Petit Le Mans weekend at Road Atlanta will open with nice weather on Thursday as high pressure sits over the southeast U.S. – partly cloudy and high temperatures in the upper 70s. However, a cold front will be approaching from the west, and as high pressure gives way, a chance of showers and thunderstorms will arrive on Friday. At this point, most of the activity is expected in the afternoon. The chance of showers and thunderstorms appears highest Friday night. I don’t think there will be any severe weather, but some gusty winds and heavy rainfall may be possible if a stronger storm passes over the track. Timing is still uncertain this far out, and could change in later forecast updates, but right now it looks like the chance of rain will be decreasing through Saturday morning.
What exactly do meteorologists mean when they talk about "a chance of rain" as a percentage? The above graphic from the National Weather Service San Angelo office explains it pretty well. Basically, the chance of rain, or "probability of precipitation", is a combination of two factors - forecaster confidence that precipitation will occur, and the area that the precipitation is expected to cover.
For our Racecasts, we give a probability of precipitation (PoP) that covers a 12 hour period (from 7 am to 7 pm) at a 5 mile radius around the race track. Essentially, it is a way for us to express how confident we are that rain will occur in that circle.
We also use different wording in our forecast to express the chance of rain. Here are the phrases that we use that correspond to different ranges of PoP:
15-24% = slight chance of rain
25-54% = chance of rain
55-74% = rain likely
75-100% = definite rain