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Why we evacuate

If there’s another call to evacuate the Houston area because of a hurricane threat, this will be part of the reason why.

Imagine a Category 3 hurricane striking the western end of Bolivar Peninsula. The storm surge would raise water levels by 6 feet in Galveston Bay and along Galveston Island, according to computer models.

Now, imagine the same storm striking a mere 20 miles down the coast, just past the Galveston seawall. The surge would push as much as 17 feet of water into Galveston Bay and 13 feet along much of Galveston Island, clipping it from behind even if the seawall buttressed the initial waves.

The two landfall scenarios just 20 miles apart would mean the difference between excellent surfing conditions in Galveston and monstrous, fatal waves of water.


For the most part, evacuations are intended to move people away from the storm surge. The question is whether the science of surge modeling can aid evacuation managers anytime soon.

The storm surge forecasting tool, known as the Sea, Lake, and Overland Surges from Hurricanes model, is accurate to within 20 percent if given perfect information about a storm’s landfall time and location. But such information is rarely perfect.

Although the National Hurricane Center’s tracking forecasts are now about three times better than they were in 1970, predictions made 24 hours before landfall still have an average error of about 60 miles.

Three days out — roughly the minimum time needed to call a mass evacuation in the greater Houston area — the error is about 150 miles.

“Can we ever be accurate to within 10 miles?” asked the hurricane center’s chief, Bill Read. “Probably not within my lifetime.”

Put another way, mass evacuations are here to stay — at least for decades.

Just something to keep in mind. On the plus side, the early indicators point towards a relatively quiet hurricane season, though as one commenter noted, there were only four named storms in 1983, it’s just that one of them was Alicia. So let’s not get too cocky just yet.

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  1. Michael says:

    the Sea, Lake, and Overland Surges from Hurricanes model, is accurate

    However, the SLOSH model is known to have spillover effects…

  2. Jeb says:

    On a more serious note, the decision to evacuate has to be weighed against the risks of sheltering in place and the hardships of evacuation. The real measure for evacuation should be TS Allison. If your house flooded during Allison or if the flooding prevented you from getting to essential services, then you should probably evacuate.

    Everyone should take a realistic assessment of the risk of flooding in their neighborhood before choosing to evacuate.