MIKE MOSS SAYS: Charles, Data from a couple of stations in the Sanford area is the closest available to your location. Based on those records, the latest Spring occurence of freezing air temperatures has been about May 11th, while the earliest date with no further freezing temperatures was March 14th. Note that tracking the occurence of frost per se is a little more difficult, since frost may develop at ground level or on plant surfaces in the presence of a surface-based temperature inversion, while the measured air temperature (taken at a height of around 4-5 feet) remains above freezing. Air temperatures in the Sanford area have been observed to fall below 36 degrees as late as June 4th, which could support some pockets of frost.
It's worth mentioning that with the shallow layers of cold air associated with late season frost or freeze episodes, microclimate effects and very localized terrain variations can have important implications, so your particular location may or may not fall in line with results around Sanford. To look up the data for other stations around NC, you can go to http://www.sercc.com/climateinfo/historical/historical_nc.html, select the station of interest, and then find the "Spring Freeze Probabilities" link along the left hand side. This will give you a cumulative probability chart for falling below a given temperature on a given date. There is also a "tabular data" link below the chart that will show the same info in a table that makes it easy to check the average (50% probability), earliest and latest "last occurences" of specific temperature thresholds.
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