Atmospheric Rivers, Hype or Science?

If you have lived in the Western US for any length of time you have experienced periodic episodes of moderate to heavy rain, snow or a combination of both at least once a year and sometimes more. Up until a few years ago, if it was rather mild and wet, you heard the term “Pineapple Express” used both in the National Weather Service as well as the news media. This term would refer to a supposed stream of moisture and mild, sometimes windy and wet pattern that would bring moderate to heavy precipitation especially to locations west of the major mountain ranges of the Cascades and Sierra Nevada. Nowadays it is very unusual to hear the term “Pineapple Express” used and instead we hear of “Atmospheric Rivers” often referred to as “AR” for short and often terrible flooding and/or damage from the result of these so called events. Several followers of the weather on this site have asked me to explain or clarify what exactly is an “Atmospheric River” and what causes them so that is the purpose of this article.

According to NOAA, an “Atmospheric River” is defined as “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.” When viewed on a satellite picture they look like this:

The picture above is actually what is known as a “water vapor” satellite picture which simply captures the water vapor in the atmosphere around the 600 mb (12,000 foot level) with brighter white colors indicating increased water vapor likely condensed into cloudiness and darker shades less water vapor or associated with clearer skies although this feature does not pick up low level cloudiness or high clouds above the 500 mb level very well. Using this picture which was taken in November of 2022, it can be seen that where the arrows are pointing to as an “atmospheric river” is actually the southern branch of the Polar Jet Stream. The darker area on the north side of the brighter white colors that is in the shape almost of an egg is an upper level trough of low pressure with a developing surface low located near the Northern California/Southern Oregon border just offshore. Looking further west of where the arrows are located we can follow the “atmospheric river” or jet stream and see that it continues much further to the west as well. The dark area to the south or below the arrows is an area of drier “atmospheric moisture” with the brighter white areas below that darker area indicative of the Sub Tropical Jet Stream heading into Baja California and Mexico. This then begs the question, what is causing these areas to either be darker in color or lighter in color? Is it the moisture or perhaps something else?

Here are a couple more satellite pictures taken more recently:

This is an infrared satellite picture taken of the Pacific Ocean on Sunday, March 19, 2023 in the early evening PDT. You can see a band of cloudiness extending northwest to southeast across our region in Northeast Washington and North Idaho stretching southeast into New Mexico with perhaps the more noticeable feature being the long stream of clouds stretching well out into the Pacific Ocean to the south and west of California. This is currently being referred to as another “atmospheric river” by the media and many meteorologists. For the record here is the water vapor picture for the same time frame with more of a zoomed in focus for our area:

In this picture the green and white colors represent areas of greater atmospheric water vapor at the 600 mb level or around 12,000 feet while darker areas again represent less atmospheric water vapor. According to the definition given at the beginning of this article on what an atmospheric river is we should be able to reasonably conclude that wherever we see the brighter green colors intersecting the land or, in the case of many in the media, wherever the green and white colors are we should be seeing copious amounts of precipitation falling. Now, let’s take a look at a chart of the surface observations as well as the upper level observations for the same region and the same time frame. See the picture below:

This is from the GFS model across the North Pacific Ocean with the date located above at the top of this picture taken at the same time as the previous two satellite pictures. The green areas are areas of precipitation, black lines are isobars which simply connect areas of equal surface pressure, the red “L’s” are surface areas of low pressure, blue “H’s” areas of surface high pressure with the blue and red dashed lines atmospheric thickness values with red broken lines associated with higher or warmer atmospheric thickness values and blue broken lines associated with lower or colder atmospheric thickness values. The blue and red numbers along those lines are the actual thickness values themselves. If what the definition of an atmospheric river stated above is true then we should be seeing a continuous line of green and more likely dark green to even blue or purples and reds continuously stretching from our area down to New Mexico and from California all the way across the Pacific Ocean to near Japan yet that is simply not true. In fact the shades of green in this picture are all either very light green indicating very light precipitation or no precipitation actually being generated at all. Absent is a continuous line of precipitation and instead we see areas of precipitation and no continuous “atmospheric river” of precipitation. Why is this the case based on what the media and meteorologists are telling us? Heavy precipitation events, or as many in the media like to say “extreme” precipitation events are supposed to be tied to these “atmospheric rivers” and very large amounts of moisture in the atmosphere due to so called “climate change”. But is this actually true?

Upon closer examination of the pictures and charts above the reader can see that the areas of greatest precipitation amounts are actually located in areas where a few things are happening: surface convergence or air coming together at the surface, which leads to vertical motion or lift in the atmosphere that helps generate precipitation, upper level divergence or air aloft at around 18,000 feet or 500 mb spreading apart which helps keep vertical motion going in the lower layers of the atmosphere, a coupled jet stream where the blue colors and red colors are converging or coming together which helps generate and sustain surface convergence and upper level divergence, developing surface lows which are basically areas of surface lower pressure which also causes air to rise and generate vertical motion, and lastly and perhaps most important of all, upper level lower pressure which helps to drop cold air into the atmosphere and create instability which results in decreasing lower surface pressures, increased vertical motion, lift, jet stream dynamics and eventually the squeezing out of precipitation. As we can see in the satellite pictures the moisture is really not a factor in generating the precipitation as if that were the case then everywhere we see the increased atmospheric moisture we should be seeing heavy amounts of precipitation. The analogy is similar to a car. You can put 92 to 98 octane gas in your car but unless you have a trigger or spark, your car simply will not run. The same is true of the atmosphere. There is always plenty of moisture in the atmosphere but without the necessary “spark”, in this case vertical motion or other dynamics, the results will be miniscule. Something else to consider is that the media and meteorologists will often say that an “atmospheric river” will end because it will get “cut off”. If the reader thinks about a river, a river is only “cut off” by either a dam being built on the river or it runs out of forward motion and simply either empties out into a body of water or dries up. In the last picture above tracing the red broken line as well as in the water vapor and satellite pictures we can see that the jet stream is simply NOT “cut off”, the jet stream is still very much in tact but what IS missing is the vertical motion or dynamics and lift, NOT the moisture so to say that the “atmospheric river” is “cut off” is simply not true.

In conclusion this forecaster has come to the following: the bottom line is that moisture or “atmospheric rivers” are NOT the primary drivers of precipitation across the region, whether they be minor or significant, other factors are far more important which were listed above. The moisture is more of a by-product of more complex situations going on in the environment and atmosphere. Using the terms “atmospheric river” sounds a lot more dramatic and alarming than “Pineapple Express” and it is this forecasters belief that this is simply another way of inserting politics into science rather than looking at and studying the science to find answers to questions. The opinion of this forecaster is that “atmospheric rivers” are more about hype and dramatization rather than actual science. The use of such explanations is simply a lazy and simple way to try explain to the public what is going on when in reality this is far from fact or truth. Much of science is theory and not simply fact. Lastly, the transport of atmospheric moisture does not really move horizontally in the atmosphere or across the Pacific although the jet stream does. Atmospheric moisture transport is driven more vertically through lift and vertical motion. The clouds seen moving along in a satellite picture are often not producing much in the way of precipitation but are rather mid and high level cloudiness such as cirrus or alto-stratus clouds. The actual precipitation producing clouds in the lower atmosphere are not being moved along horizontally but are being generated by vertical motion, convergence, lift and dynamics, similar to the spark in a gasoline engine of a car. In the very short term there is some horizontal transport but this typically only lasts at most an hour or so before weakening and no longer becoming a factor. Think about summer time thunderstorms. If they moved horizontally then the same cell would travel very long distances. The reality however is that if you observe at thunderstorm on radar or satellite examination will show that the life cycle of a thunderstorm is usually between a half hour and an hour. Additional cells can be generated if the conditions are right in the atmosphere that may give the APPEARANCE of the same cell but a closer examination will show that is simply not the case. Hopefully this article helps clear up some of the hype of “atmospheric rivers” of what they are and are not. Feel free to contact me if you would like further explanation or information.