Most of that rain on Thursday will come in the afternoon, so there may be an hour or two of testing in dry conditions that morning before rain arrives. I expect that light rain will start to fall between 10 am and 1 pm, with a steady rain through the rest of the day and evening. The wind will be blowing pretty strong on Thursday as well, with sustained south winds of 15 to 25 mph, gusting to around 35 mph at times.
Rain should start to diminish Thursday night, and most of it will be gone by sunrise. There could be a few light showers that linger past sunrise Friday morning, but I don't expect that it will have much impact on the on-track activity on Friday. The total amount of rainfall expected from Thursday through Friday morning is between 0.25 and 0.75 inches.
Saturday should have the nicest weather for the event, with partly cloudy skies and high temperatures near 60 degrees.
Sunday is looking a little more uncertain at this point. There is some question of timing with another low pressure system that will be crossing the East Pacific. One model (GFS) shows no rain at all in the Willows area, taking the system and its rainfall well to the north on Sunday night. Another model (ECMWF) shows rain moving into the area as early as Sunday morning, which is faster than what it showed a few days ago. Given this uncertainty, I'm going to keep a slight chance of rain for Sunday afternoon, but the rain chances may need to be increased with later forecast updates if the model trends of a faster system continue. Since the race ends at noon, it is possible that the rain will hold off until after the race ends, and all 25 hours of racing may be dry. Hopefully this will become more clear in the next forecast update (which I'll probably post on Thursday afternoon).