If the storm would have moved a little faster it could have hit Manhattan as a category 3 or maybe even higher. Lots of factors come into play when predicting storm strength but the main ones are barometric pressure and temperatures. Get a storm with low barometric pressure and let it idle over warm water for a while and you end up with very strong storms, take the same storm and put it over colder water and it'll weaken.
Irene was a category 3 with 950 or so millibars at one point, that's very low and it was still gaining strength. Katrina was around 930 at it's peak and hit as a category 4, after weakening considerably before impact. Storm predicting is exactly that, predicting. There is no exact science to any of it.
Luckily for everyone the storm weakened considerably as it was over the colder waters in the Atlantic.
Irene was a category 3 with 950 or so millibars at one point, that's very low and it was still gaining strength. Katrina was around 930 at it's peak and hit as a category 4, after weakening considerably before impact. Storm predicting is exactly that, predicting. There is no exact science to any of it.
Luckily for everyone the storm weakened considerably as it was over the colder waters in the Atlantic.
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