After an initial round of rain that soaked parts of western Washington on Tuesday, the bulk of the heavy rain event will arrive on the West Coast on Wednesday.
Wednesday night and Thursday, the Pineapple Express will sag slowly south through northern and central California as the southward dip in the vigorous Pacific jet stream swings into the Golden State.
It's at that time that the heaviest rain and, therefore, highest threat of flash flooding will be in play in those areas.
Late Thursday night and Friday, the weakening atmospheric river and corresponding southward dip in the jet stream will swing over Southern California, providing a quick burst of rain for the Southland in time for Friday morning's rush hour.
Flash flooding, mud/rockslides and debris flows are most likely in these areas, particularly given saturated ground from last week's soaking. Much of the rain will also fall during a short period of time as the cold front pushes through and there is also the chance for thunderstorms, which bring additional flash flood concerns.
In fact, the National Weather Service in Monterey, California mentioned the last time the city of San Francisco picked up a four-inch-plus rainfall total in two days or less was on Feb. 3, 1998, during a strong El Nino, no less. (There is no El Nino, as yet.)
Over an inch of rain is possible in parts of Southern California late Thursday into Friday, particularly in the L.A. Basin. While these totals are not nearly as impressive, they may be sufficient to produce additional debris flows in burn areas, and may prompt more evacuations.
Blizzard warnings have been hoisted for parts of the Sierra above 6,000 feet as the combination of wind and snow may combine to create blizzard conditions. Sustained winds of 25 to 50 mph are expected in the warning area, with gusts over 80 mph possible. Some 1 to 3 feet of snow (and locally more) may fall.
Outside of the blizzard warning area in the Sierra, winter storm warnings have been issued above 5,500 feet, where 1 to 3 feet of snow may pile up from Wednesday night through Friday.
These snow levels will be at least 1,000 feet lower than last week's storm, so the impact to drivers could be much greater. Travel over the Sierra will become hazardous and some passes may close for a time Thursday, as heavy snow whipped by the strong wind gusts may produce blizzard conditions at times.