This is why Harvey and Hurricane Katrina are two very different storms

As Tropical Storm Harvey continues to rage on, causing unprecedented flooding in Houston and surrounding areas, it makes sense that forecasters and the public have started trying to compare the floods to previous extreme weather events.

Namely, the extreme flooding calls to mind another storm that devastated New Orleans in 2005: Hurricane Katrina.

Katrina is a cultural touchstone that calls to mind government incompetence and the heroism of people trapped in an impossible, deadly situation.

So, how exactly, does Harvey compare?

While the scope and scale of the flooding disaster is similar when you compare Harvey and Katrina within Houston city limits, the reasons behind the flooding are very different.

The catastrophic flooding associated with Harvey is caused by extreme rainfall slowly filling up Houston and nearby parts of southeastern Texas with trillions of gallons of rain.

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So far, about 15 trillion gallons of water have fallen from Harvey, with about 6 trillion more expected before the storm is finished. Thousands of people have already been displaced by the storm, and it hasn't yet ended.

Katrina's devastating floods, on the other hand, were caused by storm surge as the hurricane came up the coast, pushing water into New Orleans. That storm surge caused aging levees to break, allowing water usually held at bay to pour into the city all at once. The Lower Ninth Ward, for example, was immediately flooded when the nearby flood wall failed, making that flooding all the more deadly.

The rapid flooding trapped people who didn't have the means or were unwilling to evacuate prior to the storm, sometimes for days before rescuers could make their way to help.

There is no analog, really

From a meteorological standpoint, Hurricane Harvey and its associated deluge stands alone in history when it comes to rainfall totals, as well as the scale and scope of the Houston flood disaster. This is why the National Weather Service has been putting out such dire statements, bluntly indicating that this is a storm and a flood outside the experience of any Houston-area resident.

No storm of its intensity has ever hit land and stayed nearly stationary for a week, while maintaining sufficient intensity to bring severe impacts to a region.

Yes, Hurricane Katrina produced similar scenes and widespread devastation. And yes, Harvey has earned comparisons to that storm. But the two are fundamentally different, and presented forecasters and emergency managers with different communication and preparation challenges.