Last week featured the heaviest rains in the Bay Area in 1,876 days. This week’s storm should be even bigger.
All the rain-related ephemera that drought-stricken Californians had forgotten—umbrella etiquette, jumping through puddles and waterfalls—came flooding back Nov. 30. By the morning of Dec. 3, San Francisco had racked up more rain in a four-day span (3.76 inches) than it received in all of last year (3.38 inches).*
Still, last week’s rain didn’t do much to help the drought—technically, nothing changed at all. But it did set the stage for the coming storm to be a big help. Since the ground is still wet in most places, more will runoff into streams and reservoirs this time around.
This week’s storm will usher in an atmospheric river event, also known as the “pineapple express,” peaking late Wednesday and Thursday. The National Weather Service office in the Bay Area has predicted “the strongest storm of this season to date,” and possibly the strongest since 2008 or 2009, with potential results including “downed trees, power lines, flooding and mudslides.” Wind gusts could exceed 80 mph at higher elevations.
Heavy rainfall could fall at rates up to an inch an hour and could exceed 8 inches in the mountains. The high Sierras are in line for up to 4 feet of fresh snowpack. The National Weather Service notes there’s potential for this storm to stall out over the Bay Area as well, in which case the risk of mudslides and dangerous flooding could quickly increase. Just offshore, ocean temperatures are much warmer than usual, which the NWS says could argue in favor of the more intense rainfall scenario.
Seasonal rains are now technically above average in Northern California, at 112 percent of normal. Despite that, the state’s largest reservoir, Lake Shasta, barely budged higher after last week’s heavy rains. Before the storm, it was at 23 percent of capacity; after, it reached 25 percent of capacity. The rainfall was about 15 or 20 percent of what would be needed to end the drought—but that doesn’t factor in refilling the reservoirs to guard against future dry spells. The state’s snowpack—arguably a more important measure of water storage than the reservoir system—is still running below average for this time in December.
The rain is coming thanks to an eastward shift in the rain-blocking ridge that dominated the Pacific Coast last winter and helped intensify the drought. That shift is helping to open the door for major storms to approach the state from the tropics.
No matter how much rainfall the storm brings, this year’s heat will have already left its mark. Amid controversy over the drought’s place in climatological history, there’s now a 100 percent chance of 2014 becoming California’s warmest on record.
*Correction, Dec. 9, 2014: Due to an editing error, this post originally included the wrong date for recent rainfall. It occurred Dec. 3, not Nov. 3.