Is a race that is started dry red-flagged if rain starts falling?
Does it matter if it is a Supersport race or not? (i.e. in a Superbike/GP/GT race riders are possibly on slicks)
There are variables
Is it the first rain of the day? was the option of rain tires already available? The track conditions itself; is it known for standing water? slick spots? lightning? diminshed visibilty?
Normally we try to address what the rain contingency is, if we expect rain, at the riders meeting.