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Believe The Meteorologists. Harvey Is Incredibly Dangerous.

Update (Aug. 26, 1 a.m.): Harvey made landfall late Friday evening between Port Aransas (just northeast of Corpus Christi) and Port O’Connor, Texas, as a Category 4 hurricane with maximum sustained winds of 130 mph. Wind speeds should decrease as Harvey encounters land, but the threat from the storm is far from over. Harvey is forecasted to come inland before hooking back toward the coast over the next few days. Through Wednesday, the area from Corpus Christi to the Texas-Louisiana border is expected to receive at least 15 inches of rain, and some spots may get 40 or more inches. This stretch of coastline includes Houston, where over 20 inches of rain is anticipated.

Hurricane Harvey is slated to become the first major hurricane to hit the United States in over a decade early Saturday morning, when it is expected to make landfall near Corpus Christi. But although the country hasn’t seen a storm like this in years, this one is likely to hit as forecasted and could be very deadly. In other words, you should believe the meteorologists.

The National Hurricane Center’s forecast is staggering. Harvey could last through early Wednesday and beyond, as its current track has it coming inland and then looping back toward the coastline. Winds of greater than 39 miles per hour are possible from Brownsville, at the southernmost tip of Texas, to the Houston area near the Louisiana border, a span of about 300 miles. And many areas within this cone are expected to face far worse winds, as the storm is forecasted to be a Category 3 hurricane (meaning sustained winds of 111 mph to 129 mph) when it makes landfall. Hurricane Katrina was also a Category 3 storm when it hit Louisiana in 2005.

But water, not wind, is responsible for most hurricane deaths. A storm surge (the leading cause of hurricane deaths) of 6 to 12 feet is possible along most of Texas’s middle coast, from an area south of Corpus Christi to Sargent, about 70 miles southwest of Houston. At least 15 and possibly more than 35 inches of rain (the second leading cause of hurricane deaths) is forecasted to fall from south of Corpus Christi all the way to coastal southwest Louisiana and as possibly as far inland as the San Antonio area.

A 2016 investigative series by The Texas Tribune, ProPublica and Reveal detailed why a high storm surge and heavy rain may be particularly devastating along Texas’s Gulf Coast. In recent years, intense, rare flooding has frequently hit areas outside of officially designated floodplains. The people who live in those areas aren’t required to have flood insurance, which can leave them in dire financial circumstances after the rains come. Even those who are covered buy their flood insurance from a federal program that’s deeply in debt after decades of charging rates that don’t reflect the true risk of flooding. Scientists believe that climate change, which is causing more frequent and more intense storms, has led to a growing risk of catastrophic flooding in the area, a risk further intensified by an increase in urban development, which has paved over prairie lands that were previously able to absorb some of the water.

Even as flood risks have grown, our ability to predict them has improved. Because the storm is less than 24 hours from landfall, the chance that it will wind up missing is minimal. Over the past 27 years, the average 24-hour Atlantic hurricane forecast went from being off by an average of 100 miles to being off by less than 50 miles. Harvey is expected to hit roughly in the middle of Texas’s 350 miles of coastline — a swing of 50 miles north or south would change which areas were most affected but wouldn’t spare the state entirely. Freeport and Galveston are about 50 miles apart, and both of these cities are already well within the heavy rain forecast.

It’s also unlikely the wind and therefore the storm surge will not occur as expected. Although wind forecasts have not gotten appreciably better since 1989, they’re still pretty good in the 24 hours before a storm hits. The maximum sustained wind speed forecasts in the day before a storm have been off by an average of a little more than 10 mph over the past 27 years. If Harvey lands with a wind speed 10 mph lower than currently projected, it would still be a significant hurricane.

Harvey’s path would cause the most damage, both in human and economic terms, if it struck the city of Houston and its ports head on — that area is the most densely populated part of the Texas coastline, and it is also home to a huge concentration of refineries and chemical plants. But there is little infrastructure in place to protect communities in that area. Harvey’s expected path also crosses hundreds of colonias — unincorporated residential areas that often lack basic services such as drinkable water, roads and electricity. Both the colonias and the surrounding cities are home to hundreds of thousands of undocumented people, who may face questions about their immigration status at checkpoints along evacuation routes.

Neighboring Louisiana is also preparing for dangerous floods. In New Orleans, a network of pumps is supposed to move water out of the city, but three of five turbines that power the pumps are reportedly not working, and more than 10 percent of the pumps themselves are down for repairs. The city is developing emergency evacuation plans, and Louisiana’s Governor John Bel Edwards has already declared a state of emergency.

History suggests that Harvey will probably be costly and deadly. Hurricane Ike in 2008, for example, was similar to Harvey in both size and landfall location. Ike hit in the Galveston area (north of where Harvey is expected to land) with 110 mph winds, which is a bit slower than the ones Harvey is expected to generate. Ike produced less rainfall than what is expected with Harvey, but Ike’s storm surge was greater than the one forecasted for this weekend’s storm. Although forecasters and officials knew Ike would be a major storm long before it hit (unlike Harvey, which dissipated and reformed, giving forecasters little time to predict its path), Ike was responsible for 74 deaths in the state of Texas alone. It also caused about $34 billion1 worth of damage in the state.

In terms of rainfall, Tropical Storm Allison in 2001 is perhaps most similar to Harvey’s forecast. Like Harvey is expected to do, Allison came inland and then looped back toward the Texas coast. The storm dropped nearly 40 inches of rain on the hardest-hit areas in eastern Texas. It also caused 23 deaths and about $6 billion worth of property damage in Texas.

To find the last major hurricane that took roughly the same track as Harvey is expected to follow, you have to go all the way back Hurricane Celia in 1970. Celia produced maximum sustained winds of 130 miles per hour at landfall and a storm surge of more than 9 feet. But while Harvey is expected to move slowly, which drives up rainfall totals, Celia was fast-moving and dropped only a little more than 7 inches of rain. Perhaps as a result, Celia was responsible for a comparatively low 11 deaths and about $3 billion in damages.

Chances are that Harvey will cause a lot more physical damage than Celia. Hopefully, though, people will take the forecasts seriously. That way, even if property is lost, the loss of life will be kept to a minimum.

Footnotes

  1. All damage estimates adjusted to 2017 dollars.

Harry Enten is a senior political writer and analyst for FiveThirtyEight.

Ella Koeze is a visual journalist for FiveThirtyEight.

Anna Maria Barry-Jester reports on public health, food and culture for FiveThirtyEight.

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