Historic Houston flood: 16 inches of rain in under 12 hours — and it's not over.

A deluge of colossal proportions struck the Houston and Galveston, Texas, areas on Sunday night, with nearly 2 feet of rain falling in some spots.  

The rain has led to widespread severe flooding, prompting the rare issuance of a "flash flood emergency" warning from the National Weather Service. 

Houston Mayor Sylvester Turner called the rainfall and associated flooding “unprecedented” for the city. City offices and schools are closed on Monday due to flooded roadways.

The flooding forced Ed Emmett, the county judge for Harris County, which includes Houston, to declare a disaster on Monday morning.

Inside this storm system

A cold front that is barely moving east is helping the warm, moist air to rise, cool and condense into clouds and precipitation. Near the heaviest rain bands, onshore winds from the southeast were converging with winds from the north, accentuating the rainfall rates. 

A key factor contributing to the flooding in the Houston area is that the storm system is barely moving. Instead, the heavy thunderstorms are sitting atop the city, continuously dumping extreme amounts of rain. 

As a study released on April 15 found, when weather systems get stuck in traffic jams, extreme events can often result.This storm was well forecast, with the heavy rainfall threat discussed at least three days in advance. However, pinpointing the specific areas that will receive the heaviest rains is a difficult task, even with all the tools of modern meteorology. 

The heavy rains have caused rivers and creeks to rise, and prevented the low-lying cities drainage system from clearing water fast enough to spare homes from flooding. Reports indicate that the flooding in some spots is comparable, if not worse than, flooding that occurred during Tropical Storm Allison in 2001.That storm caused 22 deaths and more than $5 billion in property damage in Harris County alone, according to the Harris County Flood Control District. Southeast Texas also saw extreme flooding in May and June of 2015. 

It’s likely that this event will run a tab in the hundreds of millions, if not eclipsing the $1 billion mark given the amount of property involved.

Across much of the U.S. and around the world, extreme rainfall events are becoming more common as the world warms and the atmosphere holds more moisture.