While the eastern half of the country waits for the weekend for exciting weather, the West Coast will be impacted by several storm systems before the week is done. This series of low pressure systems will, however, be the set up for the weekend’s big storm. Until then, we’ll focus on the beginning of these systems as they make their week-long trips across the country.

To read more about this upcoming weekend storm, check out the past week’s Sunday Storm, written by our Head Meteorologist Josh Feldman, here.

Let’s first begin by asking, what is an atmospheric river? According to NOAA, an atmospheric river (AR) is a flowing column of condensed water vapor in the atmosphere that’s usually responsible for producing significant levels of rain and snow. ARs are very common for the western US, and are best known for the very common Pineapple Express. The Pineapple Express is a commonly reoccurring AR that’s characterized by a thin line of high moisture transport that extends from near Hawaii to California. This particular AR is generally monitored very closely due to its correlation to heavy rain, flooding, and snow that it generally brings to the West Coast.

Above is a water vapor satellite image from the GOES West. From this image, you can see two areas of enhanced moisture: the first extending into Baja California, and the second reaching into the Pacific Northwest. While these are areas of high moisture, they don’t always give the full story.

An AR is an area of enhanced integrated water vapor transport, or, intense moisture that is being moved from place to place. Above is NOAA’s Automated AR Detection from GFS forecast fields. Areas overlayed with a grey line are areas where an AR has been detected. Zoomed out from the previous image, we can see that those two areas of high moisture correlate to two ARS, both originating from the same place, between 20 and 25 degrees N.

The next question is, what does this mean for the incoming storm system? When wrapped up in a cyclone, this amount of high moisture will mean heavy rain and snow being dropped over most of the NW US. Precipitation has already begun as moderate rain on the windward side of the Cascades, and light snow over and just east of the mountains. Along the WA coast, flood and high wind warnings are to be heeded today as the bulk of this system passes early this afternoon.

In the mountains, heavy snowfall and accumulations of 1-2 feet can be expected from today into Wednesday. Travel across the Cascades is not recommended, and certainly not without snow tires and chains to help cars maintain grip. By this afternoon, the bulk of precip will have moved into the Northern Rockies where 6-10″ of snow is likely to fall until Wednesday night.

High snowfall rates and wind gusts mean blizzard conditions and extremely low visibility, especially overnight. Travel is recommended to be very limited as conditions will be dangerous. Combined with slick, icy roads, driving in this sort of weather is definitely not advised.

By the end of the day tomorrow, the bulk of the system will have lost a lot of moisture as it moves further south. Interactions with other storms and the Gulf of Mexico later this week will likely renew this storm’s strength as it causes more trouble for the other side of the country.


Kathleen is a Meteorologist at WeatherOptics, where she works writing content for the website, providing accurate and detailed forecasts to clients, and consulting on various meteorological projects. Kathleen earned her B.S. in Atmospheric and Oceanic Sciences in 2018 from Stony Brook University. Kathleen has also done research into our changing climate by investigating theRole of Atmospheric Rivers on Arctic Amplification in 2017.

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