Almost 60 percent of California is categorized in exceptional drought, the worst category of drought in the weekly drought monitor analysis (see map below). Reservoir levels in the Golden State are well below average levels for late September.
However, an impressive Pacific storm by late September standards, shown in the satellite image at right, will wring out significant rainfall in parts of the thirsty West.
A sharp dip in the jet stream will lumber slowly into the West over the next few days.
Ahead of the attendant cold front, an atmospheric river, a long, narrow plume of deep moisture from the tropics, will be pulled into parts of the West.
Moisture in the atmosphere associated with this storm, known as precipitable water to meteorologists, could be over five standard deviations above late September averages. In English, that means this is a highly-unusually moist storm for this time of year.
However, the key to drought relief in California is not rain, but snow.
Critical to water supply in this part of the country is the buildup of winter snowpack in the mountains, whose meltwater in the spring replenishes reservoirs.
Link: http://www.wunderground.com/news/california-drought-update-rain-forecast-20140923
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