By Doug Schneider

It's looking more and more likely that there will be some rain for the second half of the 25 Hours of Thunderhill. Remember that trend of a faster system that I mentioned a few days ago? It it getting even faster as the rain is expected to arrive Saturday night.

The graphic above gives a general idea of what conditions you can expect through the race. It will start off partly cloudy, but the clouds will be increasing through the afternoon. By sunset (which is around 4:40 PM), clouds should be covering the sky and temperatures will fall into the 40s. The main weather feature that is bringing rain is a disturbance in the mid and upper levels of the atmosphere. Rain with the system is expected to start between 2 AM and 5 AM, and I'm confident that it will be raining before sunrise (which is around 7:20 AM). The period of heaviest rainfall is expected to be between 5 AM and 9 AM.

Rain should start to decrease late Sunday morning, but there will likely still be some showers around the track through the end of the race, which is at noon. The total amount of rain expected during the race is between a tenth and a quarter inch. 

I'll do my best to have live weather updates through the race on my Twitter account (@Race4caster). You can always check the radar using the link at the top of the website.

By Doug Schneider

Testing this afternoon at Thunderhill is going to be wet, as rain will be spreading across northern California from the late morning through tonight. Expect rain to start at the track between 11 am and 1 pm. It could come down fairly heavy at times this afternoon. It will be breezy, with south winds of 10 to 20 mph, and gusts up to 30 mph possible. The rain will continue through tonight and early Friday morning. Most of the rain should be gone by sunrise, so I expect that Friday's testing will be rain-free, although the track may be wet in the morning. The total amount of rainfall at the track from today through Friday morning will likely be around a half inch.  

Saturday's weather looks great, with plenty of sunshine. It will be chilly in the morning, with lows in the upper 30s, but highs will reach the upper 50s by the mid afternoon.

Sunday continues to have a lot of question marks. There is a good chance of seeing some rain during the day, but the big question is when. The race will end at noon, and it will be close as to whether the rain arrives before the end of the race. I have a 50% chance of rain mentioned through the day, I think that the morning will have lower rain chances than that, but I can't rule it out entirely. While I said in earlier forecasts that all 25 hours would likely stay dry, I'm not as confident about that happening today. There is still no clear consensus among the models about the rain, so stay tuned for updates as we get closer to Sunday.

Our radar link at the top of the page is set to cover Thunderhill through the weekend.

I plan on having another update Saturday morning, unless there is a major change before then.

By Doug Schneider

The forecast that I posted on Sunday hasn't changed very much, and it still looks likely that there will be some rain for the Thursday testing sessions.

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).