Southern California is about to be hit hard with heavy rain of 3-9 inches and powerful winds. Flooding and slides are inevitable. But it will have a silver lining: any talk of drought in California should be ended as the last below-normal reservoirs are filled.
Let's start with the 2-day precipitation totals (ending 4 PM Sunday) from the wonderful NCAR high-resolution ensembles (the ensemble mean--the average of the ten ensemble members are shown). Totals exceeding 3 inches covers a wide area from LA north, with some locations in mountains north of Santa Barbara getting 7-9 inches. That amount of rain would be huge anywhere, but in dry southern CA that is immense. For a number of locations, this will be the greatest rain event in years.
The reason southern CA is so wet, while the Northwest is relatively dry, is because the jet stream has been deflected way to the south. This is illustrated by an upper level forecast map for 4 PM Friday, with the sold lines presenting the heights of the 500 hPa pressure surface (around 18,000 ft). A deep trough of low heights (pressures) is found along the west coast, with the jet stream/strong winds heading into southern CA.
Equally impressive is the latest infrared satellite image (Friday AM). A extremely moist frontal band is headed right into the LA Basin, which will bring both heavy rain and strong winds. There is clearly instability (convection/thunderstorms) embedded in the cloud mass.
The latest radar image from Vandenberg AFB shows extensive moderate rain over the region, with the yellow/orange areas being the most intense.
And now we have the problem--southern CA is ringed by high terrain and as the moist flow climbs the barriers, precipitation intensity is greatly increased (by 5-10 times is not unusual). This air is relatively unstable, so there is also the risk of thunderstorms with locally intense rainfall. The result is the potential for flash flooding and slides, with debris-laden regions that have recently burned of particularly concern. The National Weather Service has a flood warning out for the Santa Barbara area and a flash flood watch extending down the coast. It is not going to be pretty down there.