What's Behind California's Wild Weather?

As the skies cleared over the Golden State last week, many wondered whether a vengeful god had unleashed the rain and snowstorms that killed 28 Californians and caused more than $100 million in damage, mostly in the southern part of the state. Downtown Los Angeles logged a record 17 in. of rain over a 15-day stretch (bringing precipitation for the season to a record 22.51 in.), while a mudslide in nearby La Conchita killed 10 residents and swallowed 15 houses. To the north, around Lake Tahoe, roads and cars all but disappeared beneath as much as 19 ft. of newly fallen snow.

What caused the outburst of meteorological fury? This time not much of the blame goes to the usual suspect, El Ni??o, which has produced only a slight warming of the tropical Pacific since August. A more significant culprit, say meteorologists, is a less well-known phenomenon called the Madden-Julian Oscillation (MJO), named after the pair of scientists--Roland Madden and Paul Julian--who discovered it in 1971. The MJO is a globe-girdling disturbance that sweeps across equatorial waters at intervals of 30 to 60 days. Under its influence, says climatologist Wayne Higgins of the National Oceanic and Atmospheric Administration (NOAA), the atmosphere at mid-latitudes can undergo dramatic rearrangement. The result: a classic configuration called the Pineapple Express, in which the jet stream steers powerful cyclonic systems over warm waters near Hawaii, where they tank up with moisture before slamming into the West Coast. That is what happened around the first of the year, when back-to-back storms swept through California, Nevada, Colorado, Arizona and Utah and then went on to savage Ohio.

Violent as these storms were, they also did some good. For the first time in recent memory, snowpack in much of the West is running well ahead of average, and that signals relief for the drought-plagued region. Even so, says NOAA drought specialist Douglas LeComte, it's too early to proclaim the long dry spell completely over. Snow is still scant in the northern Rockies and the Pacific Northwest, he notes, adding, "It will take more than one wet winter to refill the reservoirs along the Colorado River." --By J. Madeleine Nash