It has been a long, generally warm and dry summer and early fall. The Central Coast and much of California has been paying for it with deep drought and significant fires. It appears we may have to brace for a very quick turnaround to high rain rates and concerns about heavier rains falling in burn areas.
The transition begins Friday for the Central Coast with the first in a series of decaying cold fronts to roll thru the area. The first front Friday should cause a few showers but generally less than .10" for those that experience any rain at all. Another weak decaying front is likely over the weekend but with less potential to drop rain.
Later Sunday into Monday something much more significant is possible. For the last several days numerous models have all picked up on a potential atmospheric river event. An atmospheric river is a stream of deep moisture transported by the jet stream from over the ocean into the landmass. When this aligns with low pressure and a cold front these rivers of moist air can produce high rain rates and also prolonged rain events lasting a day or longer.
This is exactly what models are suggesting for Sunday into Monday locally (potentially lingering into Tuesday). The American GFS model likes up to 1.5" locally but indicates most of the area with lower amounts while the ECMWF model shows a .5-1.5" median rain event. It is too early to make precise predictions but it is clear the potential for moderate to heavy rain is there. I'd say a .5-2.5+ inch rain event is a good ballpark to start with. Higher elevations facing a SW wind would likely see the highest amounts, like the SLO county north coast. Fortunately, there are not a lot of fresh burn scars for the Central Coast but clearly, the Alisal Fire is a newer scar and needs to be watched. Elsewhere in CA, to the north, the number of fires and huge burn scars align with more events and more significant rain forecasts where debris flow and flooding issues and concerns are high. This far out expect both forecast refinements and a number of related watches and advisories.
Until then, looks pretty quiet. Onshore flow and plenty of clouds Thursday will keep everything mild. Friday the first cold front will drop temps but not much rain. I think on balance the weekend is quiet until later Sunday.