An inversion... 850mb would be very high (5000ft)...with inversions the air can be warmer not that much above our heads..that is why they use helicopters to pull that warmer air down...Have no idea what under ideal conditions the temp could drop to.
In this setup i wouldn't worry too much...do you recall last May 15th what your low was with that setup???
Obviously local conditions will have a huge impact on that (frost pocket/vs hill top)... Plus soil temps, moisture, dewpoint// any bit of wind/cloudiness..
This is why at least the main weather sites (NWS) tend to refer to climo for that date. The models around here always kick out much lower numbers then what usually happens and the NWS always "corrects" them to match what usually happens with these sort of atmospheric setups.