|Wind gusts forecast for 1:00 p.m. (tomorrow afternoon)|
"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:
*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;20725:apafmf>.
http://www.esrl.noaa.gov/psd/atmrivers/questions/ for a summary of into