Atmospheric Rivers

Figure 1: Visible satellite imagery from December 19, 2019 (via NOAA)

The Pacific Northwest is known for getting a lot of rain, especially in the winter months. But sometimes, the amount of rain that falls can even shock the most veteran Pacific Northwesterners. In these situations, it can seems like the rain never ends. Oftentimes, this type of situation can be classified as an atmospheric river.

An atmospheric river is defined as a narrow band of moisture originating from the tropics. They bring with it persistent and sometimes heavy rainfall, which oftentimes leads to flooding, not only in rivers but in urban areas as well.

To conceptually understand them, think about the Columbia River. Water is steadily transported from one region to another. An atmospheric river is similar, but instead of liquid water being transported on land, water vapor (the gaseous form of water) is being transported in our atmosphere.

Figure 1 shows a visible satellite image for an atmospheric river event that occurred in December 2019. The “train” of clouds is clearly visible, which is aimed directly at Washington state. The tweet above shows a water vapor image for that same event. The blue colors show the areas of high moisture, which are again clearly aimed at Washington.

So how do these happen? There is a specific setup that leads to an atmospheric river event for the PNW. As Cliff Mass describes in his blog about the December 2019 event, an atmospheric river occurs when there is “low pressure over the northeast Pacific and high pressure west of California.” The following diagram shows a highly simplified schematic of this.

Figure 2: The atmospheric setup that leads to an atmospheric river in the PNW

Air rotates around low pressure areas counter-clockwise, while air rotates clockwise around areas of high pressure (things get a bit more complicated when on land due to friction). The configuration in figure 2 leads to strong southwesterly winds (from southwest to northeast), bringing in air from the tropics, such as Hawaii. Because of the air’s origin, an atmospheric river is also called a Pineapple Express. This air is much warmer, and as a result, can hold more moisture in it (conversely, colder air doesn’t hold as much moisture… that’s why Antarctica is technically a desert!). Thus, there is the potential for much more precipitation to fall in the area. Temperatures also tend to be higher than average during an event due to the warmer origin of the air, but sometimes this only occurs higher up in the atmosphere rather than at the surface.

Impacts of Atmospheric Rivers

Flooding. Lots of flooding.

Okay, maybe that’s a little vague. The severity of atmospheric rivers actually vary, which means that the amount of flooding varies, as well. Scott Sistek at KOMO 4 News describes the scale that atmospheric rivers are rated with here.

Sometimes they can be beneficial, especially if they are aimed at a location experiencing a drought. However, problems still can arise if TOO MUCH rain falls on these areas.

Other times, however, atmospheric rivers can be more hazardous than beneficial. Rivers on the ground (had to make that distinction ;)) begin to rise from the excessive rainfall, and sometimes snowmelt (because of warmer temperatures) can even contribute to river rises. If this rise continues, areas surrounding rivers can flood.

Here are some examples of flooding and high waters from the December 2019 event, all shared via Twitter.

As I mentioned before, river flooding isn’t the only problem associated with atmospheric rivers. Urban flooding can also be a big issue. Here are a couple examples of this, again from the December 2019 event.

This atmospheric river event even turned my driveway into a river.

As you can see, atmospheric rivers can be quite damaging, and December 2019 was a significant one. In fact, December 20, 2019 became the 5th wettest day on record at Sea-Tac airport with a total of 3.25 inches. Scott Sistek wrote about some more mind-blowing statistics regarding this system and can be found here.

The most important thing to remember, however, is to never drive through flooded streets. You will hear this phrase from other meteorologists, and I’ll repeat it here:

Turn around, don’t drown!