Early this week, a cold front will pass through the state of Florida, and a large high pressure system will build in from the north through the rest of the week. This will result in dry conditions (no rain and low humidity), lots of sunshine, and temperatures cooler than normal. Normal temperatures for Sebring in mid-March are a low in the mid 50s and a high around 80. Temperatures for the start of the event will be around 10 degrees below normal. As the center of the high pressure moves off the east coast of Florida by race day, temperatures will get warmer, closer to normal, and there may be a slight increase in clouds and humidity. But likely not enough for any chance of rain.
There is good model agreement on this pattern, so I'm pretty confident that this forecast will hold up through the week, and probably won't have significant changes as we get closer to the race. I plan on having the next update posted on Tuesday.