A quick series of storm systems approaching the West Coast from the Pacific will bring some beneficial rainfall to most of California and Oregon over the next week. Populated areas of northern California and western Oregon could see several inches of rain, with more at higher elevations.
The bulk of the precipitation will begin towards the end of this weekend and continue through next week, associated with a couple of weak surface lows coming ashore Oregon and Northern California. The biggest low will develop well off the coast of Oregon this weekend and approach northern California on Monday and Tuesday, and it's this low that will be responsible for the bulk of the precipitation that falls.
Widespread rainfall is expected across all of California—yes, even Los Angeles—largely as a result of the low siphoning tropical moisture north into California from the west coast of Mexico. The above animation shows mean sea level pressure (contours) along with precipitable water (or "PWAT," depicted by the colored gradient) as predicted by the GFS model beginning today through the end of next week.
Precipitable water shows the amount of rain that would fall, in inches, if all of the moisture in the atmosphere were to condense and precipitate to the ground, hence the name. Higher precipitable water values indicate more moisture in the atmosphere; values range between one and two inches across most of California for a brief period of time on Monday and Tuesday of next week.
This region of the western United States needs every drop of water it can get.
More than half of California is experiencing "exceptional drought" conditions as of Thursday, with 55% of the state in desperate need for water. 94% of the state is seeing at least a "severe drought," with 79% of the state in an "extreme drought." Drought conditions are slightly less severe—but still considered extreme—across much of Nevada and interior sections of Oregon.
The region has seen some rainfall over the past two weeks, but it's not enough to put a dent in the deficit. Most of the hardest-hit areas haven't even seen an inch of rain from recent precipitation. The upcoming rainfall should give many hilly areas from San Francisco north an inch or two. Residents will take whatever they can get—rain is rain.
Aside from the rain, one of the most important factors to replenishing the region's water table comes in the form of snowfall in the higher elevations. This morning's run of the GFS model shows the highest elevations along the Sierra Nevada receiving between one and two feet of snow from this upcoming event. This morning's snowfall analysis from the NOHRSC shows zero natural snow cover in the mountains of California, which is unusual for this time of the year.
[Images: Maps by the author, model animation via WeatherBELL]