“Sunrise doesn’t last all morning
A cloudburst doesn’t last all day …
All things must pass
All things must pass away.”

~ George Harrison, musician, “All Things Must Pass”

Did you read last week’s Crescenta Valley Weekly? What about Weather in the Foothills? If so, it may have arrived wet. The weather forecast did not include accurate information. Thursday morning surprised all of us with a sudden and intense thunderstorm. There had been absolutely nothing from NOAA meteorologists just the day before about this thunderstorm. Apparently, they were as surprised as us common folks. All in all, .10 inches of unexpected rain fell! One last spring shower slipped in just under the wire; Sunday, June 20, was the first day of summer.

While most Southern California stormy weather occurs with winter storms, each summer also brings periodic thunderstorms to the region’s mountains, deserts and far inland valleys. We’ve all seen the beautiful and billowing clouds – cumulus nimbus – rise to great heights over the San Gabriel Mountains on the north side of the Crescenta Valley.

Most common is on hot August days; they can’t be mistaken for a winter thunderstorm. They’re produced by a North American monsoon. Monsoons are analogous with India and Southeast Asia and for good reason; the annual rainfall averages 390 inches! In comparison, the lighter Southwestern version, primarily impacting Utah, Colorado, New Mexico and Arizona, produces about 14 inches with 50% falling during the summer monsoon. We get the mostly wrung-out leftovers.

Monsoons are driven by the sun heating up the land and the Pacific Ocean at different rates with land surfaces warming more quickly than the ocean. The warm land creates low-pressure zones as hot air rises. Once this pattern establishes across the region, there’s a shift in the winds. Moist air moves northward during summer from the Gulf of California. Fall brings cooler air and monsoons drop off the weather radar.

The often difficult to predict thunderstorms cause life-threatening flash floods, but if one is cautious, the monsoonal rains are just an inconvenience. They tend to be brief and spotty with intense, local storms drenching some neighborhoods but not others. Stormwaters flood streets and sometimes structures. At one time the water was essential in the life of the Native American tribes living in the southwest. Times change …

Monsoonal conditions, plus a weakening onshore flow, will impact our weather. Expect triple digits and humidity. The befitting single word definition is “sultry.”