12:55 PM MDT, Wednesday, July 11, 2018
Although it would be an exaggeration to say wet and cool weather is coming Thursday & Friday, it’s accurate to say that the Front Range region should be less hot with a little boost in the chance for clouds and thunderstorms. The Southwest monsoon is resulting in increased thundershower activity in western Colorado and down in the desert, and some of that moisture is likely to move over the Front Range region, at least in the mid levels. Significant rainfall Thursday and Friday will probably be spotty, but gusty wind and brief showers will probably affect a larger portion of the region.
After a return to generally hot and dry weather on Saturday, a cool front on Sunday will probably boost the chance for periods of clouds and thunderstorms again.
What is the monsoon?
The Southwest Monsoon (also called the North American Monsoon, or the Mexican Monsoon) is not nearly as strong as its Asian counterpart, but it can have an important influence on weather in Colorado during mid and late summer.
It begins when the strong June sun warms up the high plateaus of Mexico and the southwestern U.S. The heated air become buoyant and rises, and moisture-laden air from the tropical Pacific (and some from the Gulf of Mexico) moves in and triggers torrential thundershowers in the Sierras of northern Mexico. As summer progresses some of that moisture moves up into the southwestern U.S. and enhances thundershower development there too. The monsoon dies off in the late summer or early autumn as the strength of the sun weakens.
Colorado is on the edge of the monsoon influence. Some years see very little impact and in other years the pattern can allow surges of monsoon moisture that trigger slow moving thunderstorms with high rainfall rates. The monsoon is not continuous, but it can come in spurts from July to September, with mid July to mid August being the climatological peak. This week’s monsoon surge is not that impressive for the Front Range, but we still have the main season ahead of us.
I should point out that not all rain in July and August is from a monsoon pattern, but extreme rainfall is often connected to the monsoon pattern.