“Rollin’, Rollin’, Rollin’ On The River”

Atmospheric River Globe View
World View Atmospheric River | Image by PNNL

You might recognize the title of this week’s column as a line from the song Proud Mary written by John C. Fogerty and first made famous by his group Creedence Clearwater Revival. The song is about a sternwheeler riding on the Mississippi River. The topic this week is about a river, but not the terrestrial kind of river in the song. Instead it’s a river that is above us in the atmosphere aptly called an Atmospheric River.

Atmospheric River
Atmospheric River Diagram | Image by NASA

An Atmospheric River has been the topic of discussion recently by National Weather Service Meteorologists and Television Meteorologists alike. First we need to define the term so we know exactly what we’re talking about. According to the National Oceanic and Atmospheric Administration (NOAA) “Atmospheric Rivers are relatively long, narrow regions in the atmosphere – like rivers in the sky – that transport most of the water vapor outside of the tropics. These columns of vapor move with the weather, carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River. When the atmospheric rivers make landfall, they often release this water vapor in the form of rain or snow.”

Pineapple Express
Pineapple Express 1.19.19 Heading For The Pacific Northwest| Image by NOAA

A name for a specific atmospheric river over the Eastern Pacific Ocean is “The Pineapple Express.” Interestingly enough that was the exact situation at the end of last week. An atmospheric river of moisture moved from the area around the Hawaiian Islands and scooted northwestward to the Pacific Northwest. Significant rainfalls were predicted and rain was expected to become heavy at times. Advisories for flood potential were issued so that the public would be prepared for what could be flood producing rainfalls.

Water Vapor Channel
Water Vapor Channel | Image by NOAA

There is another satellite view that shows the actual channels of moisture and it is called the Water Vapor Channel. The darker areas show the concentration of atmospheric moisture, rather than just the layers of clouds, moving as a wide fetch of moisture that could produce copious amounts of rain over the ocean and particularly serious when it reaches land.

Flooding Potential
NWSFO Home Page Flooding Potential | Image by NWSFO Portland

Now that the Atmospheric River has been explained we should take a look at what actually happened when it finally reached the Pacific Northwest. The National Weather Service Forecast Office in Portland posted advisories, etc. to prepare us for the potentially heavy rainfall that was occurring. The following summarizes the rainfall from Friday January 18th through Sunday evening January 20th for four representative locations in Western Oregon.

Location          Friday          Saturday          Sunday    3-Day Total

Eugene             0.72″            0.10 ”                 0.85″           1.67″

Roseburg          0.90″            1.24″                  0.79″           2.93″

North Bend       1.11″             0.19″                 1.02″            2.32″

Medford            0.62″            1.58″ *               0.56″           2.76″

* The 1.58″ of rain that fell in Medford on Saturday was a record amount for the date. The old record was 1.4″ set in 1964.

With the limited number of locations listed interesting information can still be derived. The heaviest rainfall amounts occurred at the Coast and to the southern reaches of Western Oregon. If you look back at the two satellite images above the Atmospheric River was aimed at the more southern area of Western Oregon. This does give verification that these rivers of moisture can be reasonably predictable giving the National Weather Service enough information to issue advisories, watches, and/or warnings so that the public can be prepared for the copious amounts of rain that the Atmospheric Rivers can produce.

Let me know what you would like me to talk about or explain. You can email me at: [email protected].

Tim Chuey is a Member of the American Meteorological Society and the National Weather Association and has been Awarded Seals of Approval for television weathercasting from both organizations.

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