by Robert Stonefield
Figure 13. NWS Frost-Freeze program
Temperatures at or below the freezing point of water can seriously affect outdoor operations (e.g. gardening or construction). The probability of freezing temperatures occurring varies by location and time of year. Therefore, the National Weather Service starts their frost/freeze program (Fig. 13) based on the average (mean) date of the last spring freeze (Fig. 14), using a minimum shelter temperature of 32F for a group of counties with the similar geographical features (e.g. mountains, piedmont, etc.). This program ends on the first freeze of the autumn (Fig. 15), or the date when the first autumn freeze has almost always occurred, (90% chance based on 30 years worth of data); whichever comes first.
Figure 14. Average date of the last spring freeze across Blacksburg County Warning Area.
Figure 15. Average date of the first autumn freeze across Blacksburg County Warning Area.
When a frost/freeze event is expected 24 to 48 hours in the future during the frost/freeze season, a WATCH will be issued, highlighting the potential for such an event. Whenever the minimum shelter temperature is forecast to be 32F or less in the next 12 to 36 hours during the frost/freeze season, a FREEZE WARNING will be issued. Whenever the minimum shelter temperature is forecast to be 33-36F in the next 12 to 36 hours during the frost/freeze season, on nights with light wind and good radiational cooling, a FROST ADVISORY will be issued.
General thresholds and terminology are shown in the table below:
Figure 16. NWS Frost-Freeze terminology
During anomalous climate patterns (e.g. La Nina, El Nino/Southern Oscillation), vegetation may begin growing several weeks ahead of normal in the spring while the first freeze in the autumn may occur very late in the year, or not at all. Under these circumstances, NWS local offices have the discretion to begin the freeze/frost season early, or terminate it two weeks after the 90 percentile date in the autumn.
To view previous climate articles, visit our climate web site.