Isaac Slams Gulf with Torrential Rain, Tornadoes, and Storm Surge

30-août-12

Isaac has moved approximately 250 miles inland from New Orleans and is now over northern Louisiana with maximum sustained wind speeds estimated to be 40 mph. The storm continues to carry heavy rains and some regions are expected to receive up to 25 inches of rainfall. Nine tornadoes were reported in southern Mississippi and Alabama yesterday (August 29, 2012) and 5 more were reported today (unfiltered reports that may include duplicates). Isaac is expected to weaken to a depression by tonight as it continues north.

Isaac was downgraded to a tropical storm at 2 pm CDT yesterday, Wednesday, August 29, when maximum sustained winds dropped to 70 mph, below the hurricane intensity threshold of 74 mph. Since landfall in southeastern Louisiana on Tuesday, Isaac has been moving to the northwest at a slow pace of between 6 and 8 mph. The exceptionally slow forward speed of Isaac is leading to heavy rainfall amounts in Louisiana and southern Mississippi.

Many rivers are expected to flood along southern Louisiana; low-lying areas along the Tangiaphoa River are at risk due to potential dam failure in Tanqipahoa. Isaac is expected to produce total rainfall amounts of 7 to 14 inches, with isolated maximum amounts to 25 inches possible. These rains will continue today over northern and eastern Louisiana and much of Mississippi and southwestern Alabama. Several rivers are expected to crest over the weekend as storm runoff makes its way into the riverine system.

Isaac Rainfall - August 30, 2012

Isaac Rainfall - August 30, 2012
Source: NOAA - National Weather Service

The storm surge levels observed during Isaac were exceptionally high for a Category 1 storm. Surge levels reported on August 28 were 11 ft at Shell Beach, LA, 10 ft at Bay Waveland Yacht Club, MS, about 6 ft at Pascagoula NOAA Lab, MS and 6 ft at Mobile, AL Coast Guard station. The surge levels are now gradually receding along the Mississippi and Louisiana coastal areas. The Plaquemines levee that was over-topped during Hurricane Isaac did not fail during Hurricane Katrina (2005). No other levee breaches have been reported, thus far.

Tropical Storm Isaac Potential Storm Track - August 30, 2012

Tropical Storm Isaac Potential Storm Track - August 30, 2012
Source: National Hurricane Center (NHC)

At 10 am CDT, Thursday, August 30, the center of Isaac was roughly 50 miles south of Monroe, LA, near latitude 31.7N, longitude 92.1W. Maximum sustained winds were 40 mph, and a steady weakening is expected as Isaac continues moving inland. Isaac is forecast to be downgraded to a tropical depression by tonight. Isaac was moving to the north-northwest at 9 mph, and a turn to the north is expected by Friday. The forecast track will bring Isaac through northern Louisiana today and into Arkansas on Friday.

Tornadoes are still possible along the central Gulf Coast and in parts of the lower Mississippi River Valley through today.

Elsewhere, Hurricane Kirk becomes the fifth hurricane of the 2012 Atlantic Season. Kirk became the season's eleventh named system at 11 pm AST Tuesday, August 28 when it was declared a tropical storm. This forecast track remains well out in the central North Atlantic, and Kirk poses no threat to land.

Today at 11 am AST Thursday, August 30, Kirk was upgraded to a hurricane when maximum sustained winds reached 75 mph. Additional strengthening is expected over the next few days. Kirk was located around 1,065 miles northeast of the northern Leeward Islands, near latitude 27.2N, longitude 49.5W. The hurricane was moving to the northwest at 12 mph, and Kirk is forecast to turn to the north on Friday, and then accelerate to the north-northeast on Saturday.

EQECAT will continue to monitor this event and provide updates as more information becomes available.

Hurricane-Induced Storm Surge Modeling

EQECAT has published an article on hurricane-induced storm surge and the unique methodology for modeling this important component of hurricane losses.

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