Tropical Storm Isaac continues to intensify as it continues its slow trek through the Northern Gulf. Isaac threatens the Northern Gulf seven years following the landfall of Hurricane Katrina, and marking what will be the first U.S. landfalling hurricane in the area since 2008.
The storm is presently about 80 miles south of the mouth of the Mississippi River, and moving northwest at about 10 mph. Maximum sustained winds are 70 mph, and ongoing intensification is expected by the National Hurricane Center (NHC). The NHC anticipates landfall of Isaac’s center somewhere between the Atchafalaya Basin to coastal Mississippi, with most probable landfall near the mouth of the Mississippi River.
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The NHC is expecting landfall of Isaac as a category-1 hurricane, with maximum sustained wind speeds of 74-95 mph. Isaac is a large storm, with tropical storm force winds found 185 miles from the center of circulation. As a result, impacts will be imposed well away from the center of circulation. A storm surge of 6-12 feet will be imposed for areas of Eastern Louisiana and Mississippi.
Storm Details and Forecast Factors
Despite the ongoing intrusion of dry air, and a period of moderate wind shear, Isaac has shown decreasing surface pressure, and evidence of a consolidating and contracting wind field, all consistent with an intensifying storm. Since wind shear has relaxed this morning, an eye has started to form. While the eye of the storm is difficult to identify by visible satellite imagery, it is clearly defined by both water vapor and infrared. The storm is clearly intensifying, and should continue to do so in the near term, and thus the NHC anticipates that Isaac will reach hurricane status before landfall. The potential landfall locations are shaped by Isaac’s path along the edge of the subtropical ridge, and another building ridge to the Northwest of the storm. The exact edges of the ridge are cause for some uncertainty. Model guidance shows forecast tracks between the Atchafalaya Basin to near the Pearl River. The NHC cone of uncertainty generally follows this guidance envelope.
Watches and Warnings
- Hurricane warnings have been posted by the NHC for areas including Morgan City, Louisiana to the Alabama-Mississippi border, including Lake Ponchartrain and Lake Maurepas, and metropolitan New Orleans.
- Hurricane Watches have been posted by the NHC for Intracoastal City to Morgan City, Louisiana.
- Tropical Storm Warnings are in effect for the Mississippi-Alabama border to Destin, Florida, and from Morgan City to Cameron, Louisiana.
- Tropical Storm Watches have been posted by the NHC for east of High Island Texas to just west of Cameron, Louisiana.
Wind. Tropical storm force winds will occur this morning for the Northern Gulf, and hurricane force winds this afternoon. Tropical storm conditions at minimum are expected along the Florida Big Bend and Panhandle, Alabama, Mississippi, Louisiana, and east Texas.
Flooding. Excessive rainfall amounts are expected with accumulations in excess of 7-14 inches from the extreme west Florida Panhandle to southern Louisiana. Local amounts as high as 20 inches are possible with this slow-moving storm. Significant lowland flooding is expected. Inland flooding will be further compounded.
Tornadoes. Inland tornadoes will be an ongoing threat to the northern Gulf today.
Storm Surge. A storm surge of 6-12 feet is expected by the NHC along the Northern Gulf for southeast Louisiana and Mississippi. The storm surge is greater than one would expect for a typical category-1 storm, largely due to Isaac’s large windfield, and further amplified by the dramatic bathymetry of the North-Central Gulf. The Northern Gulf coastline is particularly vulnerable to storm surge. The storm surge will further compound inland flooding due to suppressed drainage in river basins.
Many offshore platforms have been evacuated as a precautionary measure. Losses due to storm surge, inland flooding, and wind can be expected.
Evacuations were ordered for some low-lying areas. Across the region, people boarded up homes, stocked up on supplies and prepared for the storm. Schools, universities and businesses are closed for affected areas.
President Obama has declared a state of emergency for Louisiana, enabling federal support to save lives and protect public health and safety. Engineering improvements to the levee system on Lake Pontchartrain are emphasized by New Orleans Mayor Mitch Landrieu, in an effort to reassure residents about Isaac’s threat. Evacuation of New Orleans is unlikely. The storm surge of 6-12 feet will impact a coastal area very vulnerable to storm surge. The surge will also prevent watershed drainage, which will further compound inland flooding. Evacuations have been ordered for several communities outside the levees in South Louisiana, as well as the entire parish of St. Charles, west of New Orleans.
Mandatory evacuations have been announced in low-lying areas in Alabama and Mississippi, and shelters have opened all along the coast. The evacuations were also announced in several communities outside the levees in south Louisiana, as well as for the entire parish of St. Charles, west of New Orleans.
At least one confirmed tornado has been reported for Vero Beach. Over 9 inches of rain fell in West Palm Beach, Florida pushing their monthly total to a record 22.28″.
The storm has already forced the evacuation of workers from 346 oil and gas platforms in the Gulf of Mexico, which are responsible for 17 percent of domestic oil production and 6 percent of natural gas production, though it has so far had little effect on the price of commodities. Once ashore, the storm could impact low-lying fuel refineries along the Gulf Coast, accounting for about 40 percent of U.S. refining capacity.
The Haiti death toll is now at 19 and 5 more have died in the neighboring Dominican Republic. The regional death toll from Isaac stands at 24. Inland flooding, flash-flooding, and mudslides caused the displacement of thousands, many of whom were impacted by the recent earthquake.
Source: National Hurricane Center (NOAA), Associated Press, Agence France and Reuters.
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