Bouncing between the extremes of drought and flood, the weather whiplash in Texas continued this weekend. For the second time this year, torrential rain caused widespread flooding across the Lone Star State.

The city of Corsicana, south of Dallas, saw more than 18 inches of rain between Friday and Saturday. Flash floods caused extensive damage and even derailed a Union Pacific freight train. In Houston, where they received 8 inches of rain on Saturday and Sunday, bayous swelled out of their banks and flooded roadways. Local officials say Buffalo Bayou near the downtown area rose 20 feet in just 12 hours.

The cause of this prolonged rain event involved the interaction of a few key atmospheric players. First, an area of high pressure over the east coast – with a clockwise circulation – pushed tropical moisture across the Gulf of Mexico and into Texas. Then, there was a strong upper level low – with a counter clockwise circulation – over the southwest and a cold front moving southeast. These added lift to the atmosphere. When the warm saturated air was forced to rise, it cooled. Since cool air holds less moisture than warm air, the moisture was wrung out of the atmosphere in the form of intense rain. Then, on the heels of all that, remnants of Hurricane Patricia from the Pacific Ocean traveled across Mexico and into Texas. It brought even more tropical moisture into the mix.

Ironically, much of Texas was in a drought just last week. It was considered a “flash drought” as it developed very quickly this summer after intense rains and catastrophic flooding in May brought the previous drought to an abrupt end. From drought to flood to drought and back to flood, Texas certainly has had a wild ride with weather this year.

Freight train derailed by flood waters near Corsicana, TX. Credit: NBC News