[6] As the most widespread flood event there since Tropical Storm Allison in 2001, it caused a state of emergency to be declared in nine counties.

[7] In mid-April 2016, a large, slow-moving upper-level low emerged from the Four Corners region in Rocky Mountains over the Central United States.

[8][9] Simultaneously, a ridge became anchored over the Eastern United States with another low to the east, creating an omega block—a stagnant weather pattern.

Southeasterly flow from a low level jet fueled the system with ample moisture, leading to widespread rainfall rates of 2 in (51 mm) per hour.

[11] At 4:39 a.m. CDT, a flash flood emergency was declared for parts of Colorado, Waller, Grimes, Montgomery, Harris, and Austin counties,[12] later expanding to Fort Bend.

[17] The torrential rains resulted in widespread, severe flooding across Houston and surrounding suburbs—the worst since Tropical Storm Allison in 2001.