Pineapple Express Bringing Significant Rains to Drought-Stricken California

A very moist “Pineapple Express” flow of air from the Hawaiian Islands will impact California through Sunday, likely bringing enough precipitation to make a noticeable dent in the state’s dire drought conditions (though the exceptionally dry and hard soils caused by California’s driest year in its history are forcing the heavy rains to run off faster than usual, reducing the amount of moisture that can soak into the soil.) Some locations may see more rain in a four-day period than they have had during the previous eight months. NOAA’s Weather Prediction Center is calling for most of Northern California to receive more than 2” of precipitation through Sunday, with many higher elevation areas expected to get 4 - 6”. Up to three feet of snow is predicted to fall in the Sierra Mountains, though it appears much of the precipitation will fall as rain, reducing the benefit of the moisture during the coming summer months (when Sierra snow melt provides an important source of water.) As of Thursday at 1 pm PST, Big Sur had received 2.14” of rain, which triggered a rock slide onto Highway 1.


Figure 1. Total precipitable water (TPW) for Thursday, February 6, 2014. TPW is how much rain (in inches) would fall at a given location if one condensed out all of the water vapor in a column above the location into rain. For reference, 1 inch = 25.4 mm. A narrow “Atmospheric River” of moisture is seen extending from the subtropics near Hawaii into California. Image credit: University of Wisconsin SSEC.

Drought far from busted
This weekend’s Pineapple Express is a marvelous break from the extraordinary dry conditions that have gripped California for the past thirteen months. If one could put a monetary value on the moisture from this storm, I speculate that it would easily be worth a billion dollars. But the state is in such a deep precipitation hole that it needs at least six more events like this over the next two months to pull them out of drought. Between January 1, 2013 and February 5, 2014, the San Francisco Airport received just 4.24” of rain, which is 21.19” below normal for the period—by far the driest such period in their history. The last time San Francisco had more than 1” of rain was Christmas Day, 2012. Thursday’s new Drought Monitor product showed that drought conditions in the state had remained almost the same as the previous week, with 94% of the state in drought, and a slight expansion of the area in the worst category of drought—exceptional—from 9% to 10%.


Figure 2. Comparison of how much rain is needed to relieve drought conditions in Central California, via Twitter from NWS Sacramento.


Figure 3. Amount of precipitation needed in one month to end drought conditions. In San Francisco, more than 18” of rain is needed in one month, and the average annual rainfall in the city is about 20”. Image credit: NOAA.

Atmospheric rivers: California’s big drought busters
Narrow bands of copious moisture originating in the subtropics like this weekend’s “Pineapple Express” are called “atmospheric rivers”, and are responsible for about 30 - 50% of California’s yearly precipitation. A strong “atmospheric river” transports an amount of water vapor roughly equivalent to 7.5 - 15 times the average flow of liquid water at the mouth of the Mississippi River. As discussed in a blog post today by Climate Central’s Andrew Freedman, research by Michael Dettinger of the U.S. Geological Survey and the Scripps Institution of Oceanography, published in December 2013 in the Journal of Hydrometeorology, found that atmospheric river events can effectively end major droughts in California within just one month, pulling the state from a significant precipitation deficit to a surplus. Wunderground weather historian Christopher C. Burt has a post on the last drought-busting atmospheric river event, in March 2012. I don’t see that happening this month, though. The latest 2-week run of the GFS model shows the state returning to relatively dry conditions beginning on Monday, with a ridge of high pressure dominating the weather for the remainder of the week. The most recent 1-month and 3-month forecasts from NOAA’s Climate Prediction Center call for higher than average possibilities of dry conditions into the spring of 2014. California’s best hope of busting the drought lies in the formation of an El Niño event next winter. The warm waters that El Niño events bring to the Eastern Pacific typically shift the jet stream to a position over California, bringing numerous low pressure systems and the occasional atmospheric river during the winter rainy season. The latest February 6, 2014 El Niño outlook from NOAA gives some hope that this will happen:

”An increasing number of models suggest the possible onset of El Niño. Strong surface westerly winds in the western Pacific and the slight eastward shift of above-average temperatures in the subsurface western Pacific potentially portend warming in the coming months.”

Have a great weekend, everyone, and I’ll be back Monday with a new post.

Jeff Masters