originally posted 7/30/2014
An active monsoonal pattern over the past few weeks has brought some unusual summer weather to the Bay Area featuring some rather atypical conditions, with clouds, rain and even thunderstorms dotting parts of Northern California. What causes this weird weather?
Monsoons; defined as a seasonal wind that shifts direction from season to season and often brings abundant rain and thunderstorms during one time of the year and hot, dry weather at other times.
Monsoons happen in several locations around the world, including along the Brazilian coast of South America, in Sub-Sahara Africa and across northern Mexico and the Desert Southwest of the U.S. The monsoon first begins in northern Mexico in May. The summer sun evaporates water from the Gulfs of Mexico and California and creates humid conditions over the land, which produces rain. By early July, the temperature and pressure differences are usually big enough for the monsoon flow to make its way across the U.S. and Mexico border.
The North American monsoon occurs over northwest Mexico, Arizona, New Mexico, and parts of Colorado and Utah. Typically, the annual monsoon season arrives in the U.S. early-to-mid July and lasts through early-to-mid September. On average, about half of Arizona receives about half of its annual rainfall during the monsoon. Occasionally this monsoon moisture will reach the mountains of Northern California creating afternoon thunderstorms over the Sierra’s and even reach the Bay Area bringing lighting and rain.