East side of the Rockies

[Sarah Berling]

I live on the West side of the Sandia mountains, which are a part of the Rocky Mountain chain. My question is: Why is the East side of the Rockies so much more moist than the west side, especially since air tends to move in a west to east motion? Wouldn't the air parcels be pushed up along the west side of the mountain, thus creating clouds there, which would up the moisture? I know I have something really confused, but I don't know what.

 

[Ben Cotton]

This is just a guess, so when it turns out I'm dead wrong, please don't laugh. I'd guess it is due to Gulf of Mexico moisture advection. Air from the west would primarily be moistened by the Pacific Ocean...by the time it gets to New Mexico, it's done the topography thing for a while and has probably lost a good portion.


?
Ben

 

[Darren Addy]

Sarah,
You are correct about the air parcels being pushed up on the west side of the rockies which causes precipitation (called "orographic lift"). In fact, I believe that to the east of the Rocky Mountains there is an area that is in what is called a "rain shadow" for this very reason. The moisture from the west gets squeezed out by the lifting and dumped in the mountains themselves. That rain shadow can extend quite a ways, over half-way into Nebraska, if memory serves.

However, the moisture that fuels spring time storms normally comes from the gulf where it is sent up by southerly winds. The mountains, as well as the generally west to east motion of weather systems, and the Coriolis Effect conspire to keep it on the east side of the Rockies.

I believe that the severe weather experienced in "Tornado Alley" is primarily due to the MIXING of the various air types in the spring, as well as access to that moist Gulf Air "fuel".

Meteorological-types feel free to correct/clarify my explanation as needed!

Darren Addy
Kearney, NE

 

[Jeff Snyder]

Originally posted by Darren Addy

However, the moisture that fuels spring time storms normally comes from the gulf where it is sent up by southerly winds. The mountains, as well as the generally west to east motion of weather systems, and the Coriolis Effect conspire to keep it on the east side of the Rockies.

Darren Addy
Kearney, NE

Darren has the gist of it, save for the above-quoted statement. As mid-level flow from the west (southwest or northwest as well) crosses the Rockies, there is upslope flow along the western slopes (more correctly the windward side, but in this case it is the western side), which means that air must rise in elevation to cross the mountains. Since rising motion results in cooling, much of the available moisture may indeed condense out as precipitation. Meanwhile, after the air crosses the mountains, there is downsloping along the lee of the Rockies. In other words, the westerly winds move "down" the terrain. This downsloping leads to warming by compression, which in turn leads to thermal expansion (though there are vorticity responses as well). In the end, we end up with a thermal ridge juxtaposed with a pressure trough. Assuming high pressure over the southeastern US, the pressure gradient between the high to the east and the low pressure along the foothills (termed a "lee trough" or lee cyclone) creates southerly flow across parts of the plains. This flow can help advect Gulf moisture from the GoM (called "return flow").