Colorado and the North American Monsoon
What is the monsoon, when is monsoon season, and how does it impact Colorado?
In this article, we define the North American Monsoon, describe its formation, dispel several myths around it, and look at some of its common weather patterns using several different years as examples. We’ll also discuss its potential future in the face of climate change.
As we get into summer, you’ll hear the term monsoon used ever more frequently, especially during stretches of wetter weather. Parts of Colorado (not the entire state) often experience a secondary wet season beginning in mid-July that corresponds with the formation of the North American Monsoon.
However, the monsoon’s impacts are often quite different than described in casual conversation. Afternoon thunderstorms and long stretches of wet weather in the summer do not necessarily have anything to do with the monsoon, especially before the end of July.
In fact, the monsoon can actually bring significantly drier weather to Colorado and the desert southwest. Colorado saw record wildfires in the incredibly dry summer of 2020, which was often misattributed to the lack of the monsoon. The reality was that the North American Monsoon was in full swing and its impact on large-scale weather patterns brought about our dry summer.
The North American Monsoon
Let’s start with the definition of the term monsoon.
monsoon: a seasonal reversing wind accompanied by corresponding changes in precipitation.
At a basic level, this means that the monsoon is simply a change in the prevailing wind direction, which usually brings with it a different kind of weather.
The monsoon is not:
- Afternoon thunderstorms
- Precipitation that occurs in summer
- A short-term weather pattern (e.g. something that occurs over the course of a day or even a couple weeks)
An active monsoon pattern can enhance the first two points above (afternoon thunderstorms and summer precipitation), but these phenomena on their own are not the monsoon.
Though ‘monsoon’ most commonly refers to seasonal wind changes in southern and southeastern Asia (featuring both a dry monsoon and a wet monsoon), other parts of the world experience monsoons as well.
The North American Monsoon is primarily focused on central Mexico and northward to the desert southwest of the United States, with limited impacts elsewhere.
In spring, surface winds over Mexico typically flow towards the ocean (both the Pacific — Gulf of California — and the Atlantic — Gulf of Mexico). As the season progresses into May, the jet stream begins to move north, and intense solar radiation starts to heat up the high terrain of central Mexico (the Mexican Plateau). The land eventually becomes so hot that prevailing winds flip, and air over the cooler, moister oceans starts to flow inland. The North American Monsoon has formed.
This usually triggers some large-scale changes in the atmosphere. A subtropical high (the Monsoon Ridge) often builds over Mexico and moves northwards, reaching the United States by mid-July where it then elongates. Sometimes, surface heating can create a Monsoon Low just south of the Mexico border, which can further enhance moisture transport.
The size, intensity, and position of these two features help determine whether we see enhanced precipitation or not. These features remain largely stationary until the monsoon fades away in mid-September as the days grow shorter and the jet stream moves south again.
The monsoon has a decreased impact on the weather as you move north of the Mexican Plateau, and some areas, like northern Colorado, are relatively unaffected by its formation.
Impacts of the North American Monsoon
The below image shows a common weather pattern associated with the North American Monsoon. This idealized pattern pulses low-level moisture northward from the Gulf of California (and to a smaller extent, upper-level moisture from the Gulf of Mexico), which drives enhanced precipitation over the desert southwest later in the summer.
This moisture is converted into precipitation almost entirely due to convection — thunderstorms, which mostly form due to orographic processes, e.g. air moving over the mountainous terrain of the southwest. The monsoonal weather patterns are typically not very good at producing precipitation on their own, and instead only provide pulses of enhanced moisture which the terrain converts into storm activity.
However, the pattern associated with the monsoon can manifest itself in a variety of ways, some of which actually block moisture from reaching New Mexico, Colorado, Arizona, and Utah. The monsoon still exists, but its associated large-scale atmospheric features aren’t conducive for moisture transport into the desert southwest.
Let’s take a look at 2020, which was a very bad year for fires across the entire desert southwest. The monsoon drove the establishment of the typical subtropical high over the southern United States, which initially (mid-to-late July) looked promising:
However, this high pressure broadened and strengthened westward, establishing itself prominently over southern Arizona and New Mexico where it remained for much of the rest of the season.
The strength and size of this fairly stationary high pressure system effectively blocked moisture from flowing northward into New Mexico and Colorado, thanks its associated anticyclonic (clockwise) winds. Moisture from the Gulf of Mexico also had trouble reaching the desert southwest, due to predominantly southerly flow:
Subsidence beneath the broad region of high pressure increased atmospheric stability, which made thunderstorm development difficult, even if there was some moisture available. So interestingly, the monsoon was actually responsible for manifesting atmospheric conditions which caused us to experience such a dry summer.
This is splitting hairs, but should help hammer home the point: a common claim for the 2020 summer was that “we didn’t get a/the monsoon.” The more accurate claim is that “the monsoon season was unusually inactive,” where ‘inactive’ refers to the lack of northward moisture pulses.
Understandably, it’s more straightforward to just refer to periods of wetter weather in later summer as “the monsoon” — but this seems to commonly be a shorter, colloquial way of saying “summertime precipitation.” In that context, the term “monsoon” loses its utility for short-term and long-term forecasting.
Monsoonal moisture surges often result in widespread, intense thunderstorm activity that breaks the usual rules in regards to lightning safety, such as summitting by noon — but if all thunderstorm activity becomes “the monsoon,” then the word essentially becomes useless in better highlighting the nature of storm activity on certain days.
(In reality, you could be funny and point to to giant plumes of smoke from wildfires later in summer and also call it “the monsoon” without being any more technically incorrect, so long as the monsoon created a weather pattern that blocked moisture from reaching the state.)
A prime example are the several days of wet weather that are currently ahead of us here in late June 2021: there’s already discussion across social media referring to this stretch of moisture as “monsoon season.” But the monsoon’s typical weather patterns are still weeks away from impacting the United States — it’s not even July yet. Our current pattern is still dominated by mid-latitude storm systems, as opposed to stationary features associated with the monsoon.
A mid-latitude storm is what is driving our next few days of colder, wetter weather, and is not related to the monsoon in any way — instead, it’s similar to the wet wintry storms that we get in the spring. Some key features are that temperatures associated with this system are unusually cold, storm motion for much of the event will be fairly southerly, and cloud types and precipitation will be widely stratiform in nature. These features are not typically associated with storm activity driven by the monsoon: monsoonal storms tend to be warmer, feature more northerly motion, and are arranged in powerful complexes. The dangers to recreationalists — namely, the possibility of lightning early in the morning, as well as ice and snow — are not quite of the same nature as storms associated with monsoonal moisture.
Typical Monsoon Weather Patterns, and What They Mean For Colorado
Let’s look at some large-scale atmospheric setups driven by the North American Monsoon, and how they impact Colorado’s weather. Again, parts of Colorado aren’t typically that affected by monsoonal moisture. Much of northern and northeastern Colorado do not feature any sort of second “precipitation peak” in July, August, and September.
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Southern Plains High
This is the “classic” and most common monsoon pattern, which brings plentiful moisture to New Mexico, Arizona, eastern Utah, and southwestern Colorado.
In this pattern, high pressure is predominantly located over the southern Great Plains, which helps provides an additional source of upper-level moisture from the Gulf of Mexico, in addition to the pulses of low-level moisture from the Gulf of California.
Let’s look at August 2006. The geopotential height maps can be used to assess pressure: higher heights = higher pressure. The anomaly is the geopotential height compared to average, which shows how this pattern’s position and strength different from what’s “typical.” The precipitable water is a glance at how much water is in the atmosphere, with greens, reds, and yellows indicating more moisture than usual.
Geopotential Height
Winds and Precipitable Water Anomaly
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Great Basin High
This monsoon pattern is less common, and really only enhances precipitation in New Mexico and southern Colorado.
Precipitation is provided by powerful storm complexes that form along frontal boundaries where cooler air pushes into the region from the northeast. These storm complexes move southwest and bring rain to New Mexico, Mexico, and sometimes parts of Arizona. Outside of southern Colorado, this pattern usually brings drier air from the north into much of the state.
Let’s look at September 2009.
Geopotential Height
Winds and Precipitable Water Anomaly
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Four Corners High
This pattern is similar to the Southern Plains high, but a stronger center of high pressure is located over the Four Corners.
This pattern is okay for Arizona and New Mexico, but tends to block decent moisture from reaching Colorado, aside from perhaps the San Juans. Overall, this pattern provides less moisture to the desert southwest as there is less upper level moisture from the Gulf of Mexico across the western part of the region and typically a bit of widespread subsidence.
Using July 2008 as an example, you can see much of the southwestern United States did not pick up a lot of moisture in this pattern, but some moisture from the Gulf of Mexico did make it to southern New Mexico and Arizona.
Geopotential Height
Winds and Precipitable Water Anomaly
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Blocking High
This is perhaps the overall driest monsoon pattern and is what we saw in summer 2020. A large area of high pressure is centered over the desert southwest, which suppresses the flow of moisture in addition to the formation of thunderstorms.
Storms that do form tend to be stationary, which causes flash flooding. July 2007 features a blocking setup, though it wasn’t nearly as bad as 2020 thanks to it being located further northwest.
Geopotential Height
Winds and Precipitable Water Anomaly
Again, because parts of Colorado aren’t actually that impacted by monsoonal weather patterns, some areas of out state actually didn’t do too bad during July 2007’s blocking high, despite much of the desert southwest doing poorly.
August 2020, on the other hand, was a disaster for the entire region.
Geopotential Height
Winds and Precipitable Water Anomaly
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The pattern is a little different each year, and though atmospheric features associated with the monsoon are fairly stationary, they tend to progress and change over the course of the season. Even with productive monsoon patterns, moisture tends to come in pulses, which means some drier days in between periods of more intense or widespread thunderstorm activity. What sort of setup will we see this year?
Climate Change and the North American Monsoon
As average temperatures across the globe climb, water scarcity is becoming a major concern in the American Southwest. With winter precipitation expected to decrease, the increase in precipitation each summer associated with the monsoon becomes even more crucial. Scientists have been extensively studying the potential impact of climate change on the North American Monsoon.
The current understanding is that the monsoon is not in much danger of disappearing. Extensive modeling has found a large amount of agreement that the overall amount of precipitation associated with the monsoon is unlikely to drastically change. Instead, it’s expected that the monsoon will start later and end later.
For Colorado, these changes appear to be fairly insignificant as opposed to the Mexican Plateau or southern Arizona and New Mexico. One major caveat is that the expected overall increase in atmospheric stability due to global warming could manage to suppress the storms that are driven by monsoonal moisture, which would certainly be problematic, but there is plenty of uncertainty around this.
For more information, check out The response of the North American Monsoon to increased greenhouse gas forcing (Cook and Seager 2013) [PDF].
Forecast by
Thomas Horner (Twitter: @thomaschorner)
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