It’s been a very long time since Southern California saw a storm, born over the waters of the North Pacific, deliver measurable precipitation in May! We in the Southland are accustomed to May being dreary, for sure, but usually the culprit is a persistent eddy off the coast that pumps warm spring air over cold ocean water– we call it the marine layer; May Gray; June Gloom.
But this week’s storm is an entirely different beast.
Here’s a map showing total precipitable water (TPW) over the Eastern Pacific:
Pay particular attention to the tongue of blues and oranges stretched across 30 degrees North. This is a very clearly defined “atmospheric river” – a stream of moist air flowing from the West/Central Pacific. These are common in the winter; California usually experiences 2-3 per winter. This past winter, a very strong, persistent atmospheric river dumped over 10 inches of rain on Los Angeles in December.
This “atmospheric river” is the reason we’re getting an exceptionally rare late-May rainstorm tonight through Wednesday. This river is transporting moist, energetic air across the Pacific, some of which will coalesce into a defined low-pressure system overnight and plow inland.
The Los Angeles/OC area is due to see anywhere between 1/4 and 3/4″ of rain. Keep in mind, the historical average rainfall for May is generally less than 0.25″.
The rain begins tonight. Enjoy!