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There have been showers near the northern coast of France today, but they have stayed north of Le Mans. On Friday the weather pattern will feature a stationary low pressure system to the north of Ireland, and another moving from the western Mediterranean Sea to eastern France. The second low will bring rain to central and eastern France, but I expect that it will stay far enough away from Le Mans that I can keep the forecast dry. The weather for Friday's parade should be really nice, with mostly sunny skies and temperatures in the lower 70s F or lower 20s C.
On Saturday the low pressure system to the north will bring a cold front and upper level trough across northern France. As it approaches Le Mans, it will be losing its punch and only a few isolated showers are expected in the area. If one of these showers does hit the track, it will likely be too light to have much impact, and they may arrive before the race begins. The most likely time for any showers is around noon, give or take a couple hours. Winds will be stronger on Saturday, blowing from the southwest at 10 to 15 mph or 16 to 24 km/h.
High pressure will build over France on Sunday, providing nice weather for the finish and pleasant afternoon temperatures in the mid 70s F or lower 20s C.
Sunrise at Le Mans is around 6 am, and sunset is around 10 pm.
I'll have a more detailed forecast through the race posted on Saturday before the start.