Using climatology to predict seasonal growing degree days

Growing corn in the northern Plains is hampered by various difficulties, often precipitation, but also a lack of heat to spur growth and development. This latter situation has been the case in much of the northern Plains three times during the 2004 – 2009 growing seasons. During episodes where development is delayed, producers become concerned about the number of GDDs which can be accumulated between the current time and the first freeze. Growing degree days are a common temperature measurement, which when accumulated over a season, can predict crop development. While predicting the number of GDDs from a point in the growing season to the first freeze is not possible with sufficient skill, using climatology we can provide an expected number based on whether conditions will be warm to cold during the rest of the season or if the freeze will range from early to late.

The web interface allows the user to choose the current day or any day during the growing season at a point in South Dakota (chosen from National Weather Service Cooperative Observer Stations). The application then calculates the number of potential GDDs between that date and the average freeze date, and the 25th and 75th percentile freeze dates to provide a range of dates for the first freeze. Likewise, the number of potential GDDs between the chosen day and the freeze dates is calculated for all years available in the period of record at the station. The 25th, 50th and 75th percentile of accumulated GDDs are also chosen to represent cool and warm summer. The final result is a 3 x 3 matrix of potential scenarios between summer temperature conditions and freeze dates to allow the user to determine a range of potential and likely accumulations of GDDs for the remainder of the season.