Atmospheric Rivers and the storms of December

Wind gusts forecast for 1:00 p.m. (tomorrow afternoon)

As I logged into Cliff Mass's blog to do my homework for this post, I saw that he had updated his latest post as follows:

"BIG NEWS UPDATE at 10:15 AM Wednesday:  At 10 AM, Seattle-Tacoma Airport reported 65F, the WARMEST TEMPERATURE EVER OBSERVED AT SEA-TAC FOR THE MONTH OF DECEMBER.  I repeat this is the warmest temperature every reported for any day in December in the entire climatological record.  Amazing.  Undoubtedly true of other Northwest sites as well.

"I ([Cliff] had to laugh today when I saw the front page of  the National Weather Service's Seattle forecast office web site. 
They had FOURTEEN watches, warnings, and advisories. 

"I have never seen so many.   Something out of a disaster movie or reminiscent of the plagues that hit Egypt before the Exodus.   High Winds!  Floods! Small Craft Advisory!  High Surf!  Gales! Storms! Rough Bars!  All that was missing were tornadoes, hurricanes, lice, and darkness.  Oh, I forgot, we have darkness living in Seattle during the winter.

"But it is getting very clear that the Oregon coast is going to be ground zero for a major onslaught of wind.  Hurricane-force gusts. "

All of this is being treated by the popular press as the result of an "atmospheric river," (AR) as if that was a new concept, but it's not! Two MIT researchers, Zhu and Newell, 1998*) first described the phenomenon. They found that most of the water vapor in the global conveyor belt is carried in 4-5 long narrow  water-vapor-rich sections that are only about 400 km wide. A much older term describing California storms is the "Pineapple Express" applies to a subset of atmospheric rivers that have a connection into the tropics near Hawaii. When the AR''s draw in moisture from the tropics, they can be extreme. Here's a link to a previous post that I did on atmospheric rivers. It relates to Japanese fire bombs during WWII.

A plot of the amount of moisture in a vertical
atmospheric column for an AR in 2010
(from Cliff Mass, here)

The AR's are rich in water vapor, and because of the pressure gradients that develop in cyclones/hurricanes, they are associated with strong winds. The winds will force the water vapor up and over topography, leading to condensation of the vapor and precipitation in the form of rain or snow. According to the NOAA site referenced below, 42 AR's impacted California during the winters of 1997-2006, resulting in seven floods along the Russian River watershed northwest of San Francisco, a major "New Year's Day Flood" in 1997 that caused over $1 billion in damages, and contributions to other California storms in the Merced and American Rivers. An AR hit the Pacific Northwest in 2006, producing heavy rainfall, flooding, and debris flows with damage excepting $50 million. You can find a list of NOAA's "notable AR's" here.

Here's a quote from an article by Dettinger and Ingram that illustrates what one of these rivers can do:

"The intense rainstorms sweeping in from the Pacific Ocean began to pound central California on Christmas Eve in 1861 and continued virtually unabated for 43 days. The deluges quickly transformed rivers running down from the Sierra Nevada mountains along the state’s eastern border into raging torrents that swept away entire communities and mining settlements. The rivers and rains poured into the state’s vast Central Valley, turning it into an inland sea 300 miles long and 20 miles wide. Thousands of people died, and one quarter of the state’s estimated 800,000 cattle drowned. Downtown Sacramento was submerged under 10 feet of brown water filled with debris from countless mudslides on the region’s steep slopes. California’s legislature, unable to function, moved to San Francisco until Sacramento dried out—six months later. By then, the state was bankrupt.

A comparable episode today would be incredibly more devastating. The Central Valley is home to more than six million people, 1.4 million of them in Sacramento. The land produces about $20 billion in crops annually, including 70 percent of the world’s almonds—and portions of it have dropped 30 feet in elevation because of extensive groundwater pumping, making those areas even more prone to flooding. Scientists who recently modeled a similarly relentless storm that lasted only 23 days concluded that this smaller visitation would cause $400 billion in property damage and agricultural losses. Thousands of people could die unless preparations and evacuations worked very well indeed."

Finally, on another note that picks up on a few previous posts (http://www.geologyinmotion.com/2014/10/update-on-this-years-el-nino.html;  wondering if we are in an El Nino year, Japan's weather bureau just announced that they find that an El Nino has emerged for the first time in five years, and is likely to continue into the winter. This is the first declaration by a major meteorological bureau of the "much-feared El Nino phenomenon." The pattern emerged between June and August and they signs of it in November as well. An El Nino year leads to drought in some parts of the world, flooding in others. 

It should be an interesting few months!

*Zhu, Y, and R. E. Newell, 1998: A proposed algorithm for moisture fluxes from atmospheric rivers. Mon. Wea. Rev., 126, 725-735, doi:10.1175/1520-0493(1998)126<0725:apafmf>2.0.CO;2.

http://www.esrl.noaa.gov/psd/atmrivers/questions/ for a summary of into