There will be some dry air over Florida on Thursday, which will keep the chance of showers and thunderstorms low. Afternoon showers are pretty common this time of year at Sebring, so just a 20% chance is lower than normal. Still, if one of these showers moves over the track, it could produce brief heavy downpours. Sunshine in the morning hours will raise temperatures well into the 90s, with very humid conditions. The heat index could rise to the 100-105 range in the afternoon.
An upper level tropical low pressure trough will be moving from east to west across the Atlantic and across Florida on Friday and Saturday. This system will bring deep moisture and instability to the area, making afternoon showers and thunderstorms likely both days. The main impact from any showers ans storms will be lightning and heavy downpours that could produce a very high amount of rain in a short period. With the high probability of rain and the potential for it to be heavy, I have the expected impact as Moderate, as ponding of water on the track could result in some delays. The increased cloud cover will help hold temperatures back to around 90 degrees.but that won't be much consolation with the high humidity.