MIKE MOSS SAYS: Rudi, It's worth noting that readily available records for the occurrence of frost are not maintained, and instead the records we have on hand are for the occurrence of freezing air temperatures measured at about 4-5 feet above the ground. Based on those records, we find that the temperature at Raleigh-Durham airport has fallen to 32 degrees as late as May 10th, in 1977. This date can vary sharply across a short distance though, depending on local terrain, urban versus rural locations, vegetation and other influences. For example, a station just southwest of Raleigh reports a last freeze date of April 28th, while the latest freeze on the campus of NCSU was April 23rd. A site in Durham lists May 11th as the latest freeze date. In all these cases, it's likely that frost at ground level or on exposed surfaces has occurred on rare occasion as much as a week or so later than the last freeze date.
You can check this data for other stations around the southeast, plus equivalent "first freeze" information for the Fall, by going to this page at the State Climate Office web site:
http://www.sercc.com/climateinfo/historical/historical.html
Just click the state and then the station you'd like to investigate, and look for "Spring Freeze" and "Fall Freeze" probability links along the left side of the page. When you see the graph with probability listings, click the "tabular output" link, and you will see a table that includes the dates at which selected temperatures have a given percentage chance of occuring on or after that date, along with the earliest and latest recorded occurrence of each temperature threshold. For example, RDU has only a 20% chance of falling to 32 degrees or below after we reach Apr 18th, and a ten percent chance after April 24th.
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