Is a ‘Pineapple Express’ headed our way? We certainly need the rain

A nearly stationary cold front off the Northern California coastline late this past week pulled in abundant lower-level subtropical moisture that stretched across the Pacific past the Hawaiian Islands.

This low-level subtropical moisture combined with the persistent southerly winds along the coast and the lack of convective dynamics produced persistent heavy drizzle/light rain Thursday and again Saturday in the coastal regions of San Luis Obispo and northern Santa Barbara counties.

Yes, you can classify this weather as dreary. But as many readers have told me, every precious drop of rain is needed and should be welcomed. Personally, and I know it sounds a bit strange, I love dreary and overcast days.

This heavy drizzle/light rain produced impressive amounts of precipitation over time, especially along the southwesterly facing coastal mountain slopes. In these areas, the southerly winds drove the moist and mild air mass upward along the Santa Lucia Mountains and wrung out the moisture from the sky like squeezing a wet sponge or mop — in other words, orthographic enhancement.

This condition created extreme differences in precipitation totals throughout the Central Coast. For example, Cal Poly reported more than 1 inch of rain, while just a short distance away only a few hundreds of an inch of rain reached Atascadero. As of Saturday morning, Cambria at the Walter Ranch recorded the most rain at well more than 2 inches, while the San Luis Obispo Botanical Garden across Highway 1 from Cuesta College recorded 1.6 inches since Thursday.

This event reminded me of December 2010, when another plume of subtropical moisture produced tremendous amounts of rainfall. In fact, 2010 was the last year the Central Coast received above-average precipitation. However — if you remember — before 2010, the Central Coast was in the middle of another drought that persisted for three years. Nacimiento Lake dropped to 26 percent of capacity by January 2009.

Currently, the lake is at 23 percent capacity.

Many feared that the 2010-11 rain season was going to be another one of below-average precipitation, but thankfully, an occluded front extending thousands of miles across the Pacific stalled over the Central Coast. This rain event started Dec. 18 and continued into the first day of winter Dec. 21, 2010. Over about four days, as much as 10 inches of rain fell from the sky, with many Central Coast locations measuring totals ranging between 6 and 8 inches — meaning a few local communities got nearly half of their annual amounts of rainfall in just a few days.

Rocky Butte had the most at 11.3 inches, followed by the PG&E Energy Education Center in Avila Valley, which recorded 10.14 inches. This system caused flooding at the North Beach Campground in Pismo Beach and areas throughout Oceano and Avila Beach.

This occluded front was later classified as an “atmospheric river.” Along the West Coast, it is informally called the “Pineapple Express.” The Pineapple Express is a subset of an atmospheric river event that originates in the tropical waters near Hawaii, hence the pineapples. Meteorologists refer to these as “the hose.” These systems can transport ridiculous amounts of water across vast expanses. In fact, they can carry more freshwater than the Amazon River.

If it wasn’t for 2010’s atmospheric river, we would be experiencing 10 straight years of below-average rainfall.

I’m grateful to see that the longer-range models indicate that wet and unsettled weather will continue through the end of December with a slight chance of an atmospheric river developing Thursday, providing the Central Coast with desperately needed rain.

▪  ▪  ▪ 

Note from PG&E: Residential customers have a new way to take control of their energy bills this winter with PG&E’s Energy Alerts, allowing customers to better manage their energy usage and budgets. To lean more, visit www.pge.com.

This story was originally published December 10, 2016 at 8:03 PM with the headline "Is a ‘Pineapple Express’ headed our way? We certainly need the rain."