figure 1. Hurricane Sandy over Jamaica. The large 55-mile diameter eye hit the island at 3:20 pm EDT, and crossed over the eastern tip of the island. The eye has been distorted into an odd triangular shape, due to interaction with the land area of Jamaica.
Figure 2. Predicted 24-hour rain amounts from Hurricane Sandy for the period ending at 8 am EDT Thursday, October 25, 2012. The prediction is based on microwave satellite data of precipitation. Image credit: NOAA/NESDIS.
Near-term forecast for Sandy
Sandy doesn't have much time over water before it makes landfall on the southeastern coast of Cuba near 10 pm EDT this Wednesday night, and the strongest the storm is likely to be then is a 90 mph Category 1. Passage over the rugged terrain of Cuba should weaken Sandy's winds by 20 - 30 mph, and will likely destroy the hurricane's eyewall. It will be difficult for the storm to rebuild its eyewall and regain all of that lost strength, in the face of the high wind shear of 20 - 30 knots it will encounter Thursday and Friday. However, the loss of the eyewall will cause Sandy's radius of tropical storm-force winds to expand, spreading out the winds over a wider area of ocean, and increasing the storm surge threat. This large wind field will likely drive a storm surge of 5 - 8 feet in the Bahamas, which is more characteristic of a storm with winds 20 mph higher. I expect that Sandy will be a 65 - 70 mph tropical storm as it traverses the Bahamas, and the storm will make its closest pass by Nassau around 10 pm EDT Thursday.
Figure 3. This Maximum Water Depth storm surge image for the Bahamas shows the worst-case inundation scenarios for a Category 1 hurricane with 85 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.
Sandy: a potential billion-dollar storm for the mid-Atlantic, New England, and Canada
The latest set of 12Z (8 am EDT) model runs are in, and they portray an increased risk to the U.S. and Canadian East Coasts for early next week. The GFS model, which had been showing that Sandy would head to the northeast out to sea, now has changed its tune, and predicts that Sandy will double back and hit Maine on Tuesday evening. The ECMWF model, which has been very consistent in its handling of Sandy, now has the storm hitting Delaware on Monday afternoon. These models are predicting that Sandy will get caught up by the trough approaching the Eastern U.S., which will inject a large amount of energy into the storm, converting it to a powerful subtropical storm with a central pressure below 960 mb and sustained winds of 60 - 70 mph. Winds of this strength would likely cause massive power outages, as trees still in leaf take out power lines. Also of great concern are Sandy's rains. 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, as predicted, 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. Another huge concern is storm surge flooding. 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.
There remains a lot of model uncertainty on where Sandy might go, and I still give a 30% chance that the storm will have a minimal impact on the U.S. An extra set of balloon-borne radiosondes is going to be launched at 2 pm EDT on Thursday all across the U.S., which should help tomorrow evening's model runs make better forecasts of where Sandy might go. Extra radiosondes will be launched every 6 hours through Saturday afternoon.
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