Currently, Tropical Storm Isaac remains just under hurricane strength with winds of 70 mph. The pressure has been steadily dropping throughout the day indicating an intensifying storm, currently at 981 mb, down from just below 990 mb earlier this morning. Often, the pressure will drop first, and the winds will respond a little later. However, Isaac has been fighting off dry air to its south which is becoming entrained into the circulation:
This is retarding development for the time being, but as Isaac moves to more to the northwest, this should diminish and intensification should continue as the storm moves into a more moist environment with very warm sea surface temperatures and little in the way of wind shear to disrupt the circulation.
Isaac is forecasted to remain heading in a northwesterly direction, and the model spread is in rather good agreement, bringing Isaac near or over the city of New Orleans, probably as a category 2 storm, perhaps a cat 3. Here’s the most recent GFS from 18z showing a very healthy Isaac just southeast of the city of New Orleans (model images courtesy of Weatherbell Analytics, LLC, at http://www.weatherbell.com):
This model, much like the European shown earlier this morning, slows the storm down as it approaches the coast, which allows for a prolonged storm surge of 6-12 ft+, over a period of 18-24 hours. Because the storm comes in from the southeast with respect to the city, the surge will come from the east from the Gulf and up the Mississippi river and into Lake Pontchartrain:
This slow movement also allows for tremendous amounts of rain over this period as well, to the tune of 10-20 inches:
These two factors, the prolonged storm surge, and heavy rains, will cause tremendous flooding in and around the city of New Orleans. If the dry air continues to win out, and development is minimal, this of course would lessen the blow. Alternatively, if the storm were to track more to the east, and make landfall over Mobile Bay, for instance, it would place the city in the western semicircle, with primarily northerly, or offshore winds. But both are felt less likely at this point. As I said this morning, although this storm will likely be weaker than Katrina when it comes to overall storm category and wind speed, the direction the storm is approaching from (southeast, not south like Katrina) and the increasing intensity as it approaches (Katrina was weakening, or decreasing in intensity), coupled with the problems of a slow moving storm described above, it could potentially be as bad as Katrina, even though it’s a weaker category 2 storm.