Hurricane Harvey hit land on Friday night as the strongest storm to strike the U.S. since before the Twitter era -- since Hurricane Wilma hit Florida in 2005. While its strongest winds are steadily weakening due to the interaction with land, the winds are not what meteorologists have been most worried about with this storm. 。

Instead, it's the water that will do the most damage, and that aspect of this storm is just getting started, even if the storm's winds weaken it to tropical storm status.。

Harvey is going to turn into an atmospheric orphan, left behind by upper level steering currents, and allowed to rain itself out over southeastern Texas for at least five days or more. The rainfall totals in the forecast are staggering -- up to 40 inches in some places between Corpus Cristi and Houston.。

This spells nothing short of an ark-requiring flood disaster.。

Hydrologists are forecasting that most rivers in Southeast Texas will crest near or above record levels during the next several days, with flooding in the Houston and Galveston area complicated by the combination of storm surge flooding from the ocean and inland flooding. This one-two punch could worsen flooding there, as water builds up and is unable to escape into the sea.
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The National Weather Service continues to use strong language to describe the flood threat, stating: "Catastrophic flooding expected over the next few days." In fact, meteorologists are having difficulty recalling such a dire flood forecast associated with a landfalling hurricane, because most storms are moving faster when they hit land. 

This slow movement will allow the storm to ring out the moisture it has soaked up from the warm waters of the Gulf of Mexico, while at the same time refilling its supply by continuing to tap into that pipeline of moisture sitting just off the coast.。

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How much rain a hurricane or tropical storm produces is largely a function of its forward speed, rather than the intensity of its winds. Even slow-moving, weak tropical storms can produce devastating flooding. 。

With Harvey, its forward motion is anticipated to be essentially nil for the next few days. This could lead to phenomenal rainfall totals, all dumping on more or less the same area.。

Rainfall forecast for the next 5 days from Hurricane Harvey, showing a large area of 25-inch rainfall amounts over Texas.Credit: noaa/nhc
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“The historical record of U.S. hurricanes gives us few, if any, analogs for a major hurricane landfall that transitions into a multi-day rainfall event as prolonged, extensive, and intense as the scenario painted by multiple forecast models for Harvey.”

Tropical Storm Allison in 2001 devastated the Houston area, but its rains were far less widespread than the forecast calls for with Harvey. That storm still caused $9 billion in damage and killed 41 people.。

Though most people think of wind as a hurricane's greatest weapon, flooding kills nine out of 10 people who perish in such storms.。

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Across much of the U.S. and around the world, extreme rainfall events are becoming more common as the world warms. In addition, scientists expect that as human-caused global warming continues and the atmosphere holds more moisture, tropical storms and hurricanes will produce heavier rains.