Where is the North American monsoon? …and the anniversary of big floods

3:00 PM, Friday, July 29, 2016

Nineteen years ago yesterday Fort Collins suffered a devastating and deadly flash flood, especially along Spring Creek, when 10 inches of rain deluged the city in 5 hours. Thirty-one years ago Monday Cheyenne had it’s biggest rainstorm when more than 6 inches of rain and a whole lot of hail fell in a few hours resulting in deadly flash flooding. This weekend marks the 40th anniversary of the Big Thompson canyon flash flood of July 31, 1976 when 12 inches of rain in under 5 hours sent a wave of water down the Big Thompson canyon that claimed 144 lives (104 in vehicles) on the eve of the state’s centennial celebration. By comparison, the September 2013 flood that also devastated the Big Thompson canyon generally had peak flows that were much lower than in 1976, but lasted much longer resulting in a great deal of erosion and riverbank collapses.


Why is late July and early August such a common period for flash floods? You may have heard of the North American monsoon (sometimes called the Southwest monsoon or the Mexican monsoon). During June and July numerous tropical thunderstorms along the mountains of western Mexico pump large amounts of moisture into the atmosphere. As summer wears on the weather pattern sometimes permits that “monsoon” moisture to surge into the southwestern United States, in some years all the way to Colorado. July through September is when this could happen, with mid July to mid August being the prime time for monsoon impacts in Colorado. And yes, the September 2013 floods did occur in an active monsoon pattern.


The monsoon this year has not had much northward extension so far. There is more moisture around today (but not really from the monsoon), and a few localities may experience heavy rain and hail, but a widespread soaking rain appears unlikely. Warm with only small chances for afternoon thunderstorms is expected this weekend. Only subtle changes in the weather pattern can allow a monsoon surge into the Front Range region, so just because we’ve made it to late July without widespread monsoon rains does not eliminate that possibility in August.


Have a nice weekend!

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