Colorado’s Monsoon Season is Here—But it Might Not Bring Enough Rain

You’ve probably already noticed Colorado’s monsoon season has arrived.

Running from roughly June 15 to September 30 each year, the North American monsoon is a seasonal wind shift that brings a stream of moisture from the Gulf of California into the typically dry and arid southwestern United States. The monsoon brings summer thunderstorms that occur on a regular basis after daytime heat sparks afternoon storms with heavy rains, lightning, and the possibility of hail and flash flooding.

During the spring months in Colorado, winds come from the West or Northwest, and because of that pattern, we tend to have drier overall weather during late April and May. As we get deeper into the summer months, our winds shift. Rather than winds coming from the West, they come from the South and Southwest, which allows moisture to be picked up from the Eastern Pacific Ocean, the Gulf of California, and the Gulf of Mexico.

The moisture Colorado sees from the monsoon decreases the further north you go, as it’s farther from the original moisture source. Southwest Colorado typically benefits the most from monsoonal moisture while places like Denver still see an uptick in daily precipitation.

The image above shows the weather patterns that occur when the monsoon is in effect. A large-scale high-pressure system sets up over the Central Plains allowing for the winds to wrap around clockwise and pick up moisture from the Gulf of Mexico. The low-pressure system (denoted by the red “L”) over the California/Arizona border forms as a direct result of desert heat and brings in additional moisture into the Four Corners region from the Gulf of California due to its counter-clockwise rotation. These two weather systems provide ample moisture in our atmosphere; couple that with daytime heat and it’s easy for storms to spark each afternoon.

Predictions for the 2020 Monsoon Season

With over 75 percent of Colorado abnormally dry right now (30 percent is in extreme drought) this year’s monsoon season is critical. During a year where ENSO (El Niño southern oscillation) is neutral with a slight lean towards La Niña (cooler than average surface waters in the equatorial Pacific), we could expect a normal to above-normal occurrence of the North American Monsoon—which is good news.

But here’s the catch: the Climate Prediction Center puts out forecasts for the country in daily, weekly, and monthly increments—and the forecast they have for July, August and September shows higher than normal temperatures expected in every area of Colorado while below-normal rain chances are expected everywhere in the state other than the far Eastern Plains. So, for places like southwestern Colorado where we’re already seeing extreme drought, this is bad news.

Our long-range forecast through the summer is looking hot and dry, meaning the drought may worsen in many areas and the fire season might be active. Gov. Jared Polis activated the State’s Drought Mitigation and Response Plan on June 24, which allows an impact task force to begin regular meetings to increase communications and find ways to help communities respond to and mitigate drought impacts.

The North American monsoon is one of our last hopes for decent summer rains before overall drier conditions move in for Fall and Winter, but unless the forecasts change it might not have the impact Coloradans hope for.