Weather Musings With KNAU Meteorologist Lee Born

Aug 15, 2017

Several weeks ago we asked listeners to email their weather-related questions to KNAU meteorologist Lee Born. The response was overwhelming. Today, we hear from two listeners who want to know a little more about how monsoons form...it's a new, occasional segment we like to call Lee Born's Weather Musings. 


Listener: Hi, Lee. This is Fred Ducat from Flagstaff. My weather question is, do the San Francisco Peaks affect our regional weather? Do they cause, prevent or modify our weather from what would normally be expected in our region? If they do have an effect, how far does the effect go from the San Francisco Peaks? Thank you, Lee.

LB: Mountains are natural places for increased precipitation for a number of reasons, most obviously, they physically – or orographically – force air upwards. Less obvious is that they are natural places for surrounding air to come together, or converge, and then rise. In summer this can be observed as monsoon storms typically initiate over the Peaks. They receive the morning sun first and are a “sky island” of heat causing an area of low pressure around them in which air then comes together and rises, leading to the first thunderstorms of the day. In winter when large Pacific storms hit the Peaks, the air mass is physically, or mechanically, lifted. In effect, this cools the air rapidly, and efficiently rings moisture out of the atmosphere. Additionally, large mountain features such as the Peaks can cause a rain shadow effect on the lee, or downwind side of the mountain, and this can be seen from Doney Park to the Little Colorado River Valley and the Painted Desert. The Peaks also have effects on the wind. During our windy spring months, westerly winds are forced up and over the Peaks and then deflected down and accelerated as they enter Doney Park and the Little Colorado River Valley. Channeling through mountain features also accelerates the wind. In fact, there is one other geographical feature of the region that plays an even larger role in our climate and weather across northern and central Arizona: can you guess what it is? The Mogollon Rim is the most significant feature that impacts the weather across the region. There is nearly three times the amount of yearly precipitation along the rim region than there is to the south or to the downslope northern side.

And now for another question regarding our monsoon…

Listener: Hi. My name is Bill Lee from Flagstaff. I’ve heard meteorologists say that the moisture for the summer monsoon comes, not only from the Gulf of Mexico, but also from the Gulf of California and the Pacific Ocean. So, why does the monsoon have so much more effect on eastern Arizona and New Mexico than it does on western Arizona and California?

LB: This is true. There are several moisture sources that feed our monsoon. The Gulf of Mexico supplies the moisture in the upper levels of the atmosphere, transported in on a subtropical ridge of high pressure known as the Bermuda High. The Gulf of California provides the low level moisture, that muggy feeling we get in the monsoon months. When the desert becomes super hot, the onshore flow from the sea becomes increasingly strong, surging moisture into the core monsoon region, being the southwest United States mainly Arizona, New Mexico and northwestern Mexico. Much as in the way we discussed in the previous question, the mountainous regions of the area are the focal points for the daily thunderstorms. Areas further west of the core monsoon region, namely California and Baja California, only receive spotty monsoon related rainfall. On occasion an area of weak low pressure will set up off the coast and actually draw in some monsoon moisture, generally leading to some thunderstorms in the Sierra Nevada Mountains. A not-so-talked-about, yet significant feeder of monsoon moisture is evapotranspiration from the Sierra Madre Mountains in Mexico which receive heavy  monsoon rains.