Satellite Precipitable Water:

This is an excellent measure of atmospheric moisture because it includes all of the moisture from the surface up to about 30,000 feet. There is little water vapor in the atmosphere above 30,000 feet. Thunderstorms are more likely to occur when precipitable water values rise above an inch and a half along the Colorado River (1.50"), above one inch (1.00") over southeast and central Arizona, and above a half an inch (0.50") along the Mogollon Rim. On the wettest days, the precipitable water can rise over 2.00 inches along the Colorado River, over 1.50 inches in southeast Arizona, and up to 1.00 inch along the Rim. During periods of high precipitable water, the risk of a flash flood occurring with a thunderstorm is significantly enhanced.

To measure precipitable water accurately, an upper air sounding (rawinsonde) must be used. However, it is impossible to send up enough weather balloons to track rapid changes in moisture over the Southwest. Instead, satellite moisture measurements and estimates from special Global Positioning System (GPS) detectors are compared to upper air data from weather balloons to derive estimates over large areas. The images shown in this window are satellite measurements, which are almost as accurate as upper air soundings. The one drawback, though, is that clouds obscure moisture at lower levels of the atmosphere. Thus on cloudy days or after thunderstorms develop, this data is unreliable, and is depicted by varying shades of "cloudy gray" on the images.