A few weeks ago, a relatively narrow plume of warm subtropical moisture stretched southwestward past Hawaii across the Pacific Ocean and brought continuous rain across the Bay Area. More than 10 inches of rain was reported at Mount Tamalpais in Marin County. In Sonoma County, Guerneville along the Russian River got 8.6 inches. The Charles M. Schulz-Sonoma County Airport reported nearly 6 inches.
PG&E meteorologist Scott Strenfel told me that the northern Sierra saw an average of 8 inches across the region. Although this is good news, the index sits at just 40 percent of normal for this water year. It would take two and a half similar atmospheric river events to return to near normal for this date and five through this water year to produce a “normal” year.
Unfortunately, San Luis Obispo received only about a quarter of an inch.
This narrow corridor of dense water vapor is referred to as an atmospheric river. The term atmospheric river hasn’t been around very long. Before I wrote this column I couldn’t find any reference to it in any of my textbooks. Turns out, the phrase was coined by researchers Reginald Newell and Yong Zhu of the Massachusetts Institute of Technology in the early 1990s.
Since December 2010, The Central Coast has seen an absence of atmospheric rivers due to the persistent ridge of high pressure over the eastern Pacific.
These rivers in the sky can stretch for thousands of miles across the world’s oceans, but are only a few hundred miles wide. At any given time, there are typically several of these narrow plumes of moisture in the northern and southern hemispheres. In the northern Pacific Ocean they are often formed by large storms (mid-latitude cyclones) drawing huge amounts of water vapor into narrow bands ahead of cold fronts. They can transport fantastic amounts of water across vast expanses.
In fact, they can carry more fresh water than the Amazon River. Along the West Coast they are informally called the "Pineapple Express." The Pineapple Express is a subset of an atmospheric river event that originates in the tropical waters near Hawaii; hence the pineapples. In the past, meteorologist simply referred to these as “the hose.”
The topography of California plays an important part in enhancing the amount of rainfall from these atmospheric rivers. Moisture-laden winds associated with this phenomena blow horizontally from the Pacific Ocean and turn vertical or upward as they hit the coastal range mountains and further eastward, the Sierra Nevada. As the air mass is lifted up over these mountains (upwind), it cools and eventually reaches its dew point temperature. When this occurs, either clouds, rain or snow will develop on the windward side of the mountain.
Like squeezing a wet sponge, moisture from this air mass is released in the form of precipitation. This is also called orographic enhancement. Nearly all the water in one of these atmospheric rivers can be dumped in California over a short period of time. They can certainly be classified as drought busters, but they also can cause floods.
In Northern California, the Russian River is notorious for flooding parts of Sonoma County. I was astonished to learn that the last seven major floods of the Russian River since 1997 were caused by atmospheric rivers. In times past, the majority of the flooding occurred from discrete cold fronts that passed in succession through the region.
As any physics professor will tell you, the warmer the atmosphere the greater the amount of water vapor it can hold. Through the end of this century, the occurrence of atmospheric rivers is projected to increase about 10 percent as the climate continues to warm. Relatively warm atmospheric rivers can raise the snow level several thousand feet, resulting in more immediate runoff from rain in the Sierra Nevada Mountains and the greater possibility of flooding.
Snow on the other hand, acts as a reservoir. As it gradually melts over the spring and summer months, it provides water for agriculture, residential, industrial, environmental and clean hydroelectric generation.
PG&E is now accepting applications for the Bright Minds Scholarship program. PG&E is awarding scholarships ranging from $2,000 to $20,000 to 100 deserving students in Northern and Central California. This scholarship is open to high school seniors, current college students, veterans and adults returning to school. All majors are encouraged to apply. Apply by this Friday, Feb. 28, 2014.