If you’re someone who regularly checks SPC outlooks, you may have noticed that the general thunderstorm category extends just a little further south and west than usual. In fact, it extends to the desert southwest – areas like New Mexico and Arizona.
Despite what many people think, rain is actually not uncommon in this area, especially at this time of year.
The period from June 15 to September 30 is known as the Windy Season. The word “monsoon” is often misused. People use it to refer to heavy rains when in reality the real definition is a change or reversal of the wind.
In general, the United States is under the influence of Western flows. This allows very little moisture to penetrate the desert southwest.
However, in the summer, the jet stream goes north and the long-standing slopes are allowed to be built in the Eastern United States. The clockwise flow around the ridge allows winds to blow from the Gulf of Mexico to the southeast or from the Gulf of California to the southwest. This flow carries moisture into the southwestern desert.
Daytime heating then allows thunderstorms to form, reminiscent of a hot summer day in the southeast. These thunderstorms have the potential for heavy rain, assuming there is enough moisture, and can lead to flash flooding.
As you can see, it doesn’t take much rainfall to exceed the Flash Flood value in this area. Burn scars, mountains and low-lying terrain (such as canyons) are particularly vulnerable to flash floods.
Monsoon season is not certain. It is highly dependent on these ridges forming and existing in the eastern half of the United States. No slopes, no beneficial currents.
Monsoons is also not exclusive to the Southwestern United States. They appear near the tropics as the ITCZ (Intertropical Convergence Zone) moves north in the summer. The best known monsoon season occurs in India and South Asia. This monsoon can be particularly strong as the Himalayas prevent dry air from entering the area, allowing humidity to reign supreme. The Sahel region just south of the Sahara in Africa is also subject to monsoons like Panama, Costa Rica, Nicaragua and western Mexico.
Global circulations such as El Nino and La Nina affect the amount of precipitation that falls during these events. Typically, La Nina style means wetter monsoon. However, this is not always the case as many other factors come into play as well.
Monsoons are important to the economies of these regions. Without that summer’s rainfall, crops would fail and water reserves would not be replenished to last through the dry months. On the other hand, excessive rainfall can also cause crop failure and flash floods have their own consequences.
Hope you enjoyed this little piece on a pretty important part of our annual weather cycle, and perhaps you learned something too!