Peter Shelton

Tales of the Monsoon

Posted in Watch columns, Weather & Climate by pshelton on July 19, 2013

Denny Hogan, Colorado boy and former snow ranger at Silverton Mountain, was reassigned by the Forest Service to California’s Lake Tahoe region a couple of years ago. He can’t get over the cloudless summers there.

“Every day’s the same!” he exclaimed on a recent visit to old friends. “It’s one blue-sky day after another. Not a drop of rain.”

I know whereof Denny speaks. I spent a week or two in the Sierras every summer of my life until I left home. I didn’t know it could be different. That’s what summer days were: sunny, warm pine-needle smell, dusty trails, a breeze glittering the lake.

Then I went to Colorado to visit my Uncle Hal and his family. It was the first time I’d been in an airplane, so that part was seared into memory. The other thing that really made an impression was a monsoon thunderstorm. Hal had driven us from Golden up into the mountains for a swim in a big hot-springs pool. It must have been Idaho Springs. The morning was crystalline and hot, the sun closer and sharper than I was used to. And I was loving that pool; it was so vast and bath-like despite its odd sulfur smell. I hardly noticed the air above the canyon walls growing dark, suddenly very dark, and then a half-seen flicker of light and BANG! – the thunder echoing off the walls as if the rocks themselves were crashing down.

Even as we scrambled out of the water hailstones like a rain of pebbles pounded the surface and danced on the concrete walk to the dressing rooms. I’d never seen anything like it, wouldn’t see anything like it again until I moved to Colorado.

The North American monsoon arrives every summer about this time, and blessedly so, following a baking-hot June with its smoke and fires and nervous fireworks bans.

The term monsoon comes, of course, from the famous Indian monsoon weather pattern that drenches much of the Asian subcontinent like clockwork every (well almost every) year. The root is an Arabic word, mausim, which means “wind-shift.” Sailors on the Arabian Sea noticed centuries ago that dry northeast winds in the winter suddenly turned southwest during the summer, bringing with them torrential rains. A similar wind-shift drives our mid-summer pattern.

All monsoons are caused by temperature differences between the land and the sea. The Indian summer heats the vast Rajasthan Desert. Ours is similarly driven by the hot, dry Sonoran/Mojave complex, stretching from western Mexico to eastern California. As the deserts heat up in June and July, surface low-pressure forms over Arizona triggering the wind-shift and sucking moisture up from the south. You can see it on the satellite pictures: moisture pumping northward from the Gulf of California.

The pattern starts along Mexico’s tropical Pacific coast and builds gradually to the north. When the plume bumps up against the high country, daytime convection builds those towering cumulus clouds, and the rainy season begins. In Asia, the barrier is the great Himalayan Range. In Arizona, the Mogollon Rim serves the same lifting/cooling function. When the moisture finally reaches Colorado (near the northern limit of most monsoons) the southern mountains especially present an impressive rain-generating barrier.

Parts of Mexico and Arizona receive 80 percent of their annual rainfall in July and August. Nearly a quarter of Telluride’s yearly total falls courtesy of the monsoon. As important perhaps as the wetting of the soil is the cooling of the psyche, the relief felt by flowers and wild things and the humans who tramp among them.

But, of course, there can be too much of a good thing.

Ellen and I were building a house in Colona during the summer of 1999, which turned out to be a wet one. That summer, like this one, started out hot and dry, scary dry. Then, in July, afternoon thundershowers began and didn’t let up through the end of August. Telluride recorded something like 53 straight days of rain during that period.

Satellite images showed a plume of moisture, like a 600 mile-long white feather stretching from the Gulf of California across Arizona into Colorado and, some days, all the way up into Idaho.

Days often started clear, then individual cells would pop up as warm moist air was forced upward by the terrain. The clouds grew out and up like giant cauliflowers. Sometimes they marched forward in multi-cell lines. By early afternoon, the cells matured as updrafts pushed them to 30,000 and 40,000 feet at their tops. Rain and hail (the freezing line inside a cell vacillates between 13,000 and 17,000 feet) mixed inside the cloud in a violent clothes-dryer effect. When the precipitation came, often in dramatic vertical shafts, outflow winds, like waterfalls of air surrounding the rain column, blew hard enough to knock a man off a ladder.

Most of these cells lasted no more than 20 or 30 minutes before dissipating. Trouble was, there were usually reinforcements forming up behind. The worst days were the result of mesoscale convective complexes (MCCs). These super-size groupings of cells organize into monster storms capable of dropping furious amounts of water. These are the cells that cover Hwy 145 in red mud, that wash thousands of tires into the Uncompahgre River.

This is what happened on July 31, 1999, in the Dallas Creek drainage west of Ridgway.

An MCC got stuck between the Uncompahgre Plateau and the Sneffels Range and dropped about three inches of rain in three hours. Little Dallas Creek, which was flowing at a lazy 130 cubic feet per second the day before, leapt up to 3,960 cfs on the 31st – almost eight feet above flood stage. The flood took out bridges and hayfields and rural roads for miles around. Things weren’t quite that bad at our home construction site. I do remember thinking, as work was cancelled for the day, that I could kayak down our driveway.

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