Whatever you want to call it, it is an unusual weather pattern, and generally a positive one for us.
Looking at San Francisco, we normally don’t get any significant rain from May through September, and about an inch in October. So far this October, we have gotten almost 2 inches in downtown and the storm coming Saturday night through Monday is expected to bring another 1-2 inches. 4 inches would be about 20% of our annual rain, which is very good given we are in a drought. In the areas this rain hits, we can call the fire season over, which is also good.
This by itself won’t take us out of a drought, we could still have a dry winter. Our wettest months are January and February, when we get almost half our annual rain (about 9" between the two months), and they are still predicting drier than normal for the winter. So we will take any rain this will give us.
These “atmospheric river” weather patterns are somewhat targeted. The area the rain hits covers maybe a 100-150 mile swath north to south. If the weather system stays in one spot, that swath will get dumped on, and north and south of it will get less or even be dry. This one thankfully sounds like it will slowly drift south, which will spread the wealth (and reduce risk of landslides some).
Oh, another negative is that the areas that had fires in last few years are at risk of land slides.
This is not expected to do anything for southern California. It wont slide that far south. Santa Barbara and south will get little to no rain. Their fire season likely will continue there for a while.
For more info on SF climate (monthly averages and the like) - check this site: Climate in San Francisco County, California
Note - for those that aren’t familiar with our weather, we have significant microclimates in this area. In the summer, there are times when it will be 60F on the coast and 100F a half hour drive inland. Drive an hour north of SF to Santa Rosa, and the annual amount of rain almost doubles. An hour south to San Jose, and it halves from SF.