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5 fast facts about San Diego County’s majestic monsoonal clouds

Cumulonimbus clouds developed over eastern San Diego County on Sunday.
Cumulonimbus clouds developed over eastern San Diego County on Sunday.
(Gary Robbins /The San Diego Union-Tribune )
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The summer monsoon season sometimes passes quickly in Southern California, depriving weather watchers the joy of watching tall, majestic cumulonimbus clouds form in the sky.

This year, though, the monsoon has been strong and persistent, spinning out one raucous thunderstorm after another. That was the story all weekend, and will be in the week ahead, says the National Weather Service.

So we decided to give you a quick primer on the phenomenon, which generally occurs from July to September.

1. The monsoon is composed of moist, unstable, humid air that originates far to the south, in the subtropics. Broadly speaking, the moisture gets pulled north from the Gulf of Mexico and Gulf of California by the circulation patterns of high-pressure systems in the US.

2. Much of the monsoon flows into Arizona and New Mexico, triggering spectacular thunder and lightning and, at times, wild rain. On other occasions, lots of the moisture bleeds into Southern California and pools up in the mountains and deserts.

3. The monsoon can unleash heavy rain, sometimes causing death and destruction. That’s what happened Thursday when monsoonal rains pounded the Las Vegas area, killing two people and sending mini-rivers of water through the city’s casinos.

On Saturday, the monsoon caused road flooding in San Diego County, along Highway 78 near Borrego Springs and on State Route 94 , near Campo. The National Weather Service issues frequent advisories during such weather, partly because many Southern California’s have little experience driving in the rain.

Forecasters are just as worried about lightning, which can heat the air that it passes through to 50,000 degrees Fahrenheit.

Then there’s the wind, which gusted more than 50 mph across parts of eastern San Diego County on Saturday, and nearly hit 70 mph in one spot.

Monsoonal moisture generated thunder, lightning and rain Sunday across parts of Southern California.
Monsoonal moisture generated thunder, lightning and rain Sunday across parts of Southern California.
(NOAA/NWS)

4. The big, puffy clouds associated with the monsoon often start out as small areas of instability. Early on, they’re cumulus clouds. But they frequently evolve into cumulonimbus clouds, becoming huge and menacing in 10 to 20 minutes. Many of the clouds that formed in San Diego County over the weekend rose 30,000 to 40,000 feet into the atmosphere before they spent their energy and slowly collapsed.

5. At the time, it was hot and humid in Southern California. The exception could be found inside the clouds. The weather service says the temperature at 30,000 feet on Saturday was minus 20 degrees.

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