The New York Times has an interesting map which shows the changing drought across the US.  You can see the growth and decay of drought in the Texas panhandle as well as the increasing severity of drought in California.  Access it here.  The text indicates the drought is increasing as a result of global warming, but climatologists know that California droughts often strength in summer due to normal lack of rain (their dry season runs from roughly April to October).  In addition, California’s climate history using paleorecords like tree rings indicate that they have gone through many periods of protracted drought in the past.  If the Earth continues to warm, however, droughts may increase due to higher evaporation rates associated with warmer temperatures, in addition to any decreases in rainfall that may occur.