The 2005 Atlantic hurricane season was the most active season on record with 28 named storms (winds of at least 39 mph) forming, shattering the record of 20 named storms set in 1933. By the time Katrina formed on August 24 (two weeks prior to the midpoint of the season), 2005 already had eleven named storms and five hurricanes – nearly the average number of storms experienced in an entire average season!
Hurricanes require very warm sea surface temperatures (SSTs), typically greater than 80 degrees Fahrenheit, to develop, with even warmer SSTs critical for intensification. At the time of Katrina’s development, SSTs were much warmer than normal across most of the tropical Atlantic, including north of Hispaniola where it formed.
Another critical ingredient for hurricanes is mid-level moisture. Higher levels of moisture help promote deep thunderstorm development which is essential for hurricane maintenance. Mid-level moisture values were much higher than normal in the region surrounding Hispaniola when Katrina was getting organized.
A third critical component for hurricane intensification and maintenance is minimal vertical wind shear. Too much shear rips off the tops of thunderstorms and also disrupts the “in-up-and-out” circulation of a well-developed hurricane. Vertical wind shear was much weaker than normal across the tropical Atlantic and Caribbean in the days leading up to Katrina’s formation.
Hurricane Katrina formed from the remnants of Tropical Depression 10 as well as an additional tropical wave. Tropical Depression 10 was first identified as an “invest”, the predecessor of a tropical depression, on August 11, then became a depression two days later. The depression dissipated into an open wave on the 18th, but remained a trackable feature as it moved just north of Puerto Rico and Hispaniola. On August 22, a new invest was initiated north of Haiti, and this quickly became Tropical Depression 12 over the eastern Bahamas, which was upgraded to Tropical Storm Katrina the following day.
Katrina reached hurricane strength prior to making an initial landfall near Miami, Florida on the evening of August 25, then intensified rapidly once it reached the Gulf of Mexico. It left the Florida peninsula with maximum sustained winds of about 75 mph (Category 1), reached 115 mph (Category 3) 30 hours later, and after another 24 hours, on August 28, it had exploded into a large and powerful Category 5 storm in the central Gulf of Mexico with incredible 175 mph sustained winds in the eyewall.
To put the “large” size in perspective, here is a simple schematic comparing the average diameter of tropical storm and hurricane force winds for Atlantic hurricanes (center) to that of Katrina (right) and Andrew (left). Katrina’s diameter of hurricane-force winds at peak intensity was over twice the average, and over three times the size of Andrew’s when it was at the same intensity.
One of the primary reasons Katrina rapidly intensified over the Gulf of Mexico was due to very high levels of upper ocean heat content in the Gulf. Ocean heat content is a measure of the depth of warm water. A high ocean heat content means that warm water extends fairly deep.
The very warm ocean eddy that Katrina tracked over in the central Gulf was associated with the Loop Current which transports warm Caribbean water northward towards the Gulf of Mexico and Florida Keys. As you can see from the track, it intensified as it passed over the high ocean heat content region, and began to weaken once it exited that region.
Vertical shear was quite low throughout Katrina’s lifetime. On August 28, when Katrina was at its peak intensity, vertical shear throughout most of the Gulf of Mexico was below 12 mph (10 knots).
The combination of the very warm, deep ocean and light upper-level winds helped fuel Katrina’s intensification into the monster that it became.
Thankfully, the storm weakened quite a bit by the time it reached the northern Gulf Coast. It’s hard to imagine the result if it hadn’t.
The peak winds at its second landfall on the southern tip of Louisiana had decreased to 125 mph, but when combined with its enormous size, was still more than enough to generate a record-smashing 28-foot storm surge in Mississippi.
The devastating flooding in New Orleans was not a bona-fide storm surge like the ones observed in Mississippi and Alabama that morning. It was the result of the failure of levees that were designed to hold back high water. And they did… for a while. However, once they began to fail, the nightmare quickly unfolded as Lake Pontchartrain flowed into the low-lying portions of the city.
The storm details and aftermath associated with Katrina were long-known to be a significant vulnerability in the U.S. Studies and documentaries done years before Katrina occurred basically outlined exactly what happened… it was a ticking time bomb. And it could certainly happen again.
More Katrina from The Washington Post