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As an example of an atmospheric river, see http://www.wired.com/2014/03/california-atmospheric-river-rain/.

These events have been known to hit areas like California and Bergen, Norway.

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    $\begingroup$ I guess because winds are mostly westerly at the mid-latitudes. $\endgroup$ – gerrit Apr 15 '14 at 21:42
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    $\begingroup$ Atmospheric rivers rely on moisture flux or moisture advection to transport the stream of water vapor from the tropics to the mid latitudes. Winds are the are the agent of transport at this scale, so indeed, @gerrit is correct. The winds are westerly at the mid-latitudes in the Northern Hemisphere, so this is the direction we typically see the development of moisture rich precipitable water plumes. See i.stack.imgur.com/2GAqt.png $\endgroup$ – DrewP84 Apr 15 '14 at 22:09
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    $\begingroup$ I should also add that it is typically stronger winds associated with an extratropical cyclone within the mean westerlies that are the source of the stronger winds/advection/moisture flux. $\endgroup$ – DrewP84 Apr 15 '14 at 22:43
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Extratropical cyclones in the Northern Hemisphere rotate around the earth in the mid-latitudes in a mean westerly flow. This means that, on average, the track of a storm between 30°-60°N moves from west to east. Ahead of these extratropical cyclones is a strong frontal boundary with very powerful winds, sometimes up to hurricane strength near the boundary layer. Moisture is advected (transported by wind) from the tropics (a very wet place, relatively speaking) by these powerful winds ahead of the extratropical cyclone. The moisture then creates a narrow channel of extremely moist air along the frontal boundary. Finally, the atmospheric river moves with the mean westerly flow as the frontal boundary/extratropical cyclone shifts east. Therefore, to answer your question, it is because extratropical cyclones on average move from west to east that ARs impact west coasts in the Northern Hemisphere.

Atmospheric River preceding Upper low in Mid-latitudes off of US West Coast Atmospheric River preceding Upper low in Mid-latitudes off of US West Coast

Another way to think of this is that tropical systems (those that develop between 0°-30°N) initially move from east to west in the easterlies. Now ask yourself how often do the mid-latitude west coast's of northern hemisphere continents fall victim to category 3-4-5 hurricanes? Caveat: remnants from weakening tropical depressions often move into the mid-latitudes before impacting these regions.

Track of tropical cyclones in the opposite direction of Atmospheric Rivers Track of tropical cyclones in the opposite direction of Atmospheric Rivers

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