Isaac's Forecast Challenges

By: Bryan Norcross , 2:51 AM GMT on August 26, 2012

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Isaac is on track tonight, and the center should be in the vicinity of the Key West tomorrow (Sunday) afternoon. The tropical core is embedded in a huge rotating circulation that extends hundreds of miles across, which has been a distinguishing feature of Isaac since its pre-depression days. This is why warnings and watches have been posted for most of the Florida peninsula.

The strength of Isaac when it gets to the Keys in tricky. In favor of strengthening: it's over very warm water, moving away from land, strong thunderstorms are wrapping around the center, and the upper air is moderately okay. Working against it: big circulations intensify more slowly, part of the circulation is over land, thunderstorms have developed each night and weakened, and the upper air is only moderately okay. It's likely to be at or near hurricane strength when it gets to the Keys, but no rapid intensification is expected.

This storm brings back memories of Hurricane Georges in 1998, though Georges was a bit stronger and more organized when it ran into eastern Cuba. The center went over Key West as a Cat 2, and pushed damaging storm surge against the Keys. Folks in the Keys should be ready for a Georges-type event, and hope Isaac doesn't get that strong, which it probably won't.

Then there's the storm surge forecast for the Florida west coast. Storm surge science is excellent. Given a certain intensity, track, and radius of maximum winds... the surge in affected locations can be forecast accurately. The problem is, you rarely know those things very far in advance, and slight differences in any of the parameters can make a big difference in the surge.

The NHC is forecasting a worst case of 5 to 7 feet of surge on the southwest Florida coast and 3 to 5 feet in and around Tampa Bay. If the storm arrives at high tide, and the storm intensifies as forecast, these worst-case values could be reached. That's seriously high water for low-lying areas near the water, of which there are many. This is a potentially very dangerous situation.

But, as discussed yesterday, if the storm takes just the right (wrong) track, like Hurricane Dennis in 2005, a wave can set up in the shelf water accentuating the surge along the west coast and even more so in the Panhandle. This will have to be watched very closely. Again, little variations and it doesn't happen. Here's a simulation from the NHC that shows Dennis' surge-enhancing wave develop over the coastal shelf and travel north.

The storm surge from Dennis is below.


And then there's landfall. The trend in the computer forecast models has been to move west toward Mississippi and Louisiana, with the European now the outlier to the east. Previously it was the outlier to the west. This is why the cone is wide at 3 days out.

There is a big high pressure system over the Rockies and Plains that is forecast by the American GFS model to build across the South just as Isaac gets near the northern Gulf coast. That blocks the storm and pushes it westward. The European doesn't build the high as strongly or quickly east. Normally we would trust the European model for atmospheric-pattern forecasts, but it's been especially flakey this year, so we take the average as a good forecast, and admit that we don't know.

The NHC is forecasting a Cat 2 storm at landfall on the northern Gulf coast, but it's easy to see how things could come together to make it much stronger. Everybody from the Florida Panhandle to Louisiana should be on alert. Pay attention... and be prepared!

See you tomorrow on The Weather Channel.

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6. TropicalAnalystwx13
7:13 PM GMT on August 26, 2012
Thanks, Mr. Norcross.
Member Since: July 6, 2010 Posts: 113 Comments: 33116
5. wxchaser97
4:23 AM GMT on August 26, 2012
Thank you Mr. Norcross, Isaac is starting to get his act together. Could definitely be a bad situation for the gulf coast.
Member Since: March 16, 2012 Posts: 127 Comments: 7975
4. mgwxta19
3:55 AM GMT on August 26, 2012
Nice analysis of a very uncertain situation. Hoping models come into better agreement tomorrow. Will be interesting to see how much it strengthens passing by the Keys. That high pressure system may steer this one way or another. And interesting to compare it to Hurricane Dennis; noticed NWS Nashville made that comparison a couple days ago as well. Certainly not a _boring_ storm!
Member Since: July 10, 2012 Posts: 12 Comments: 0
3. originalLT
3:19 AM GMT on August 26, 2012
Thank you Mr. Norcross. I believe Isaac is beginning to look like "less of a mess" now Beginning to see T.Storms trying to wrap around it's apparent center as it slowly gets away from the Cuban coast.
Member Since: January 31, 2009 Posts: 0 Comments: 8010
2. TheDewd
3:06 AM GMT on August 26, 2012
Thanks for the expert analysis. I've lurked here for years and you made me create an account and post. Thanks for all you do.
Member Since: August 26, 2012 Posts: 0 Comments: 12
1. PurpleDrank
3:01 AM GMT on August 26, 2012
Great Job back in '92 Bryan!

Member Since: August 17, 2010 Posts: 1 Comments: 730

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This is the official blog for Bryan Norcross, Hurricane Specialist at The Weather Channel.

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