‘Pineapple Express’ storm to soak area
And you thought winter was over.
Although sunny skies are expected in the beginning of the work week, a “Pineapple Express” storm is expected to slam into the Bay Area beginning Thursday night and produce widespread rain across the region, according to the National Weather Service.
The storm has the potential to be one of the wettest systems the Bay Area has experienced this rainfall season, according to the weather service. This “atmospheric river” is expected to produce steady rain for 48 hours, possibly resulting in flooding of rivers, creeks and streams.
“Rainfall totals, generally speaking, could be up to 3 or 4 inches,” said Rick Canepa, a meteorologist with the weather service.
A Pineapple Express is a type of atmospheric river storm, named because rivers of rain — long, narrow bands of highly concentrated moisture — form in the Pacific Ocean and barrel eastward until they hit land. This storm is drawing tropical
moisture from Hawaii, Canepa said.
While the weather service is forecasting general rainfall totals, it’s too early for meteorologists to predict the storm’s bullseye.
“It’s still in the early stages in terms of predicting more precise locations where the rainfall will end up,” Canepa said.
Still, on Monday the weather service posted on social media a chart of all-time wettest starts to April in San Francisco, suggesting this storm could crack the top 10. To achieve that, San Francisco would need to register 2.96 inches of rain by April 11.
On average, San Francisco receives 1.46 inches of rain in April.
Early estimates for rainfall totals include 2 to 4 inches in the Santa Cruz Mountains, 3 to 6 inches in the North Bay and 1 to
2 inches in most urban locations around the Bay Area, according to the weather service.
Canepa said rainfall totals may not be as robust in the South Bay, which often falls victim to a “rain-shadow” effect, a phenomenon in which a mountain range or other topographic barrier causes prevailing winds to lose moisture on the windward side. As the storm moves east, the Santa Cruz Mountains could absorb most of the rainfall, leaving San Jose with just a few drops in comparison.
About half of California’s annual precipitation rains down from atmospheric rivers, said Doug Carlson, a spokesman for the California Department of Water Resources. About eight to 12 atmospheric rivers barrel through the state in an average winter, but few have come through during this historically dry season. An unusual 43 atmospheric rivers graced the West Coast last year and helped bring about Northern California’s record rains, Carlson said.
But this year, all of Northern California remains in a rainfall deficit for the water season, which began Oct. 1, largely because of an extremely dry December and February. Rainfall totals across Northern California generally range from 60 to 75 percent of normal, including Oakland (64 percent), San Francisco (64 percent) and San Jose (57 percent).
The city in Northern California closest to its seasonal rainfall average is Eureka at 90 percent (30.21 inches of rain).
The forecast coincides with the state’s April 1 manual snow survey, an event done every month in the winter at Phillips Station off Highway 50, not far from Lake Tahoe.
Monday’s measurement found the snowpack at 32.1 inches deep, or 49 percent of the historic average. In the Sierra Nevada, the statewide snowpack as of Monday was at 52 percent of the historic average, a major improvement after a rainy march. On Feb. 26, the snowpack was at 22
percent.
“Clearly while we had a good March, it was not adequate to get us up to a really good outlook, with respect to the water supply,” said Frank Gehrke, chief of snow surveys for the Department of Water Resources.
Gehrke noted that this week’s storm will be primarily a “warm event, so most of the precipitation is going to immediately run off and end up in the reservoirs and then be released on downstream.”
While California can always use the rain, a warm storm might actually melt away some of the latest snow. A warm Pineapple Express wouldn’t produce snow below 8,000 feet, according to Carlson.
“That would mean the precipitation is falling as rain rather than snow,” he said, “and that is never a good scenario.”