From NASA JPL, video animation follows.

Wet weather is again hitting drought-stricken California as the second and larger of two back-to-back storms makes its way ashore. The storms are part of an atmospheric river, a narrow channel of concentrated moisture in the atmosphere connecting tropical air with colder, drier regions around Earth’s middle latitudes.

The storm that arrived on Feb. 26, 2014, and the one about to hit, are contained within the “Pineapple Express,” an atmospheric river that extends from the Pacific Ocean near Hawaii to the Pacific coast of North America, where it often brings heavy precipitation. This next storm is expect to be the largest rain producer in Southern California in three years.

This animation, created with data acquired by the Atmospheric Infrared Sounder (AIRS) instrument on NASA’s Aqua satellite, shows the total amount of water vapor contained in the atmosphere for most of the month of February if it were all to fall as rain. Typically, the atmosphere over Southern California and most of the continental U.S. in winter holds only about 0.4 inch (10 millimeters) or less of water vapor. However, much wetter air lies tantalizingly close in regions to the south and west. The largest amounts of atmospheric moisture, up to 2.4 inches (60 millimeters), are associated with a persistent band of thunderstorms circling the tropics. These thunderstorms are the source of several atmospheric rivers apparent in this animation. One atmospheric river arises near Hawaii around Feb. 10 and comes ashore in Central California a few days later, bringing the largest Sierra Nevada snowfall of the season to date. Other atmospheric rivers can be see originating in the Gulf of Mexico and extending into the Atlantic on the right side of the movie; the northward movement of tropical water vapor is important in winter storms in the eastern U.S. and Europe. The animation concludes with the current Pineapple Express. Moisture from around Hawaii has surged northeast, and the persistent, dry air immediately west of Baja California has been replace by air with up to 1.6 inches (40 millimeters) of water vapor. The next storm will bring that moisture ashore, where it will be forced upward by coastal mountains to fall as heavy rain. Up to 8 inches (20.3 centimeters) of rain is predicted in some parts of the Los Angeles area by March 2, bringing possible flooding and landslides to recent wildfire burn areas.

The recent cold conditions in the eastern U.S. are also apparent in this movie as very dry regions. Because cold air can hold relatively little water (less than 0.4 inch or 10 millimeters), cold region are always dry. So, the eastern U.S. has some of the driest air in this animation. However, high pressure systems also dry the atmosphere by forcing down air from above.

That descending air expands and warms, but retains the low moisture amounts it had when it was higher and cold. So, cold Minnesota and warm Mexico have similar water vapor amounts in this movie.