Monsoon moisture spurs Colorado flash floods


Radar-estimated storm total precipitation ending at 6:05 am MT. (NWS Denver/Boulder modified by CWG)

Sections of northern Colorado are under water after torrential rains swept through the state Tuesday night. According to the National Weather Service in Denver/Boulder, rainfall amounts reached upwards of 3.5 inches north and west of the cities, and radar estimates show that five to six inches likely fell in Logan and Washington Counties.

The Denver airport set a 24 hour rainfall record on Tuesday, with 1.80 inches of rain, breaking the previous record for the date of 1.44 inches, which was set in 1989.

Rainfall will continue today, and though thunderstorms are not expected to be as intense as Tuesday’s, some areas of heavy rain are possible. A flash flood watch is in effect for much of the area until 6 p.m. MT.

According to CBS4 meteorologist Chris Spears, late July and early August is the most common time for flash flooding to occur in the Denver area. The combination of moisture-rich air from the North American monsoon combined with weak upper-level winds can lead strong storms with heavy downpours to develop and essentially park over the area. “Add in any other element, such as a boundary from an old cold front,” writes Spears, “and it’s almost the perfect storm for a lot of rain to happen in a short period of time.”

Some residents thought it might be easier to break out the rafts than to try and wade through the water.

At one on-ramp to I-25 in Denver, drivers decided to roll the dice and see whether or not they would make it through the deep water.

Video from Main Street in Windsor, Co. shows heavy rain and flooding.

Angela Fritz is an atmospheric scientist and The Post's deputy weather editor.
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