Countdown to Katrina

NOAA Flyig inside the eye of Katrina

The data is fed to the Hurricane Center's supercomputers, which generate a prediction cone - a 5-day forecast of the hurricane's likely track. Initial predictions show that Katrina will cross into Florida and then head out again to the Gulf of Mexico. Katrina is still only a tropical storm. The key question is, will she become a hurricane?

At the bottom of the scale is a category 1 with wind speeds of up to 95 mph. A category 5 hurricane is the most feared with winds of more than 155mph: whole roofs are sheared off and houses destroyed. Hurricanes deliver a deadly double blow - not just high winds but a massive bulge of water called the storm surge. The high winds push down on the ocean's surface, causing the water to rise like an unnaturally high tide. This wall of water is so dangerous that 90% of deaths in hurricanes come from drowning.

Thursday 25 August, T-4 days

Today, Katrina finally grows into a category 1 hurricane: she forms an 'eye' - and she's heading straight for Southern Florida. Just three hours later, Katrina hits land. But with no warm water to fuel her fury, she dies down. There's more warm water ahead in the Gulf of Mexico, though, and beyond lies New Orleans…

The city was built between the Mississippi River to the south and Lake Pontchartrain, connected to the ocean, to the north. Over the aeons, the Mississippi's annual floods have deposited silt to create vast tracts of boggy marshlands. New Orleans itself was constructed on the only natural high ground, which would become the French Quarter.

New Orleans couldn't grow until an engineer, Baldwin Wood, designed a complex system of pumps and drainage canals to drain the surrounding marshlands. Much of his system is still working today. But draining the land over the years has caused the city to sink. Initially the growing city was protected with earthen levees. In 1927, though, the Mississippi burst its manmade banks in a catastrophic flood that killed an estimated 500 people and destroyed more than a million homes. It was a turning point. After that, the Army Corps of Engineers took control of the levees. Now there is a 1200-strong team - and it's their responsibility to build and repair the levees to protect against flooding.

New Orleans has two types of levee: the original earthen levees and more recently built concrete and steel floodwalls. They are designed to withstand a category 3 hurricane. But with Katrina growing in the Gulf, would category-3 levee protection be enough?