Atmospheric rivers are long, narrow bands of moisture in the atmosphere that extend from the tropics to higher latitudes. These rivers in the sky can transport 15 times the volume of the Mississippi River. When that moisture reaches the coast and moves inland, it rises over the mountains, generating rain and snowfall and sometimes causing extreme flooding.
Recently…a team of researchers from Scripps Institution of Oceanography and the Army Corps of Engineers in the first systematic analysis of damages from atmospheric rivers due to extreme flooding. We found that while many of these events are benign, the largest of them cause most of the flooding damage in the western U.S. And atmospheric rivers are predicted to grow longer, wetter and wider in a warming climate.
RTFA. More info. A scale for evaluating the dangerous potential in these rivers. Another wake-up call for this nation.
Eideard 
“Pacific Storm Parade Could Tap Atmospheric River : A classic winter jet stream is set up over the North Pacific Ocean.” (Weather Underground January 28, 2020) https://www.wunderground.com/article/forecast/regional/news/2020-01-23-pacific-storm-parade-atmospheric-rivers-washington-oregon
“Atmospheric rivers (ARs) are temporally ephemeral filamentary features in the lower troposphere that horizontally transport large quantities of water vapor (on average, more than double the flow of the Amazon River) and can cause extreme precipitation events on west coasts of major landmasses due to orographic lift over mountainous topography.” https://advances.sciencemag.org/content/5/12/eaax4631
During the next few weeks, leadership in NOAA (the National Oceanic and Atmospheric Administration (NOAA) and the National Weather Service (NWS) will make a key decision regarding the future organization of U.S. numerical weather prediction. A decision that will determine whether U.S. weather forecasting will remain third rate or advance to world leadership. https://cliffmass.blogspot.com/2020/01/the-future-of-us-weather-prediction.html
(Washington Post 2/15/20): Dennis has become one of the most powerful nontropical cyclones on record in the North Atlantic, packing hurricane-force winds and turning a vast swath of seas into a churning, ship-sinking cauldron with individual waves topping 100 feet. The storm slammed into Britain just one week after deadly Storm Ciara hit with high winds and heavy precipitation, prompting flooding fears. https://www.washingtonpost.com/weather/2020/02/15/storm-dennis-one-atlantics-most-powerful-bomb-cyclones-churns-up-100-foot-waves-slams-uk/
Dennis is the result of the atmospheric version of a corporate merger as two bomb cyclones spinning in proximity merged into one behemoth on Saturday, reaching historic proportions.
It occupies an extraordinary stretch of real estate, extending all the way from south of Iceland, southeastward into Britain, and southwest across the Atlantic and into the Gulf of Mexico.
A nearly 5,000-mile-long cold front connects Storm Dennis to a plume of tropical moisture, known as an atmospheric river.
As of Saturday afternoon, Dennis contained a minimum central air pressure of 920 millibars after putting on an extraordinary display of rapid intensification, known as bombogenesis. This pressure reading was roughly equivalent to what would be observed in a Category 4 or 5 hurricane.
NOAA weather map 2/15/20: https://pbs.twimg.com/media/EQ2B_YLX0AAdt1z?format=jpg
A NASA satellite image of Storm Dennis currently dominating the northern Atlantic Ocean (click to enlarge). A nearly 5,000-mile-long cold front connects Dennis to a plume of tropical moisture, known as an atmospheric river.
‘Atmospheric river event’ crashes into California spraying ‘fire hose’ of six inches of snow an hour and torrential rain down the West coast – as millions across Plains, Midwest and South brace for blizzards, tornadoes and severe storms https://www.dailymail.co.uk/news/article-11525523/Atmospheric-river-event-crashes-California-West-coast-millions-brace-storms.html
Excellent visual information
updated version, 7AM MST, tomorrow morning, 18th