Here’s a slice of weather life many Minnesotans have no idea about. I know I didn’t know about the monsoon when I moved to Arizona in the late ’90s.
The North American Monsoon is a phenomenon that brings precious moisture to northern Mexico and the southwestern deserts of the USA. Many communities such as Tucson, Arizona receive half of their annual rainfall during the next few weeks.
The storms can be sudden and violent. A typical monsoon day features a clear morning sky. By noon, puffy white cumulus clouds are billowing into the sky above area mountains called “sky islands.” As the storms build and gain momentum, they roll into the valleys bringing heavy downpours vicious lighting and often damaging microburst winds.
The word “monsoon” comes from the Arabic word mausim, or season. Many people think a monsoon refers to the rainfall associated with storms. The reality is the term monsoon actually refers to the seasonal shift in mid-level winds that brings the moisture into the region. In places like Tucson and Phoenix, the monsoon usually begins around July 7th, and lasts into September.
For many years and while I was in Tucson, the NWS used a definition to start the monsoon of 3 consecutive days with dew points at or above 54 degrees. (55 degrees for Phoenix) This was considered a good indicator of the increased moisture that came with the seasonal wind shift.
Today, local NWS offices have declared any rainfall between June 15th and September 30th to be “Monsoon rainfall.” While in Tucson, I was a strong advocate for keeping the dew point designation as a launch for the monsoon. My reasoning is that this unique and important “5th season” in Arizona is too important to be left to a date on a calendar, and too distinct.
The moisture or fuels for the summer monsoon comes form two distinct sources. The Gulf of Mexico and the Gulf of California are critical in providing moisture to fuel storms. You can find out much more about Arizona’s monsoon here.
People in the desert southwest look forward to the precious rainfall and cooling breezes of the summer monsoon. There’s an old saying in Arizona; “You’re as welcome as rain.” Rain is a good thing in the desert, so that’s considered a compliment. Our Minnesota analogy might be something like; “You’re as welcome as a warm sunny day in March!”