LEAD: So far, 2015 has been the tale of two extremes when it comes to rainfall across the U.S.
1. NASA's GPM satellite network shows accumulated rainfall since January 1st of this year. Heavy flooding rains in parts of the east. But it has been very dry over California this year.
2. In fact a California drought has been going on since 2012. Ground water supplies are low. California's 4-year dry spell has left the state short by 20 inches of rain, an entire year's worth of rain.
3. Persistent high pressure off California has blocked Pacific rainstorms. What is needed are a series of "atmospheric rivers", often called the Pineapple Express, similar to December 2014, when 3 inches of rain fell.
TAG: About 20-50% of California's rain comes from the "atmospheric rivers" that pump warm tropical moisture over California.