Monday, August 27, 2012

Tropical Storm Isaac: August 27, Update A

I heard from a few people yesterday that they were concerned that Isaac is another Katrina, because everyone is showing the path of the two storms, which is similar as they cross the Gulf, and because it’s almost 7 years to the day since Katrina hit. You should all definitely be prepared, but there is a BIG difference between the two… and that’s the position of the Loop Current and associated eddies etc.

(Science alert) The easiest way to see where the areas of deep warm water are is to look at the sea surface height (because in the summer, the sea surface temperature is almost uniformly warm). We have satellite instruments called altimeters that can measure this. As you know, if things are warm they expand and if they are cold they contract. It’s the same for water. If we have an area where the water is warmer over a great depth, it ‘expands’ and is higher on a map of sea surface height compared to an area where the water is colder. When a storm passes over these warm (high) areas, they intensify because they have warm water to churn up from the deep instead of cold water. (end science alert)

Here is a proxy for the sea surface height map with the path of Katrina and her intensity:
You can see the correlation very easily – she got stronger as she went over the warm areas of the Gulf (i.e. over the Loop Current). And here is a map of the sea surface height yesterday:
Isaac is marked with an ‘X’ (this was his location about 2 hours ago, but you get the idea) and I added an arrow to show his approximate forecast track. He is going to be moving over an area which has some deeper warm water, but as he is currently only a strong Tropical Storm and this area of deep warm water is not very big, he won’t blossom into anything like Katrina’s cat 5.

Although he’s passing over some deep warm water, the atmosphere is inhibiting his development for now. It looks like he didn’t completely get away from that dry air to the south and there is some winds shear that you can see the water vapor satellite images. These are inhibiting his development. As he moves farther north, he will begin to interact with land again, so that should also help to keep his intensity in check.

He is currently at 26.1N, 85.3W, heading NW at 14mph, so slightly slower than yesterday. The center of cone forecast track takes him to New Orleans with landfall tomorrow morning as a hurricane. I’m not 100% sure of this, but the track hasn’t changed in the last couple of advisories so I’ll go with the official track – the models are generally quite decent within 24 hours of landfall. I was asked what I thought about leaving New Orleans. I don’t live there so it is difficult for me to advise – I would go with whatever your local emergency managers say because they have the best overall local picture. Although he is not a strong storm, the water is being pushed onshore and is currently about 1 foot above normal (from tidesonline).

More later,
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DISCLAIMER: These remarks are just what I think/see regarding tropical storms - not the opinion of any organization I represent. If you are making an evacuation decision, please heed your local emergency management and the National Hurricane Center's official forecast and the National Weather Service announcements. This is not an official forecast. If I "run away, run away" (Monty Python), I'll let you know.

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