Spring makes early arrival | TahoeDailyTribune.com

Spring makes early arrival

Patrick McCartney

Six weeks ago, Lake Tahoe reached its highest level in 70 years after the wettest two months in the past century.

More than 30 inches of precipitation fell in South Lake Tahoe and Tahoe City during December and January, creating millions of dollars of flood damage and burying U.S. Highway 50 under 40 feet of mud.

Disaster officials publicly begged for dry weather.

It arrived with a vengeance.

Since Feb. 1, less than 1 inch of precipitation has fallen at the Lake Tahoe Airport, including just 8 inches of snow. With temperatures in the basin skirting the 60-degree mark, the heavy snowpack has receded at lake level, although substantial amounts have survived on the upper slopes.

Dry weather is expected to persist for at least another week, with a storm front this weekend not likely to produce much more than a stray shower.

So, who turned off the spigot?

According to climatologists, the abrupt right-turn by the weather is all too typical for California.

“This is what California and Nevada weather is all about,” said Bill Mork, California’s state climatologist. He explained the change as a function of the constantly shifting global pattern of high-pressure systems and low-pressure storms.

While a powerful jet stream carrying tropical moisture plowed into California in December and January, a strong high-pressure system has clung tenaciously over the West Coast in the past six weeks, diverting most storms to the north.

“It’s not unusual for the atmosphere to change modes and go from a wet regime to a dry regime,” said Tim Brown, a climatologist with the Western Regional Climate Center in Stead, Nev.

Yet, some evidence exists that cooler water temperatures in the tropical Pacific Ocean may have given a boost to the wet weather. Temperatures were somewhat depressed, even if it wasn’t a full-fledged La Nina, which are believed to stimulate storms in the Pacific Northwest and cut off precipitation for the Southwest.

La Ninas are the opposite of the El Nino condition, which is caused by the warming of the tropical eastern Pacific. El Ninos are associated with drier conditions in the Pacific Northwest and wetter weather in the Southwest, including Southern and Central California.

Brown said the effects of the global shift in weather systems is less pronounced in western Nevada and central California.

“In this part of the country, there’s not a well-defined signal,” he said.

Yet, Mork said that some data show that a cold episode in the Pacific coincided with the wet two months, a pattern that has been repeated over the decades.

“We know that three of the major floods of the last 40 years were during a cold episode,” Mork said. “The clues I’m getting now suggest that it is in transition, and that by this summer temperatures in the Pacific will be back to normal, and possibly pushing toward warm water.”

Despite the extended dry period, automated readings Thursday indicated that the water content of the Tahoe Basin snowpack is still 24 percent above average for the date, and that the basin has received 171 percent of the precipitation for the season.

Despite the prediction of dry weather for at least another weak, meteorologist Tom Cylke of the National Weather Service in Reno said mountain residents should not expect spring to stick around.

“I’ve lived here long enough to know it’s hard to say that it will remain dry for the rest of the season,” Cylke said. “People, especially Californians, get lulled into a false sense of spring in the mountains. But it’s a lot different than the coast.”

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