<p>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. <strong>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.</strong></p>

<p><img src="https://i.sstatic.net/lKqjk.gif" alt="Atmospheric River preceding Upper low in Mid-latitudes off of US West Coast">
<em>Atmospheric River preceding Upper low in Mid-latitudes off of US West Coast</em></p>

<p>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.</p>

<p><img src="https://i.sstatic.net/3LlCq.jpg" alt="Track of tropical cyclones in the opposite direction of Atmospheric Rivers">
<em>Track of tropical cyclones in the opposite direction of Atmospheric Rivers</em></p>

CC BY-SA 3.0
2014-04-15T23:48:06.680Z
2014-04-16T14:11:10.347Z
2014-04-16T14:11:10.347Z
DrewP84
<p>As an example of an atmospheric river, see <a href="http://www.wired.com/2014/03/california-atmospheric-river-rain/">http://www.wired.com/2014/03/california-atmospheric-river-rain/</a>.</p>

<p>These events have been known to hit areas like California and Bergen, Norway.</p>

CC BY-SA 3.0
2014-04-15T21:42:11.497Z
I guess because winds are mostly westerly at the mid-latitudes.
gerrit
CC BY-SA 3.0
2014-04-15T22:09:21.997Z
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 https://i.sstatic.net/2GAqt.png
DrewP84
CC BY-SA 3.0
2014-04-15T22:43:57.703Z
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.
DrewP84
<meteorology><climate><precipitation>
Why is heavy precipitation from atmospheric rivers more likely to hit the West Coasts than the East Coasts?
InquilineKea
# Why is heavy precipitation from atmospheric rivers more likely to hit the West Coasts than the East Coasts?

(Asked by: username_0 on 2014-04-15)

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.

**Tags:** meteorology, climate, precipitation

> **Comments:**
> **username_1**: I guess because winds are mostly westerly at the mid-latitudes.  
> **username_2**: 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, @username_1 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 https://i.sstatic.net/2GAqt.png  
> **username_2**: 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.  

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## 1 answers

### Answer by username_2 on 2014-04-15. Score: 13 | Accepted answer

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](https://i.sstatic.net/lKqjk.gif)
*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](https://i.sstatic.net/3LlCq.jpg)
*Track of tropical cyclones in the opposite direction of Atmospheric Rivers*


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