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!


5 comments

   

Comments

  • Chip says:

    How does El Nino look at the moment? Any signs of it starting to shut the Atlantic down in the near term? Or, are we going to see activity into and through September, in your opinion, Levi?

    • Levi says:

      El Nino has been affecting the season negatively since the beginning. We’ve seen the African waves very strong but struggle to develop east of 70W and south of 20N, and the MDR should continue to struggle all year, meaning the Caribbean islands are less likely to deal with a strong hurricane hitting from the east. They are all developing to the north and near the U.S. and central American coastlines, which is a classic El Nino signature.

      You may say the number of storms is quite high right now and rising. This is true, and is in part due to the historic early-season burst in May and June, and the negative PDO/positive AMO pattern we are in that is favorable for Atlantic activity despite the El Nino. ACE index is near-normal right now. This season may end up normal to slightly above normal at this pace, but the evidence of El Nino’s impact on the Atlantic is quite clear and has been for months.

  • Gary Z says:

    Trolls had Isaac hitting Texas.
    Now TD12 will hit the east coast.
    It’s funny, but it never ends.

  • Jeff H. says:

    Levi,

    Good evening and happy holiday weekend. Do you see any chance of TS Leslie making a USEC landfall?

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