Monsoon season runs roughly from June 15 to September 30 on any given year. The monsoon is a seasonal wind shift that brings in an almost continuous stream of moisture from the Gulf of California into the normally dry and arid southwestern United States.

The weather that we see is largely controlled by the winds in the atmosphere. During the Springtime in Colorado, we see winds that are coming from the west or northwest. Because of that wind pattern, we tend to have a drier overall weather pattern during late-April and May.

As we get deeper into the summer months, our winds begin to shift. Rather than winds coming from the west, they start to 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. This is thanks to a large-scale high pressure system that sets up over the Central Plains allowing for the winds to to wrap around clockwise and thus picking up that aforementioned moisture. We can see this occur when we look at our dewpoints. With more moisture in the atmosphere, our dewpoints begin to rise and stay consistently high thanks to this continuous wind direction. Couple that with daytime heat and it’s easy for storms to spark each afternoon.

NWS Boulder Depiction of a Typical North American Monsoon Setup

The image above shows the typical setup that occurs when the monsoon is in affect. Something else that is notable from the image above is the Low pressure system (denoted by the red “L”) over the California/Arizona border. This is called a monsoon low but it also referred to as a thermal low or heat low which essentially is a localized area of low pressure that forms as a direct result of desert heat. The addition of this thermal low aids in bringing moisture into the Four Corners region from the Gulf of California due to its counter-clockwise rotation.

What Does This Mean for Colorado?

Colorado benefits from the monsoon but moisture decreases the further north you go or as you get further from the original moisture source. Southwest Colorado will benefit the most from monsoon moisture where places like Denver will definitely see a change in daily precipitation chance but it is not as notable. Areas near the Nebraska or Wyoming borders almost see no influence from the North American Monsoon.

The 2020 Monsoon Season

This years monsoon season is being relied on to bring drought-relieving moisture for our area as drought is gripping over 75 percent of the state currently. During a year where ENSO (El Niño southern Oscillation) is neutral with a slight lean towards La Niña, like this year, we could expect a normal to above-normal occurrence of the North American Monsoon. Good news. The Climate Prediction Center puts out forecasts for the country in daily, weekly and monthly increments and they include all of this data. 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 where there is an equal chance of seeing above or below-normal precipitation. Bad news.

Our long-range forecast through the summer is looking hot and dry which means the drought may worsen in many areas which allows the fire season to be active. The North American Monsoon is our last hope for decent summer rains before drier conditions move in for Fall and Winter.

~ Rain or Shine
I’m Andy Stein

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