Good thing it's less unstable than average

By: Stu Ostro , 2:57 AM GMT on August 29, 2012

Isaac has had two characteristics that have been a constant throughout its life: its struggles to strengthen, and its big size. The latter unfortunately more than compensates for the former. Isaac is a large and dangerous tropical cyclone and represents about as serious a threat as one will ever see from a low-end Category 1 hurricane in this location.

But what about the former? Here are a current map and time series graphs of vertical instability in the atmosphere relative to average, showing it being much less than average over the Gulf and Caribbean. I'm not sure to what extent this is dry air vs. something else going on in the atmosphere, but it is consistent with the difficulty Isaac has had in having and maintaining deep, symmetric convection.

Images credit: Regional and Mesoscale Meteorology Branch (RAMMB)

If instability had been above average, Isaac may very well have become a higher, perhaps a much higher category of hurricane. Which is scary, given its size and how dangerous it is as it is. The wind will be relentless, and result in widespread power outages continuing to accumulate and create adverse conditions for crews trying to restore power quickly. A long-duration strong onshore flow onto this very surge-prone coastline will cause the storm surge to be much higher compared to that from a small Category 1. The size and slowing movement of Isaac will produce excessive rainfall amounts, and flooding of that nature in addition to storm surge flooding.

And, having said that about Isaac's struggles, it is, alas, at the last minute the healthiest it has been. Look at the difference in symmetry between 24 hours ago last night (top image below) and tonight (top image), and the central pressure is down to 968 millibars... It's going to be a long night and a long couple of days ...

Images credit: NOAA/NESDIS

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3. Bogon
2:14 PM GMT on August 29, 2012
Your observations support the theories of doctoral student Michael Ventrice at SUNY in Albany. His chart shows that Isaac developed under a wave of rising air (convectively-coupled Kelvin wave) associated with the Madden-Julian Oscillation. Most of Isaac's life, after he entered the Caribbean, was adversely affected by the ensuing descending phase of the wave. Isaac had enough size and momentum to keep going despite the interference.
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2. redagainPatti
6:11 AM GMT on August 29, 2012
but... west of NO is also not good as that side of a hurricane is the worse with the winds.. praying for those in the pathway..
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1. originalLT
4:38 AM GMT on August 29, 2012
Thanks for the report Mr. Ostro. As other bloggers have pointed out, at this late hour, Isaac has taken a pretty major "jog" to the West, and does not appear to be moving NW. Just saying. Even if it starts the NW motion again, it's center will be further away from NO.
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Proud to be a weather-obsessed weather geek! \m/ Senior meteorologist at The Weather Channel. If not a meteorologist, would be a DJ ♫

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