Hurricane Isaac

Pre-Landfall 1 | Summary
Posting Date: August 22, 2012, 11:20:00 AM

Current Conditions

Tropical Storm Isaac is not expected to increase beyond tropical storm strength as it tracks directly over the French island of Guadeloupe later today. Isaac’s winds are likely to impact several islands as many are situated close to one another in that area, including Dominica, Montserrat, and Antigua. Despite low wind shear and warm sea surface temperatures (SSTs), Isaac has not strengthened beyond a tropical storm since its formation off the African coast on August 17. A mass of dry air that had been lying north of the storm’s track prevented much development; however, Isaac is now entering conditions more conducive to storm strengthening as it moves into the Lesser Antilles. The dry air has dissipated, and the warm SSTs in this area are allowing Isaac to become more organized.

Exposure at Risk

In Guadeloupe, reinforced masonry is the dominant construction type for both residential and commercial structures, with the remainder of the building stock consisting of unreinforced masonry and wood frame buildings. Reinforced masonry is also the most common material used for residential buildings in Dominica and Montserrat, although these islands have more unreinforced masonry and wood frame buildings of all occupancy classes. Antigua’s building inventory contains a large amount of unreinforced masonry, with wood frame and reinforced masonry making up the remainder of residential homes. Sheet metal roofs are very common in many of these territories, including Antigua. Dominica, Antigua, and Montserrat are all members of the Organization of Eastern Caribbean States (OECS), which has brought about building standards for these territories based on the Caribbean Uniform Building Code (CUBiC). Many member territories, including Dominica and Antigua, have taken steps to officially adopt these standards and given them the force of law. Though enforcement in the region is often limited, the awareness of the need for hurricane resistance has been increased in Dominica since Hurricane David ravaged the island in 1979, leading to improvements in hurricane damage mitigation. Although significant damage is not expected in any of the areas currently affected by Isaac in the next 24 hours, some occasional damage may be seen to metal roof coverings or building envelopes. Only the most vulnerable (and likely uninsured) properties would see any major damage. Damage from flash floods or mudslides, as well as storm surges, is also possible.

Forecast and Intensity

Isaac will likely pass well south of the U.S. Virgin Islands as a strong tropical storm or weak hurricane, and its track will be within 100 miles south of Puerto Rico. By Friday, after the storm passes just offshore of Santo Domingo, it is expected to move over the Dominican Republic and Haiti as a weak hurricane. Over the weekend, Isaac may become a threat to Cuba as it moves over Guantanamo, and the storm could affect southern Florida by Monday. Isaac is expected reach the status of a Category 1 hurricane; the likelihood of this happening depends on how long the storm lingers over land in Hispaniola. Although remaining over land prevents the storm from intensifying, it allows more precipitation to fall in one area. Rainfall accumulations of 4-8 inches are possible in the Lesser Antilles and up to 6 inches could fall in Puerto Rico. Flash floods and some mudslides are possible. Storm surges in these areas could be up to 1-3 feet above normal tide levels and could bring dangerous waves and rip currents to the windward and leeward islands. By Friday, rain and flood damage is likely to be seen in Hispaniola whose high mountains will produce an orographic effect that enhances the amount of precipitation. 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 impact of this storm. The AIR U.S. Hurricane team is also analyzing any potential threat to the U.S. where there is some concern about the storm impacting the Republican National Convention, which begins on Monday. AIR is also monitoring another system, Tropical Depression 10, which could impact Bermuda next week according to the National Hurricane Center.

Pre-Landfall 1 | Downloads
Posting Date: August 22, 2012, 11:20:00 AM