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August 2012

Isaac Makes Landfall – Causing Flooding in Louisiana

   Posted by Levi at 3:38pm on August 29, 2012

Couldn’t get a video out today, so it is just a written post.

Hurricane Isaac finally moved ashore near Houma, Louisiana a couple of hours ago after dancing within a couple dozen miles of the coast for most of yesterday. Isaac is moving off to the northwest, but very slowly, and has not weakened significantly yet due to the center being over mostly marshes. The storm surge has been up to 11 feet high near New Orleans at Shell Beach due to the prolonged onshore flow, and the surge up the Mississippi River has caused a levee to be overtopped in Plaquemines Parish, which has put entire houses under water as of this morning. As we’ve talked about for the last few days, Isaac’s slow movement due to a fragile steering pattern which brought him on a rare track into this area is going to result in 12-20 inches of rain in parts of southern Louisiana and Mississippi. Up to 10 inches have already fallen in New Orleans, with around a foot still forecasted on top of that by the HPC.

It may be another 24 hours before the rainfall starts to let up in the New Orleans area, though areas farther east such as Mobile, AL may see it let up sooner. In about 24 hours a ridge building to the east of the storm should accelerate it northward into Arkansas and Missouri, providing beneficial rains in drought-stricken areas there. Until then, inland flooding is going to be a huge problem.

Isaac is like a somewhat weaker version of Hurricane Ike in 2008, which was also a large storm with an unusually low pressure for its maximum winds. Isaac got down to 966mb before landfall, and is now up to 972mb, but as we also saw with Hurricane Irene last year, it doesn’t take a major hurricane to cause life-threatening problems. A pressure this low means a lot of air is getting forced upward, and no matter how “light” the winds are compared to the pressure, the rainfall potential is massive, and flooding will be what Isaac is remembered for. Category 1 strength winds for an unusually long period of time are also capable of doing as much damage as Category 2 or 3 winds over a more typical short period of time. The effects of Isaac are only about halfway over for many people in Louisiana right now.

Elsewhere in the Atlantic…Tropical Storm Kirk has formed in the central Atlantic, and will be recurving out to sea well away from land, and is not a threat.

Invest 98L, another large area of monsoonal low pressure, is moving westward across the eastern Atlantic, and could be a threat for development in a few days. Currently all models agree this system should pass well north of the Antilles Islands, and may recurve harmlessly out to sea. The pattern remains active, and additional threats for development are expected over the next few weeks as we go through the peak period of the hurricane season in early September.

We shall see what happens!