The Southwest Monsoon Climate Transition

Climatologically, there is a remarkable climate transition that occurs between southeastern Arizona, which is strongly influenced by monsoon rains, and southwestern Arizona and southern California which are considerably drier.  This transition is clearly evident in the mean precipitation for the two months of July and August.  The mea precipitation for these two months varies from 2–9+ inches over southeastern Arizona (depending on elevation), but is less than an inch (or even half inch) in the valleys of southern California, with a amaximum of only about 2 inches on the highest peaks near Palm Springs.

Data Source: Oregon State University
PRISM Climate Group

Better yet, let's look at the mean precipitation for July and August at individual stations (valley locations) going east to west across southern Arizona and California:

Tucson: 4.57"

Organ Pipe Cactus National Monument, AZ: 3.16"

Yuma, AZ: 0.70"

Imperial, CA: .42"

Impressive.  Imperial and Yuma are lower, but the change in elevation plays a secondary role in the precipitation contrast, as can be inferred by the analysis in the image above.

There are, however, times when southwest Arizona and southern California can get in on the action.  Tomorrow may be one of them.  There is a strong easterly wave presently moving across northern Mexico and southern Arizona that will move over southwest Arizona and southern California tomorrow.  

This should bring a good chance of showers and thunderstorms to the lower Colorado River and Imperial Valleys, which are some of the hottest and driest places in the United States this time of year.  

The loop below shows the march of high precipitable water air into the Imperial Valley.  Precipitable water increases from about 15 mm over the Salton Sea at 1200 UTC this morning to over 50 mm tomorrow.  

GFS Surface wind, sea level pressure, and precipitable water
analysis and forecast from 1200 UTC 29 Jul – 0000 UTC 31 Jul 2012.

Gully washers are a real possibility.  Wish I could be there.