About - Monsoon:
Across Arizona and New Mexico, the wettest single month of the year will likely be July or August; on average, these two months account for up to half the precipitation that falls in a year. Communities in the Southwest depend on the monsoon–to moderate hot summer temperatures, to relieve demands on water resources, and to drive vegetation productivity.
Percent of total annual precipitation occurring during the monsoon season, July–September ( based on 1979–2020, data CPC Unified rain-gauge-based dataset). Figure by climate.gov.
The monsoon high, or monsoon ridge, is a feature of the atmospheric circulation that plays a key role in determining how a given monsoon season plays out. It is characterized by an area of upper level high pressure–an anticyclone–driving clockwise flow of the atmosphere that builds and shifts north in the summer, ultimately covering much of the continental United States, usually some time after the summer solstice. When the center of the clockwise flow is far enough to the north, weak, easterly to southeasterly flow over the southern U.S. and Mexico allows air moistened by the Gulf of Mexico and the Gulf of California to move into the Southwest. The core of the North American Monsoon region lies to the south along the coasts of Mexico, and storm activity begins there earlier in the season and acts to push the moisture further and further north. Once the moisture has moved into the Southwest, the position of the center of the monsoon high determines where thunderstorms will occur, and whether they will organize and form stronger, longer-lasting systems. Near the center of the high, storms are often suppressed; farther away, conditions can be more favorable.
Conceptual diagram of key circulation features of the North American Monsoon System.
Links / Resources:
- National Weather Service - Tucson: Monsoon
- National Weather Service - Tucson: Monsoon Information Index - A comprehensive list of short articles explaining different aspects of the SW Monsoon.
- Arizona and the North American Monsoon System - UA Cooperative Extension Article by Mike Crimmins (CALS Climate Extension Specialist and CLIMAS PI)
- MadWeather Blog - Another perspective on regional/Southwest weather/climate models from Bob Maddox.