At 11 p.m. Friday, Harvey was centered about 30 miles east-northeast of Corpus Christi, tracking toward the northwest at 8 mph. A weather station at Aransas Pass – northeast of Corpus Christi and within storm’s eyewall, reported sustained winds up to 111 mph and gusts to 131 mph in the previous hour. In Rockport, Tex., the National Weather Service logged reports indicating “numerous structures destroyed”, and “buildings collapsed with people trapped inside”. The full extent of the damage will not be known until daylight.

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In addition to damaging winds, the National Hurricane Center said it expects “catastrophic and life-threatening” flash flooding along the middle and upper Texas coast. An incredible amount of rain, 15 to 30 inches with isolated amounts of up to 40 inches, is predicted because the storm is expected to stall and unload torrents for four to six straight days. In just a few days, the storm may dispense the amount of rain that normally falls over an entire year, shattering records.

“Let’s set the expectations: Texas is about to have a very significant disaster,” said Brock Long, FEMA administrator.

The storm is also predicted to generate a devastating storm surge — or raise the water as much as 13 feet above normally dry land at the coast.

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“In all these years, it’s rare that I’ve seen a hurricane threat that concerns me as much as this one does,” said Rick Knabb, hurricane expert at the Weather Channel and former director of the National Hurricane Center.

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The National Weather Service office in Corpus Christi, near where the storm is expected to make landfall, said that due to the combination of flooding from storm surge and rainfall, “locations may be uninhabitable for an extended period.” It warned of “structural damage to buildings, with many washing away” and that “streets and parking lots become rivers of raging water with underpasses submerged.”

Hurricane, storm surge and flood warnings plastered coastal and inland portions of East Texas Friday evening, and tropical-storm-force winds are forecast to spread well into the interior of Texas Friday night.

The five-day “cone of uncertainty,” an illustration of where the storm may track, is squashed down to a circle, indicating that the storm is expected to stall, unleashing its wrath over the same general area through the middle of next week.

Until Harvey, Texas had not been hit by a hurricane since 2008, when Ike crashed ashore near Galveston as a Category 2 storm. Harvey brings together some of the worst aspects of three recent Texas hurricanes wrapped into one: Ike’s devastating storm surge, the destructive winds of Category 4 Hurricane Bret in 1999 and the disastrous rains of Tropical Storm Allison in 2001.

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Harvey could be a storm Texans remember for many years to come. It became the first Category 4 storm to hit the state since Carla in 1961, and the first in the U.S. since Charley in 2004.

The rain

The rain forecasts are extremely ominous. “Somebody is going to get a rainstorm to tell their grandkids about,” said Bill Read, a former director of the National Hurricane Center.

Areas along the middle and upper Texas coast may see 15 to 30 inches of rain, with a few areas receiving as much as 40 inches, although it is impossible to pinpoint exactly where the heaviest rain will fall.

“Millions of people from Corpus Christi to Houston will get more than two feet of rain when all is done, with Southern Louisiana getting up to a foot of rain,” said Louis Uccellini, director of the National Weather Service.

So much rain is predicted that the National Weather Service had to add a new high-end category to its forecast map, illustrating the potential for more than 20 inches, and over a sizable area:

Dangerous flooding could expand five to six counties inland from the coast.

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The European model simulation run Friday afternoon simulated isolated areas of 50-60 inches of rain, which would be an unimaginable disaster.

Houston, the nation’s fourth-largest city, could receive 20 inches or more of rain from the storm, depending on exactly where it tracks — with the heaviest moving in Saturday or Sunday and then continuing into early next week. Matt Lanza, a meteorologist based in Houston, said 20 inches would be “devastating” for the city, depending where it fell. A worst-case scenario, Lanza said, would be for this amount of rain to fall just northwest of downtown as “all that water has to push through the bayou networks across the city into Galveston Bay.”

The Weather Channel’s Knabb pointed out 9 out of 10 people who die in landfalling tropical systems in the U.S. die in water, from either flooding from heavy rain or storm surge.

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“Promise yourself that you’re not going to drive your car over a water-covered roadway or drive your car around a barricade when the road is closed,” Knabb said. “Most people who die in inland flooding die in their cars.”

Especially late this weekend and into early next week, areas of western and southern Louisiana could also be hit with double-digit rainfall totals.

Storm surge

The Hurricane Center predicts 6 to 13 feet of water — above normally dry land — inundating coastal areas immediately to the east and north of the landfall location. That amount is based on the assumption that Harvey makes landfall as a Category 3 hurricane. It is critical that affected residents heed evacuation orders.

Keep in mind that the timing of normal astronomical tides is a factor. If the highest storm surge arrives at or near high tide, the total “storm tide” will be maximized.

Compound effect of rain and storm surge

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In coastal areas, the combination of double digit rainfall and a storm surge that raises water levels (above normally dry land) for days (because the storm will stall) may result in massive water buildup.

“The collision of these ingredients looks like a train and car headed for an intersection on the tracks,” writes Hal Needham, an international storm surge expert. “If Galveston Bay is raised by several feet for many days, and a wide area of Texas real estate receives 15-20 or more inches of rain, where is that rain going to go? Runoff into Galveston Bay and Galveston Harbor will be severely impeded and this will create a catastrophic compound flood event.”

The area around Corpus Christi could face similar issues.

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Wind

Harvey was producing maximum sustained winds up to 130 mph when it came ashore, strong enough to cause widespread power outages and destruction to homes and businesses.

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At 11 p.m. Friday, tropical-storm-force winds expanded 140 miles away from the storm center, while hurricane-force-winds were confined within 40 miles of the center.

A projection from modelers at several universities indicates the potential for approximately one million outages in affected areas.

In addition to the violent straight-line winds inflicted by the storm, tornadoes could also spin up within its spiral bands as they push inland.

Uncertainties

There are important uncertainties about storm’specific track and exactly how long it will linger in Texas. Both of these factors have significant implications for where the heaviest rain falls and how long the deluge persists.

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Right now, it is important for Texans in the path of this storm to understand — irrespective of where the storm makes landfall — Harvey’s footprint will be enormous because of the large expanse of its heavy rain field and long duration. Preparations should be brought to completion hastily.

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A note about what is considered a major hurricane