A) The conditions are good enough for a formation lap and a normal start
B) It's too wet and they will start behind the safety car, dry up the track and then do a normal standing start.
B will no doubt ensue chaos.
It's important to note an important rule change for this year - all the laps made behind safety car before the standing start DO NOT COUNT [and the 2hr time limit is activated only when the start is made]. After the successful race start, all following laps under safety car are counted.