Why storm experts are so scared about Hurricane Harvey’s rain

Hurricane Harvey is a huge, life-threatening storm. It struck the Central Texas coast Friday night and brought with it 130 mph winds and several feet of storm surge. But perhaps more concerning is the rain still to come.

“Catastrophic and life-threatening flooding is expected across the middle and upper Texas coast from heavy rainfall of 15 to 30 inches, with isolated amounts as high as 40 inches, through Wednesday,” the National Hurricane Center reports in its forecast. That’s almost an entire year’s worth of rain over the course of a few days.

And, it’s kind of mind-boggling. So I called up Hal Needham to help put in in perspective.

Needham is a scientist (a geographer by training) and consultant who studies storm surge risk. He’s based in Galveston, Texas, and writes about storm surge science on his blog. He says when a storm like Harvey comes along, we tend to think too simply about storm risk: We fixate on wind speed, or storm surge height, or rain. But what makes this particular storm risky (and complex) is how all those elements mix together.

My interview with Needham is below, edited for length and clarity.

Brian Resnick

What worries you most with Harvey?

Hal Needham

Harvey is a hybrid type of event we haven’t seen before. The storm surge itself has not been forecast to be that catastrophic in the Houston-Galveston region. It may reach 3, 4, or 5 feet. A lot of people are saying, “Wow, that’s so much less than Hurricane Ike [in 2008].”

But the really unusual thing about Harvey is it’s expected to stall near the coast for days. We may have Galveston Bay elevated by several feet for many days [from the storm surge]. On top of that, we may see 20 or more inches of rain in the extended region. To get that much rain, over a wide area, and have the costal areas elevated with storm surge — the waters are going to struggle to drain. Some areas are going to flood that haven’t before in recent memory.

That combination may be confusing to people. You can have two storms with the same amount of rain, and one of them floods your house and the other does not.

In this case, we have complex combined risk of storm surge and heavy rainfall. If people just look at one or the other, they may underestimate their risk.

Brian Resnick

Why is Harvey likely to stall by the coast?

Hal Needham

Hurricanes are steered by upper-level winds. And these winds are basically going to break down and stop steering the storm as it arrives here on the Texas coast. It may drift south, north. What does look pretty confident — in several models — is that it should stall for several days.

Brian Resnick

Here’s something I can’t quite wrap my mind around: The forecast is calling for 30-or-40-plus inches of rain in some areas. I have no mental reference for what that means.

Hal Needham

Let’s put it in context. Much of the Northeast Corridor — Washington to New York and Boston — maybe receives maybe between 40 and 45 inches of rain a year. Think of all the rain you get in July through Christmas and put that in a couple days. It’s a lot of rain.

Brian Resnick

Backing up a bit, here’s a simple question: What’s a storm surge?

Hal Needham

A storm surge is when the onshore winds of the hurricane elevate the level of the ocean. The entire column of water in the ocean is rising, due to onshore winds and lower pressure. We have less pressure pushing down on the surface of the ocean, and the water can actually rise a little. But most of the force creating the surge is the onshore winds.

When you hear about a 6-foot storm surge, the sea is actually rising by 6 feet. On top of that, we get large destructive waves.

Brian Resnick

Looking at this storm, the potential for surge, and the geography of the Texas coast, which areas are you most concerned about?

Hal Needham

The storm will make landfall in the Central Texas coast, and it’s going to produce the largest storm surge since Hurricane Carla in 1961. So it’s been more than 50 years that we’ve seen a storm surge this high in some of these regions. Maybe a 6- to 12-foot storm surge.

Brian Resnick

Why does the “more than 50 years” concern you?

Hal Needham

The biggest concern is public awareness and risk perception [with] this area around the coast.

Brian Resnick

Do the areas south of Houston and Galveston also face the same threat: combined storm surge, huge rain, and nowhere for all that water to drain?

Hal Needham

Absolutely.

Brian Resnick

I’ve read that officials in the Houston-Galveston area have discussed building a “costal spine” or a dike in the bay to keep storm surge waters out of the cities. The plans haven’t gone very far. Would such barriers be helpful to protect the area from this storm?

Hal Needham

That would be an extensive flood barrier along the coast. There would be a gate, a flood barrier for Galveston Bay, and the idea is that it could potentially keep back one of these storm surges.

[But here,] we have a moderate storm surge. We also have many feet of rain. If we put [in] a barrier and shut it, we may keep the saltwater out and the freshwater in.

Brian Resnick

So it’s not a simple answer.

What’s the most dangerous aspect of this storm as far as human lives are concerned?

Hal Needham

Just the prolonged nature of it, that it’s really going to go into the middle of next week. That it’s just going to park, and perhaps we’re going to see many feet of rain. There will be some substantial wind damage along the Central Texas coast, but when we look back on this, in historical context, I think it will really be the flood that’s the main story.

If it just passed through, it would be a wind and moderate surge event for the Central Texas coast, and it would be gone and we could start the cleanup. But it’s going to last quite a bit.

Sourse: vox.com