Hurricane Isaac
Status: Closed
Pre-Landfall 2 | Summary
Posting Date: August 23, 2012, 11:00:00 AM
Current Conditions
Tropical Storm Isaac is moving westward through the eastern Caribbean Sea toward the island of Hispaniola, which comprises the Dominican Republic and Haiti. Yesterday, Isaac drenched the northern islands of the Lesser Antilles. Currently less than 200 miles south of Puerto Rico according to the National Hurricane Center’s (NHC) 11:00AM EDT Advisory, Isaac is churning toward the southern coast of the Dominican Republic at about 15 miles per hour. Early this morning, an Air Force reconnaissance aircraft found that Isaac had a poorly defined inner core, with a large area of only light winds around its center. Based on that information, other data gathered by the aircraft, and similar findings by a NOAA flight and buoy readings, the NHC lowered Isaac’s maximum sustained wind speed to 40 mph.Reported Impacts
Tropical Storm Isaac lashed the French island of Guadeloupe in the Lesser Antilles with heavy rain yesterday, where more than 3 inches had accumulated by midday. The island’s chief meteorologist warned of swollen rivers and flooding, but so far there have been no reports of significant damage. News agencies report similar heavy rain last night on many of the area’s popular vacation islands—but also report that many seafront businesses remained open last night even as choppy surf broke against piers and seawalls. More than 3 inches of rain was also reported to have fallen in Martinique—where sustained winds reached 28 mph—and more than 1.5 inches fell in St. Lucia. In St. Croix in the U.S. Virgin Islands, schools and government offices were ordered to remain closed today, Thursday, and yesterday sandbags were distributed to residents. Along the Christiansted Harbor on the island, piers were empty of pleasure boats and along the narrow streets of the town sandbags were stacked around the doorways of the pastel-colored historic buildings. Governor Luis Fortuno of Puerto Rico declared a state of emergency yesterday and activated the National Guard as a precaution. The U.S. Coast Guard closed all ports on the island (and also in the U.S. Virgin Islands) to incoming commercial ships and warned that all commercial vessels larger than 200 gross tons must leave the islands or obtain permission to remain in port. Schools and government agencies were also closed, as was the historic San Felipe del Morro castle in Old San Juan. Impacts have already been felt farther away: the threat of Isaac’s expected impact on Florida—which will not happen for several days—triggered a nearly 6 percent jump in orange juice prices Wednesday, rising to a six-week high in trading in New York. Florida accounts for about 40 percent of the world’s orange juice supply.Exposure at Risk
Today’s passage of Tropical Storm Isaac past Puerto Rico and the U.S. Virgin Islands at a distance of about 200 miles minimizes the threat of significant damage to the islands. Puerto Rico’s building stock stands out in the Caribbean, inasmuch as a large percentage of residential homes are made of reinforced concrete. These buildings typically have flat reinforced-concrete roof slabs, which produce a structure that is very resistant to wind damage compared to other residential construction in the region. In older urban buildings in Puerto Rico there is a greater variety of construction material used. A majority of commercial structures are low to mid-rise buildings, usually of one to six stories. Small apartments, hotels, offices and other low-rise commercial properties are usually masonry or reinforced or block concrete. A concrete building type unique to Puerto Rico, the “bunker” style, is used for both residential and non-residential structures across the island. Bunker buildings have walls made of reinforced concrete, often reinforced with steel, combined with reinforced masonry. The impact of Hurricane Georges in 1998 paved the way for the adoption by Puerto Rico of the “Uniform Building Code” the next year, a standard code developed by the International Council of Building Officials in the U.S. Enforcement of the code has been increasingly effective. Most recently, Puerto Rico has adopted a similar standard, the International Building Code. The U.S. Virgin Islands have also seen improvements to hazard mitigation since its own disastrous event, Hurricane Marilyn in 1995. Shortly afterwards, the government of the island adopted the Uniform Building Code and initiated a roofing protection program. Most structures in the islands are now built with reinforced masonry or reinforced concrete, and the island of St. Croix performed very well when Hurricane Georges came in 1998, which was a Category 2 event. Because of the generally well-constructed building stock and the islands’ distance to the storm, significant losses are not expected as Isaac passes Puerto Rico and the U.S. Virgin Islands at its current intensity. In the Dominican Republic, which Isaac is expected to reach tonight or tomorrow, urban areas such as Santo Domingo tend to have large amounts of reinforced steel and concrete commercial structures, as well as some older construction, which is likely brick or masonry construction. As in Puerto Rico, wind design standards were improved following Hurricane Georges, but the major risk accompanying Isaac is anticipated to be flooding because of heavy rainfall. In rural areas, construction quality is typically of a lower standard, with unreinforced masonry and wood being more common than in cities. These types of structures, though less likely to be insured, are most vulnerable to significant damage from flood, which can compromise structural walls and lead to loss of contents. During Hurricane Irene in 2011, for example, more than 30,000 residents were reportedly forced to evacuate their homes because of precipitation-induced flooding. In Haiti, while Port-au-Prince generally remains in disrepair from the 2010 earthquake, buildings in other cities and the countryside—as in much of the Dominican Republic—are of poor construction and subject to damage by heavy rains, flooding, and mudslides.Forecast and Intensity
The National Hurricane Center does still expect that Tropical Storm Isaac’s convective development will improve as the storm brings more wet air into its internal environment over the next 36 hours. Isaac is then expected to strengthen only slowly. However, the environmental conditions Isaac finds itself in, including low wind shear and warm sea surface temperatures, are favorable for rapid intensification—which could occur if the storm develops a well-defined inner core. Thus, the NHC forecasts that Isaac will intensify into a Category 1 hurricane before it interacts with the mountainous terrain of Hispaniola. In addition, the NHC expects Isaac to turn toward the west-northwest sometime this afternoon, continuing this movement through Saturday. On the forecast track, the center of Isaac should continue to pass south of Puerto Rico today and be near the south coast of the Dominican Republic and Haiti tomorrow, Friday. The mountains on Hispaniola rise as high as 10,000 feet and Isaac should weaken once the storm interacts with the high terrain. The storm’s precise strength and direction of movement remain uncertain after it passes over the island, but it is still forecast to cross Cuba and approach Florida. For the Dominican Republic and Haiti, Isaac’s chief threat is not so much its winds as precipitation-induced flooding. Widespread areas are expected to receive as much as 8-12 inches of rain, with some pockets receiving up to 20 inches. The very tall mountains will enhance the possibility of heavy precipitation and flood damage. It is important to note that the computer forecast models have a large spread after the interaction with Cuba—from a complete decay of the storm to a Category 2 hurricane making landfall as far west as Alabama. In addition, some of the models keep the storm farther south during the early stages of their forecasts, which then have Isaac impacting Jamaica. Also, a storm surge will raise water levels buy as much as 3 to 5 feet above normal tide levels along the Hispaniola coast where there are also onshore winds. A storm surge will also raise water levels by as much as 1 to 3 feet above normal tide levels in Puerto Rico, the U.S. Virgin Islands, the British Virgin Islands, and the Northern Leeward Islands. Near the coast, the surge will be accompanied by dangerous surf and rip currents. AIR's Caribbean Tropical Cyclone team will be monitoring Isaac closely during the next few days and will post additional information on the development and impacts of this storm. The AIR U.S. Hurricane team is also analyzing Isaac’s potential threat to the U.S., including the possibility that the storm will impact the Republican National Convention, which begins on Monday in Tampa. AIR is also monitoring another system, named Tropical Storm Joyce only this morning, which could impact Bermuda next week according to the National Hurricane Center.