What is a "Monsoon?"

The term monsoon is one that most people associate with areas in India and Asia seasonal increases (or decreases) of precipitation, but there are several parts of the world that experience what we call a monsoon throughout different parts of the year. In addition to Asia, Australia, Africa, Europe and both North and South America experience a seasonal monsoon with varying degrees of intensity.

We could talk about all of those but the truth is they are all a bit different from each other due to differences in ocean temperature, amount of sunlight and terrain. For today we will stick primarily to the monsoon experienced in the Southwestern United States in summer.

Each year, the monsoon in the Southwestern United States begins in Mexico and Arizona...

A monsoon in its simplest terms is a season reversal of traditional wind patterns that can effect weather and more specifically precipitation. But what causes the prevailing winds to reverse in the first place?

It all comes down to an imbalance of heating between the ocean and land. During the summer months strong heating occurs on both the landmass and the ocean; rocks, dirt, sand and to an extent grass thermally conduct the heat back into the atmosphere allowing the air to heat and expand in those areas more than over ocean areas. This causes low pressure to build over the landmass while high pressure develops over the ocean just off-shore as ocean temperatures remain relatively stable in comparison to what's going on over the land.

These pressure differences cause sea breezes to blow inland carrying moisture-laiden air over the landmass. In our case, that air gets lifted as it moves upward in elevation towards and over the Rocky Mountains. As we know from basic weather 101; what happens to air with a lot of moisture when you lift it into the atmosphere? You see an uptick in showers and thunderstorms - cooler air can't hold as much moisture as warmer air. Remember our basic ingredients for thunderstorms!

The Southwestern United Status Monsoon

The North American Monsoon also called the North Mexico, Southwestern U.S. or Arizona monsoon describes the pattern shift we see in mid to late summer across the United States. It's a delicate dance of upper level low and high pressure systems along with upper and mid level winds that change the overall weather pattern for a good portion of the Western and Southwestern United States. Here's a few factoids about the monsoon and how it sets up for Colorado.

Colorado Monsoon Season

  • Timing
    • Early July through Mid/late August
    • Add or subtract a week or two to each time frame, the monsoon can establish early or run late but the early July through late August is the most common time to see the effects
  • Increased Precipitation
    • Yes, but not widespread. Afternoon thunderstorms become more numerous and more moisture enters the atmosphere, but due to lack of steering winds aloft (shear) storm characteristics are mainly heavy rain and sometimes hail. Thunderstorms are more of the "pulse" variety and tend to grow and die quickly (this can give the storms the appearance of "hopping" or "missing" areas
  • Severe Weather
    • Flash flooding from heavy rain becomes the primary threat
    • We do see an uptick in hail with storms during monsoon season, it's almost a second hail season for Colorado
    • Tornadoes are rare but can still happen, typically we see a big drop-off of tornado activity after the end of June in Colorado

Colorado's Rainy Season

In Colorado we tend to see an uptick in precipitation through the form of afternoon thunderstorms in the afternoon during our monsoon season. As I said above though, the precipitation isn't widespread and generally depends on which areas see the most consistent thunderstorm activity so it can be hit or miss sometimes. In general though, most areas see higher precipitation in July and August on average.

Average precipitation by month for Denver, Colorado. Notice the uptick in July and August; this is due to the summer monsoon


Mechanics Behind the Southwestern Monsoon

High pressure to Colorado's East and low pressure to Colorado's Southwest funnels warm and and moisture into the state - the higher terrain/mountains helps lift those ingredients to form thunderstorms

Like many of our weather patterns in Colorado, the monsoon is a delicate dance of high and low pressure systems along with the jet stream and upper level winds. All of these need to set up in just the right place make the perfect monsoon setup. Since these areas are not static and tend to wobble back and forth over the course of hours or days - we can sometimes see days with lots of storms and other days with hardly any at all.

During the summer, we often see a high pressure called the subtropical high establish over the midwest which pushes air outwards in a clockwise direction. This is an important feature of our monsoon as it is the primary mechanism responsible for transporting moisture from the Gulf of Mexico up into Colorado. The southerly and Southeasterly winds can also aid to upslope our storms against the higher terrain.

Meanwhile a low pressure system sets up over Southern California or the Baja region and pushes wind around that center in a counterclockwise fashion. You can see how the two systems funnel warm air and moisture from the Gulf of Mexico or the Baja region through Arizona and New Mexico into Colorado. The higher terrain in the Mountain West plays a part as it helps to lift that heat and moisture and form thunderstorms. The higher terrain can also help this energy break through any capping in the atmosphere which would also hinder thunderstorm development.

As we move later into the summer, capping inversions become more common. This is like a "lid" on the atmosphere and any warm/moist air needs an extra lifting mechanism to help that energy push up into the atmosphere.


Flash Flooding

By far the biggest severe weather threat we see during the monsoon season is flash flooding.


Why is that you ask? As you can see from the graphic above; we tend to see a lot of moisture in the atmosphere and when these thunderstorms for they don't have a lot of wind aloft to push them along. This means storms form with a ton of moisture and because they tend to move very slowly or sometimes hardly at all, areas underneath those storms can see a lot of rain over a longer period of time.

Additionally, due to higher runoff urban areas and burn scars from fires are especially prone to flash flooding. All of that water has to go somewhere so even though the weather may be fine where you are, it's always worth paying attention to the weather and knowing if there are storms in the area. If you are in the mountains or on the plains recreating near creeks and streams this is especially important!

Monsoon 2020 Outlook


If you're already a supporter, make sure to sign in to see this content!



Hopefully this helps give a glimpse into what is a normal part of Colorado weather every year. It really is fascinating how the monsoon behaves year to year and what it does with Colorado's Weather.

If you're a site supporter - you got a special/early look at our monsoon forecast and which direction we may be leaning this year in terms of wetter, average or below average for precipitation during the period.

If you have any questions or comments feel free to leave them here on the site or ask them on my Facebook page!