The basic problem is this. Although Dolores is weakening, she is moving northwestward into an area of weak flow that could take her out to sea or have her circulation remnants move close to the SoCal coast with her moisture remnants moving into SoCal. The latest (0600 UTC) GFS brings her moisture remnants across SoCal late Sunday and Monday and eventually into northern Utah on Tuesday. Preceding Dolores is a surge of moisture into the southwest that would bring an increasing potential for showers and thunderstorms to SoCal as early as tomorrow (gasp!).
Here are a two closeups of the GFS precipitable water and 6-h accumulated precipitation forecast for for 1800 UTC (1200 MDT) tomorrow (Saturday) and 1200 UTC (0600 MDT) Monday. The former shows the early stages of the moisture surge preceding Dolores into SoCal and Arizona, the latter the moisture remnants of Dolores moving into SoCal.
All that is fine and dandy, but a look at the 0000 UTC initialized Global Ensemble Forecast System forecasts valid for 0000 UTC Tuesday (1800 MDT Monday) show that most keep the remnants of Dolores to the west, with only one bringing a strong moisture remnant to SoCal.
|Source: Penn State E-Wall|