North American monsoon
Geographically, the weather pattern is centered over the Sierra Madre Occidental in the Mexican states of Sinaloa, Durango, Sonora and Chihuahua,[2] and typically occurs between June and mid-September.
[citation needed] In early summer, the monsoon starts with a shift in wind patterns as Mexico and the southwest U.S. warm under intense solar heating.
It extends into the southwest United States as it matures in mid-July, when an area of high pressure, called the monsoon or subtropical ridge, develops in the upper atmosphere over the Four Corners region, creating wind flow aloft from the east or southeast.
The Gulf of California, a narrow body of water surrounded by mountains, is particularly important for low-level moisture transport into Arizona and Sonora.
Once the forests of the Sierra Madre Occidental green up from the initial monsoon rains, evaporation and plant transpiration can add additional moisture to the atmosphere which will then flow into Arizona and New Mexico.
Finally, if the southern Plains of the U.S. are unusually wet and green during the early summer months, that area can also serve as a moisture source.
[10] Heavy monsoon rain can lead to excess winter plant growth, in turn a summer wildfire risk.
A lack of monsoon rain can hamper summer seeding, reducing excess winter plant growth but worsening drought.
In Albuquerque, flash flooding from storms funneled into the Rio Grande Valley by the Sandia-Manzano mountain range has prompted the city to develop an extensive system of concrete-lined arroyos and retention structures, similar to the flood control channels in the Los Angeles River basin.
As shown in the panorama below, a wall of thunderstorms, only a half-hour's drive away, is a common sight from the sunny skies along the coast during the monsoon.
The exact strength and position of the subtropical ridge also governs how far north the tropical easterly winds aloft can spread.
If the ridge is too close to a particular area, the sinking air at its center suppresses thunderstorms and can result in a significant monsoon "break."
If the ridge is too far away or too weak, the east winds around the high are inadequate to bring tropical moisture into the mountains of Mexico and southwest U.S.
[5] The combination of these factors blocked moist tropical air from reaching the American Southwest, leading to below average rainfall.
However, according to NOAA, considerable research culminating in 1993 established the fact that a summer monsoon develops over much of Mexico and the intermountain region of the U.S.[3][14][15] The controversy continues in part because of the incomplete reversal of the winds during the NAM.
