National Weather Service issues grim warning calling Harvey 'unprecedented,' with impacts 'unknown'

The National Weather Service is pulling no punches when it comes to Hurricane Harvey.

The federal agency is now calling the storm battering Texas and bringing historic levels of flooding to Houston "unprecedented."

"... All impacts are unknown & beyond anything experienced," the NWS added in a dire tweet.

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One to 2 feet of rain has already fallen on Houston, with more on the way. If you are in Houston or the surrounding area, it's important to listen to local officials and find shelter in a safe place. If you're in a safe area, don't travel.

Forecasters have been warning about the extreme flooding Harvey could bring to Texas for days, saying that a mixture of warm water in the Gulf of Mexico and the slow-moving nature of the storm could combine to produce rainfall totals the likes of which we haven't seen in modern history.

Over the past few days, the National Weather Service has issued dire advisories warning of "catastrophic flooding."

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Some forecasters have started to compare the levels of flooding seen in Houston to Hurricane Katrina's impacts in New Orleans.

While the scope and scale of the flooding caused by the two storms is comparable, Katrina's flooding occurred due to extreme storm surge and Houston's floods are due to unprecedented rainfall.

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Houston is generally at risk from flooding due to heavy rainfall. The network of bayous crossing the city help to drain rainwater; however, they can also overflow and cause flooding during extreme rainfall events.

Most flood channels, bayous, and rivers surrounding Houston are now hitting record levels and rising.

The National Weather Service is now predicting that Harvey could dump as much as 50 inches of rain in parts of the Upper Texas coast, which includes Houston and Galveston, before the week is out.

"Isolated storm totals may reach 50 inches in this region," the service wrote in an update Sunday.

"These rains are currently producing catastrophic and life-threatening flooding, and flash flood emergencies are in effect for portions of southeastern Texas."