Rain is developing across the area at this moment. The key concern will be the total amount of rain and the rain rates.
The National Weather Service has a Flash Flood Watch in place for recent burn areas in Santa Barbara and San Luis Obispo counties through Thursday afternoon.
The storm system has all the ingredients to produce heavy rain: subtropical moisture and a strong parent low.
This is an "atmospheric river" event where a strong westerly flow will direct a major moisture supply into Central California. The latest analysis shows the river arriving and upper-level flow becoming divergent which intensifies lift and rain.
2-5" of rain is possible in coastal areas with 4-10" in mountains and foothills from later Tuesday through early Friday with the bulk of the rain falling on Wednesday. Not only are those amounts a concern but also the rate. .50-.75" per hour rain rates are likely at times with a 20 percent chance that rain rates of 1" per hour could develop. Even the lower end of this range carries flooding and debris flow potential.