Fall frost and September climate outlook for 2019

Laura Edwards
SDSU Extension State Climatologist

This year’s struggles with weather and climate are continuing this fall. Late planting of corn and soybeans in the spring have now combined with near average or cooler than average summertime temperatures. This combination has led to slow crop growth and the need for an extended frost-free season to ensure these crops reach maturity.

Average frost dates

Average 32° F frost dates for South Dakota fall in late September to early October. This is a concern as much of the soybean fields are still green and have not begun to turn colors or drop leaves. Fields of corn are much behind typical crop stage for this time of year. A “spot check” of locations in eastern South Dakota, assuming May 15 planting date, indicate areas that are 100 growing degree days (GDDs) behind usual, to over 200 GDDs behind average. We are currently a bit behind accumulated GDDs as compared to 2014 and 1997, but not as far behind as 2009 at this same time.

Average hard frost at 28° F occurs a little later in Eastern South Dakota, typically in the first or second week of October. A hard frost at the current crop stages would mean substantial crop loss, essentially prematurely killing the plants before they are fully mature.

Fall outlook

Fortunately, the climate outlook for the next two to four weeks is encouraging for an average, or even later, frost date. Starting next week, a warmer pattern is expected to come over the state. This could delay our fall frost for a while longer.

The other challenge this fall season will be moisture. This week, plentiful rainfall will fall across the state. The outlook for the remainder of the fall season is leaning towards wetter than average conditions overall. There will be dry periods at times, however, as current long-term outlooks show to be possible at the end of September and early October.

Regardless, it is evident that even with a possible frost occurring later than average, dry down of grain in the field could be an issue due to rainfall and/or high humidity. Field access may likely also be a challenge due to the high soil moisture across the landscape, even before the additional rainfall expected this week.

This year, more than many years, each day and week will be important to bring our corn and soybean crops to maturity, and also in planting our winter wheat crop.

Figure 1. Median date of first fall frost temperature of 32° F.
Figure 2. Median date of first fall frost temperature of 28° F.