Friday at Daytona will be a hot and humid day. Most of the day will be rain-free with scattered clouds, then as daytime heating increases in the afternoon, scattered showers and thunderstorms will pop up in the area. Since the wind will be from the southwest, the sea breeze won't push inland enough to help cool temperatures, and highs will be in the lower to mid 90s. The practice session scheduled for 6:15 to 7:15 pm may have to deal with showers and thunderstorms in the area. I'd put the chance of rain during this time frame at 40%.
The approaching front is expected to stall somewhere across northern Florida on Saturday, The exact position is uncertain this far out, but right now it looks like it will remain north of Daytona. This will keep hot and muggy conditions at the Speedway for race day, as drier air remains north of the front. The front will also provide lift that will enhance the chance of showers and thunderstorms. Timing is too uncertain to be specific at this time, and showers and thunderstorms will be possible at any time during the day. I put the impact at Moderate for now, but it is possible that could be raised to High with later forecasts, depending on how much rain is expected to fall. That will come into better focus through the week, so stay tuned for updates.