The severe, ongoing drought in California is due to a persistent high pressure ridge over the eastern Pacific Ocean. Beneath that atmospheric dome, air sinks and dries out and cloud formation is suppressed. Of more significance, this dome shunts Pacific storm systems to the north, bringing copious moisture to the Pacific Northwest and southern Alaska while leaving California sunny, warm and dry.
Halfway through winter, the rainy season in California, the State has been devoid of precipitation. The snowpack in the Sierra Nevada is near historic lows while Santa Ana winds rake the canyons of Southern California, fueling wildfires during what should be the wet season. Of course, the severe drought also threatens the water supply for that populous, agricultural State.
Drought is usually caused by a stagnant weather pattern which often produces opposite weather extremes in adjacent regions. In this case, the high pressure ridge, resulting from a northward curve of the jet stream, lies west of a persistent atmospheric trough over the central and eastern U.S.; that dip in the jet has funneled a series of Alberta Clippers through the Midwest, bringing waves of frigid air and snow. In most winters, the jet stream directs Pacific storms eastward across the U.S., drawing in moisture from the Pacific, Gulf of Mexico and Atlantic as they progress; this year, a marked undulation in the jet has robbed California of that precipitation and the deflecting ridge is expected to remain in place for another week or so, if not longer.
