MIKE MOSS SAYS: Kim, There isn't really a good way to make a specific forecast about when to expect the last freezing night, so we have to fall back on climatological history instead. For the RDU airport, the average last date for a freezing air temperature is around April 10th, with a standard deviation of about +/- 10 days (this means in a given year, we have about a 67% chance the last freezing night will be within 10 days of April 10th, a 95% chance it will occur within +/- 20 days of that time, and a 99% chance it will occur within +/- 30 days. Keep in mind that ground and plant surfaces can become colder at night than the air temperature, so frost may occur somewhat later than the last freeze. Also, very localized terrain variations can cause significant differneces in the last freeze date over a fairly short horizontal distance, which adds even more uncertainty to that average date.
For a little more info on frost and freeze issues and data resources, see this post from last Spring...
Good luck with your garden!
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