Isaac: Not a Katrina, Tracking Farther West

By , Expert Senior Meteorologist
August 29, 2012; 6:28 AM ET
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Sand is blown off the beach at Fort Lauderdale, Fla. late Thursday, Aug. 25, 2005 as Hurricane Katrina came ashore. Hurricane Katrina dumped sheets of rain, kicked up the surf and blew strong winds ashore Thursday, toppling trees and driving sand across waterfront streets as it made landfall on the state's densely populated southeast coast. (AP Photo/J. Pat Carter)

From a core meteorological standpoint Isaac is not another Katrina in terms of intensity, but it is still a dangerous storm and causing significant damage.

Isaac made landfall as a hurricane in Louisiana Tuesday night and continued to wobble west-northwestward along the southern coast of Louisiana during the early morning hours Wednesday.

Isaac made a second landfall near Port Fourchon, Louisiana and will continue to bring storm surge flooding, flooding rainfall, damaging wind, beach erosion and the potential for tornadoes to much of the central Gulf Coast area through the day Wednesday on the anniversary of Katrina's second landfall in 2005.

According to AccuWeather.com Hurricane Expert Dan Kottlowski, "The angle and slow speed at which Isaac is moving ashore will drive a substantial amount of water inland over southeastern Louisiana and southern Mississippi."

As a result a relatively weaker storm (Category 1), when compared to Katrina (Category 3 at landfall in La./Miss.), can still pack a considerable punch.

Isaac has and will continue to test the upgraded New Orleans levees.

At its closest approach, the center of Isaac will pass between 40 and 50 miles to the south-southwest of New Orleans, but may pass very close to Houma and Morgan City, La. overnight into Wednesday morning. (As of 10:00 a.m. CDT Wednesday, Isaac was over Houma, La., about 45 miles southwest of New Orleans and moving slowly away).

While we have a doomsday track from a New Orleans standpoint, we do not have a doomsday storm intensity. In Mississippi, Katrina's storm surge was a record 27.8 feet at Pass Christian. During Isaac, a surge of up to 9 feet is forecast by AccuWeather.com for the area. (The Shell Beach area of Louisiana recorded water levels about 10 feet above normal late Tuesday night).

Katrina moved up from the south over the Mississippi Delta on a curved path. Isaac rolled in straight from the southeast, so the counterclockwise flow around the storm has driven a significant surge toward Chanderleur Sound, Lake Borgne and Lake Pontchartrain early on for a number of hours. A storm surge of 3 to 6 feet is forecast for Lake Pontchartrain with the highest levels on the western end of the lake.

Since the forward speed of Isaac has slowed in recent hours Tuesday evening, it will result in a somewhat prolonged surge over Lake Pontchartrain and other areas to the south and east.

Isaac is a large storm in terms of surface area with its circulation extending out hundreds of miles from the center, especially on its eastern and northern side.

Other cities at risk for coastal flooding include Biloxi and Gulfport, Miss., Mobile, Ala. and Houma, La.

Some low-lying roads and communities have been cut off by rising surf and storm surge.

Isaac will continue bring flooding rainfall, downed trees and power outages in portions of Louisiana, Mississippi and Alabama through Wednesday.

Indirect effects from the storm, essentially repeating downpours and severe thunderstorms can still cause damage and and risk to lives in these areas, over portions of Florida, Georgia and the Carolinas.

According to Severe Weather Expert Henry Margusity, "There is always a potential for a swarm of tornadoes being spawned by a hurricane making landfall."

With a storm moving in from this direction, winds in the wake of the storm over the Lake Pontchartrain area would shift to the south. While this will keep water levels high on the lake, wind-driven wave action would shift away from the New Orleans shores of the lake.

Recall that during Katrina, strong southeast winds drove water into Lake Pontchartrain initially. And, that is happening again with Isaac to some extent. However, due to the south to north path of the Katrina, north to northwest winds on the back end of the hurricane drove the elevated lake water and waves toward the city, contributing to levee failure.

Despite Isaac's earlier struggles, warm Gulf water Tuesday allowed the system to become a hurricane as expected, prior to landfall. Weak steering flow over the United States has caused Isaac to linger near the shoreline.

Much of Plaquemines Parish, part of a mandatory evacuation area, has been hit hard with flooding due to the lingering effects of Isaac.

Precaution petroleum rigs in the central and northeastern Gulf of Mexico were shut down and evacuated in advance of Isaac.

This story was originally published on Monday, Aug. 27, 2012 at 10:00 a.m. EDT and has been updated.

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