Bonnie is currently making its way across the Florida peninsula, and is set to enter the Gulf of Mexico on Friday night. When it does so, it will encounter some of the warmest waters the Gulf has ever seen – and in some places even warmer than the waters that Katrina crossed over in 2005. So what’s the difference between Bonnie and Katrina? It's all in the upper levels.
Over the past few days, CNN Hurricane Headquarters has been monitoring an area of low pressure at the top of the troposphere, which is the place where all the weather happens in Earth's atmosphere. Tropical cyclones (the generic term we use for tropical depressions, tropical storms, and hurricanes) thrive in "low-wind" environmental conditions. This sounds counter-intuitive but it makes sense when you think about it. Strong winds in the environment where the tropical cyclone seed is trying to grow is going to rip it apart. We call this "wind shear," and it literally shears a storm apart. A tropical cyclone seed needs to plant its roots in weak winds and warm waters so it can build the vertical structure that it needs to churn into a hurricane.
The upper-level low that we have been monitoring is causing some strong wind shear over the Gulf of Mexico, and its why we didn’t see Bonnie strengthen when it was over the Bahamas, and it’s going to prevent strengthening over the Gulf of Mexico as well, assuming the wind shear sticks around. So even though the main ingredient for a strong hurricane is present (warm sea surface temperatures), we probably won’t see Bonnie strengthen much past 50 mph wind speeds.
The northern Gulf will still see relatively strong winds, especially over the Deepwater Horizon site, but it won’t be the worst case scenario: a major hurricane rolling into Louisiana with catastrophic impacts to the oil spill clean up effort.
And we shouldn’t take Louisiana’s Invisible Shield for granted though, because as the season rolls on, the wind shear tends to decrease. We can’t be sure the Gulf will be as lucky next time.