The eyewall replacement cycle that began at about 8am this morning has ended. The 8pm EDT Hurricane Hunter pressure reading was 942 mb, down 2mb from the pressure at 6:45pm. No inner eyewall was found, just an elliptical 30-40nm eyewall. We may be on the verge of a rapid deepening phase, since the shear and dry air on the northwest side of the hurricane appear to be lessening, and the hurricane is moving over a deep layer of warm water of almost 90F. The areal size of the hurricane continues to expand, and Katrina is growing from a medium sized hurricane to a large hurricane. Where the pressure will bottom out after this deepening phase is anyone's guess, and I believe something in the 915 - 925 mb range is most likely, which would make Katrina a strong Category 4 or weak Category 5 hurricane by tomorrow afternoon. The deepening phase may last longer than usual for a major hurricane, since Katrina is expanding in size and thus has more mass to spin up.
After this phase of deepening, another eyewall replacement cycle will occur, and the timing of that cycle will be worth billions of dollars and perhaps many lives. There is no way to predict when this eyewall replacement cycle will occur. Another factor will be the timing of the tides--if Katrina hits at high tide, there may be billions more in damage. Tidal range (difference between high and low tide) at Bay St. Louis near New Orleans is two feet. High tide will occur around 8am Monday, and low tide at 8pm. There is still the possibility, too, that the trough that is now steering Katrina to the north will also create enough shear to reduce her to a Category 3 storm at landfall. This is what happened to Hurricane Ivan last year.
New Orleans finally got serious and ordered an evacuation, but far too late. There is no way everyone will be able to get out of the city in time, and they may be forced to take shelter in the Superdome, which is above sea level. If Katrina makes a direct hit on New Orleans as a Category 4 hurricane, the levees protecting the city will be breached, and New Orleans, which is 6 - 10 feet below sea level, will fill with water. On top of this 6 feet of water will come a 15 foot storm surge, and on top of that will be 20 foot waves, so the potential for high loss of life is great. Given the current track and intensity forecast, I'd put the odds of this at about 20%.
What's behind Katrina?
A very large tropical wave way out in the Atlantic, 1300 miles east of the Leeward Islands, has a low level circulation, a large and increasing amount of deep convection, and an improving upper level outflow. Shear over the system is light, waters under it are warm, and I expect a tropical depression to form tomorrow from this system. If this happens, the depression will move west-northwest towards the northernmost Leeward Islands, and possibly affect them by Thursday.
Dr. Jeff Masters