August 23rd, 2012

Tropical Storm Isaac

Posted at 6:38 PM ET

issac-smallTropical Storm Isaac is presently moving west, to the south of Puerto Rico, following several days of westward motion at the base of the subtropical ridge. The National Hurricane Center (NHC) anticipates that this motion will continue for the immediate-term, followed by a gradual turn to the west-northwest this evening. This motion is expected to continue through Saturday. The center of circulation should pass to the south of Puerto Rico today, and then near or over the south coast of the Dominican Republic and Haiti on Friday. Gradual intensification is expected by the NHC over the next 48 hours, and Isaac could well become a hurricane as it approaches the Dominican Republic and Haiti.

The longer-term outlook for the United States includes the possibility of the storm impacting the Florida Keys as early as Monday, followed by landfall along the southwestern coast of Florida or the northern Gulf Coast from Tuesday to Thursday depending on how far west the storm tracks.issac-big

Hazard data illustrated in the CAT-i map was taken from i-aXs®, Guy Carpenter’s web-based risk management platform. i-aXs users can view impacted areas on any map as well as see how their portfolios were affected. Please contact your broker or GC Analytics (SM) representative for assistance or go to www.i-axs.info for further information.

Hurricane warnings have been posted by the NHC for the south coast of the Dominican Republic from Isla Saona to the Haiti-Dominican Republic border.

Tropical Storm Warnings are in effect for the U.S. and British Virgin Islands, Puerto Rico, Vieques, Culebra, and the north coast of the Dominican Republic from its border with Haiti to north of Isla Saona.

Tropical Storm Watches have been posted by the NHC for the southern Bahamas, as well as the Turks and Caicos Islands.

Hazards include tropical storm force winds for affected areas, and hurricane conditions within 24 hours for areas under a hurricane warning. Excessive rainfall amounts are expected with accumulations in excess of 8-12 inches; flash flooding and mudslides are potential hazards to life and property. A storm surge of up to 5 feet expected for the coast of Hispaniola, with dangerous surf and wave action in affected areas.

Forecast and Reasoning

The impressive points of Tropical Storm Isaac are its size, with impressive circulation and outflow despite recent issues with wind shear and dry air. Tropical storm force winds extended outward from the storm center up to 140 miles (225 km). The storm has a “lopsided” center of circulation, with evidence of a new center reforming to the south of the original one. The storm is moving into an environment to better enable intensification, and the NHC anticipates that the storm will reach hurricane status within 48 hours as it nears the Dominican Republic and Haiti.

After 72 hours, the forecast becomes more complicated.

The storm is expected by the NHC to weaken as it crosses the higher terrain of southewest Haiti and eastern Cuba, following the edge of a weakening subtropical ridge. A mid-latitude disturbance plunging into the U.S. Southeast will weaken the subtropical ridge, potentially creating a weakness through which the storm would pass. The exact placement and timing of the disturbance, at the time of interaction with Isaac, together with the exact edges of the ridge, will determine Isaac’s exact path. Not a simplistic situation to forecast by any means.

Intensity is an even more difficult forecast. Isaac will most likely leave Cuba as a tropical storm, but could reintensify after emerging into the warmer Gulf of Mexico waters, especially if Isaac tracks further west. The history of Gulf of Mexico hurricanes reminds of the potential for aggressive and rapid intensification of tropical cyclones. Examples include Camille (1969) and Charley (2004). Hopefully, the already large size of Isaac will make for slower intensification.

The storm is expected to clear the coast of Cuba sometime Sunday or early Monday (depending on its exact track) as a tropical storm.

The longer-term outlook includes the possibility of the storm impacting the Florida Keys as early as Monday, followed by landfall along the western coast of Florida or the northern Gulf Coast from Tuesday to Thursday depending on how far west the storm tracks. This extended forecast involves a very complex set of factors, with a large range of possible outcomes affecting aforementioned areas. Interests in potentially affected areas should review plans, and execute as appropriate. Needless to say, Isaac should be monitored closely, particularly by those in south Florida and along the northern Gulf Coast.

Source: National Hurricane Center (NOAA).

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Guy Carpenter publishes CAT-i reports for major natural catastrophes worldwide. These reports cover catastrophes including worldwide tropical cyclones, earthquakes, major UK and European floods and any other natural event that is likely to incur a significant loss to the (re)insurance industry. Please email CAT.i@guycarp.com if you wish to be added to the free email distribution list.

Guy Carpenter compiles RISK-i reports for major technological or man-made events worldwide. These reports cover risks to property, transport and life including explosions, fires, crashes, engineering disasters and terrorist attacks that are likely to incur a significant loss to the (re)insurance industry. Please email RISK.i@guycarp.com if you wish to be added to the free email distribution list.

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