What makes this hurricane so potentially dangerous?

Longevity, in a word. Hurricane Harvey is forecast to stay in the area for as long as four days. It will decline in wind strength so it will stop being called a hurricane by Sunday, but the wind is not the main story.

Tropical cyclones are classified by wind strength, progressively named depression, storm, and then hurricane as the wind power increases. At all stages they produce vast amounts of rain. Even if they pass through they can leave 300mm of rain in a few hours.

Harvey isn’t moving much at all now so will produce heavy rain in the same place for days on end. Forecast models give the highest figure of about 400mm of rain in Texas Hill Country, west of Austin, and as far away as southwest and central Louisiana. As much as 1000mm is forecast for the coastal lands east of Harvey’s centre (the city of Victoria).

These rainfall figures may be on the low side – the city of Victoria has already reported 417mm collected in the last 24 hours. 

Why all this talk about storm surge?

Hurricanes are like a hole in the atmosphere. Think of the views as seen from space. In the middle, air pressure is much lower than the atmospheric norm. Harvey dropped down to below 940mb and this creates an imitation vacuum cleaner. Water tries to fill from below, so effectively a hurricane pulls a dome of water with it. In addition, persistent wind piles up the water on one side of a hurricane, in this case the eastern side.

The resultant storm surge has already been measured at two metres above normal high tide at Port Lavaca. If you add waves on top of that at high tide, you create a threatening wall of water that is virtually unstoppable.

These are reasons why levees have been built along the southern coasts, but prolonged or repeated water pressure and insufficient maintenance can breach levees. In the case of this part of the Texas coast, there is an outer barrier island with entrances to inner lagoons. The land beyond is naturally low-lying. This is obviously prone to flooding and very slow to drain. 

When will it be over?

It is unusual for a hurricane to slow down as it makes landfall and then hang around the area. It looks like Harvey will be around as a recognisable feature until at least Wednesday over southern Texas. If this turns out to be the case, it will be an unprecedented event.

There is a chance that Harvey will re-cross the coast, maybe on Tuesday, back to the warm waters of the Gulf of Mexico. These were at about 30C on Friday, but will now be a little cooler because of the churning caused by Harvey. Nevertheless, anything above 27C will be like turbocharging the cyclone again. Were this to happen, Hurricane Harvey will likely make another landfall further up the coast. 

Where did it come from?

Africa. Most Atlantic hurricanes start with a cluster of thunderstorms in tropical Africa. Just like the system that brought the deadly landslide to Freetown, Sierra Leone. Then there is a long track across the warm tropical Atlantic waters. As long as water temperatures are above 27C, then when the storm cluster encounters the right atmospheric conditions, it will start to revolve and intensify.

Harvey did just this and encountered its first land in Barbados a week ago. On August 18, it crossed the island as a tropical storm. It then lost its identity, followed the breeze as a cluster of storms, crossed the Yucatan peninsula, and emerged into the Gulf of Campeche.

All this time it had been watched by forecasters with an expectation of redevelopment. That is just what happened. The Gulf of Mexico is warm – water temperatures of 29C near Campeche, Mexico and higher off the Texan coast.

Harvey quickly became a tropical storm, then a hurricane. It was steered towards the coast of Texas and strengthened more. The winds in the upper atmosphere were light so didn’t tear the hurricane apart.

And there is the enduring problem. Hurricanes are steered by prevailing winds well above our heads. There is no wind above Texas, just a howling hurricane over the south with no clear direction to go.

With files from Wunderground.com

Texas begins mass evacuations as Hurricane Harvey approaches

Source: Al Jazeera

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