Every year around July, August and September we hear about the "Monsoon" and the favorite term used by many Western US meteorologists "Monsoon Moisture" to refer to our summertime outbreaks of Thunderstorms. I myself in my early years of studying weather would also often refer to this. As I began to look more and more at patterns and understand how the atmosphere works I questioned whether or not "Monsoon Moisture" was a factor or for that matter, if it even contributed to our summer thunderstorm outbreaks. Take a look at the picture below.
This is a satellite photograph taken on August 6, 2014 of the Western US. Our region is relatively clear while there is an abundance of cloud cover arching from Central California to Central and Eastern Montana with a bright blob over the center of Idaho which happens to be a cluster of thunderstorms. Almost every meteorologist that I know of would say that this cloud cover is "Monsoon moisture" due to the monsoon flow around the "4 Corners High" that develops each year around June over the border region of Arizona, Utah, New Mexico and Colorado. This sounds reasonable, correct.....or perhaps not. So you may ask, well what does that have to do with our weather up in Northwest Montana? Very often during the summer months, you will see TV meteorologists and even the National Weather Service say that it's this "monsoon moisture" that is going to trigger or cause thunderstorms in our area. Here is the problem with that argument. The term "monsoon" simply refers to a seasonal wind shift. In late May into June, an upper level high pressure area develops across the Desert Southwest due to the intense heating of the land and as the Western US warms up, the upper high tends to migrate northward as well. In the Northern Hemisphere, the airflow around high pressure is clockwise so as that upper high drifts further north in the summer months, our weather here in Northwest Montana becomes increasingly dependent on the location of that upper high and the corresponding upper low in the Gulf of Alaska. We are often in a battle ground of sorts between these two opposing forces. In the picture above, based on the reasoning of "monsoon moisture" coming from the Gulf of California and the Gulf of Mexico as it is widely believed and reported there should be a continuous stream of cloudiness and thunderstorms that we can see clear back down to Arizona and Mexico. Yet from this picture, I don't see any cloud cover over those regions at all. So what really is the cause for the cloud cover and thunderstorm activity over Idaho in this picture? Is it "monsoon moisture" or something else? Maybe the forcing behind the cloud cover and thunderstorms is not "monsoon moisture" but rather something like surface convergence(air coming together at ground level and rising) and/or instability (cold air aloft that allows warmer air at the surface to freely rise, condense and produce clouds and precipitation).
Anyone who has been to the Nevada desert during the summer months or been to (or in my case lived in) the Eastern Sierra of California will tell you that summer thunderstorms are common even though afternoon humidity values are often only around 10 to 20 %. That does not sound like a lot of moisture to me. So you may argue and say "well it's the surface dew points that are high and that's what is triggering the summer thunderstorms." If that is true then all of the Central Valley and coastal areas of California along with Western Washington and Oregon should be seeing daily thunderstorm outbreaks because their humidity levels are higher along with their surface dew points. During the so called "Gulf surges" which are characterized by a dramatic increase in surface dew points across the lower deserts of Southeast California and Southwest Arizona during the summer months based on the explanations given above we should see an abundance of cloud cover and thunderstorms over those areas correct? The reality is though that in almost all of those cases where the surface dew points are high (by high I mean dew points in the 60s or even lower 70s) there is little to no cloud cover and what does develop is often over the elevated heat sources such as the mountains where temperatures are cooler, surface dew points are lower and moisture is lower. How is that possible since it is supposedly moisture that causes the thunderstorms to break out? The answer is surface convergence, instability and lift, not moisture. Mountains are higher than the surrounding land and thereby are able to release their heat more readily into the atmosphere destabilizing the air which in turn allows clouds to develop. If instability is present aloft (meaning temperatures are relatively cold around say 20,000 to 25,000 feet) that heated air over the mountains will keep on rising through the cold air aloft and develop into thunderstorms. Mountains also act as a focusing mechanism for which thunderstorms can successfully develop over before being moved off by some other force such as wind or outflow boundaries from other storms. An outflow boundary is simply an area of colder air produced by a shower or thunderstorm after it has matured that flows away from the storm producing it. The deciding factor in our area for thunderstorm development involves a "kicker" or something to trigger the instability and colder air aloft to generate the lift and develop thunderstorms. In fairness to the TV weather people and meteorologists who say our thunderstorms are triggered by "monsoon moisture" that is simply the easy explanation for the weather for the day but it is by no means correct. The main trigger for our thunderstorm outbreaks across Northwest Montana has nothing to do with the North American monsoon but are tied to weak upper level low pressure areas that move into our region during the summer from Oregon or strong cold upper level troughs moving in from the Pacific that bring substantial cooling after a hot spell. Often our best nighttime or nocturnal thunderstorm outbreaks are triggered by what we call a "vorticity maximum" which is simply a fancy name for a small area of upper level lower pressure and cold air aloft that is created often by a thunderstorm outbreak a day or two before usually over Nevada or California's Sierra. Because of our far north location and close proximity to the jet stream, even in the summer months, we do not usually have enough time to modify our airmass over several days to build up the instability necessary to trigger thunderstorms as do areas further south. We need triggers and the three I listed above are the ones that do that for us.
In summary, our summertime thunderstorm outbreaks in Northwest Montana are not the result of so called "monsoon moisture" but instead are caused by triggers that cause instability to increase by cooling the upper atmosphere and thereby allowing thunderstorms to develop. Most of the time, the trigger is an upper level low pressure area, be it a vorticity maximum or a closed upper level low or even a cold upper trough moving in off the Pacific. The moisture is already there in the atmosphere it is simply a question as to whether or not there is the available lift, surface convergence and instability to cause air to rise and trigger thunderstorms. The southwest Monsoon is true and does play a roll in the summer rainfall maximum found in Arizona and New Mexico but "monsoon moisture" is not a true factor for triggering thunderstorm outbreaks but is instead a byproduct of summertime airmass modification that takes place over the Southwestern US. One final thought, I have also often heard meteorologists say that it is "too dry" for thunderstorms to develop. As I stated above, having lived in the Eastern Sierra and traveled out into the Nevada desert where it is extremely dry, I have observed thunderstorms with heavy rains in these areas where it was supposedly "too dry" for thunderstorms to develop. I have also witnessed both here in Northwest Montana and when I lived in North Idaho heavy rains from showers and thunderstorms when it was supposedly "too dry". In these situations, surface dew points may have been low but the instability was present aloft and so "elevated showers and thunderstorms" developed which do not rely on or need surface moisture to develop. Most of our thunderstorms in Northwest Montana are of this variety and are not surface based which is why most of our storms occur at night.