Following an exceptionally wet August, Colorado’s Rocky Mountains are drying out as monsoonal patterns weaken and signs point to above-normal temperatures heading into September. 

Building as the interior U.S. heats up in June and July, the monsoon draws moist air from the Pacific Ocean and the Gulf of California toward the southwestern U.S. 

The seasonal pattern results in consistent thunderstorms and rainfall in the monsoon region, which primarily includes Arizona and New Mexico but also Utah and Colorado, and provides much of the region’s annual total precipitation, according to the National Oceanic and Atmospheric Administration.

“We started off with a bit of a slower onset of the monsoon this summer — June and July were fairly dry across most parts of the state,” said Zach Hiris, a meteorologist for the National Weather Service office in Boulder. 

Storms ramped up in August with much of the state experiencing near- to above-normal precipitation. Areas west of the Continental Divide saw anywhere from “normal to almost two  to three times the normal amount over the course of the month,” Hiris said. 

Dillon, for example, recorded precipitation that was 50% above normal, while areas further west towards Grand Junction saw as much as 300% or normal, Hiris said.

Parts of the Western Slope were anywhere from 1 to 2 inches above normal, said National Weather Service meteorologist Matthew Aleksa, who is based out of the agency’s Grand Junction office. 

Deep monsoonal moisture resulted in flooding in some areas, such as Redstone and Carbondale where heavy rain also resulted in mudslides, Aleksa said. 

“August was more of an active month, whereas July was more of a down month, which isn’t what you typically see with the monsoon,” Aleksa said. 

The pattern defied early forecasts, with the Climate Prediction Center in late June projecting that Colorado’s mountains had as much as a 50% chance of seeing below-normal precipitation in August. 

Moving into September, the monsoon is expected to soften, giving way to dryer days with higher-than-normal temperatures. 

Labor Day weekend is slated to see mostly warm, sunny weather for parts of the High Country and Western Slope, though some monsoonal moisture could be pulled in through the Four Corners region bringing precipitation Sunday through early next week. 

“But it doesn’t look to be as significant as a moisture surge as we saw in August,” Aleksa said. “We’ll still see some bursts here and there, but overall it is looking like the pattern is becoming drier.”

A 6-10 day outlook from the Climate Prediction Center shows western Colorado has between a 50% to 70% chance of seeing above-normal temperatures between Aug. 29 and Sept. 8. During that same time, the region is predicted to see near-normal precipitation. 

Zooming out further, a month-long forecast for September shows a 60% to 70% chance for above-normal temperatures for the mountains and Western Slope and a 33% to 50% chance of below-normal precipitation. 

“The monsoon quickly dies off as we get into the September time frame,” Hiris said. “But that comes with the caveat that it takes just one or two thunderstorms to erase those deficits.”