We’re about two weeks away from monsoon season in the desert southwest. This week’s high country storms gave us a sneak peak of the thunder, lightning and rain coming our way.
Officially, monsoon season starts June 15, although we see our first storms in the Valley on average around the first week of July. The season ends September 30. Monsoon is the Arabic word for “wind” and signifies a shift in the winds that brings moisture into our usually dry atmosphere in Arizona. Between June and September, our typical westerly winds switch to a more southerly or easterly flow and dew points (a measure of moisture in the air) begin to rise. The intense summer heat works with that moisture to bring daily storms in parts of the state.
Storms are more numerous in the high country, thanks to the lift of the mountains that helps to generate storms. Southern Arizona is also especially active, since it’s closer to the moisture source during the season. When storms form to the south of the Phoenix metro area, outflow winds from those storms can sometimes push walls of dust into the Valley. We average around 8 to 12 of these dust storms each summer, with higher numbers over the past several years because of our region’s ongoing long-term drought. These dust storms can happen anytime during our thunderstorm season, but are most numerous during the first month of the season in June and July. One of the most notable dust storms in recent history brought our state worldwide attention in early July of 2011.
Storms typically form over the Mogollon Rim during the afternoon hours and work their way down toward the Valley during the evening hours. But we’ve had plenty of overnight and early-morning thunderstorms during the season as well. Outflows from storms to the north, east and south of Phoenix also frequently collide over the Valley and generate new storms.
We typically get just shy of 3 inches of rain total in the Valley during the monsoon season, although that varies from year to year.
Our wettest day in Phoenix history came in September of 2014 when overnight storms fueled by a dying tropical system brought 3.29 inches of rain to Sky Harbor. While that was the official total from the airport, some Valley neighborhoods saw twice that much rain. Homes were flooded, schools canceled and freeways underwater.
Other dangers of the monsoon season include flash flooding and lightning.
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