Calculating the Odds of a Hard Freeze
With all of the late-planted crops across the Midwest this year and the calendar turning to August, it is time to calculate the odds of a hard freeze (equal to or less than 28°F) damaging the corn crop. (I’ll update this in early September to incorporate August temperatures.)
For this analysis, GDD accumulations were calculated from four emergence dates (April 29, May 13, May 27, and June 10) through July 31. Data from the National Centers for Environmental Information (NCEI) was used to determine first fall hard freeze (less than or equal to 28°F) probability dates to calculate the normal GDD accumulations from August 1 through the mean hard freeze date.
Table 1 describes the freeze probability based on the mean freeze date for the current normals period (1981-2010) and gives the percentage of time the event would occur before reaching the targeted GDD accumulation value. This was done by simply adding each of the individual year’s GDD accumulations from August 1 to the mean hard freeze date and applying them to this year’s GDD accumulations from the four defined emergence dates.
Table 2 uses the same methodology with emergence and GDD accumulations through the end of July, but adds on the average GDD accumulations based on the 1981-2010 normals period from August 1 to the mean hard freeze date. These accumulations were derived from the Useful to Useable GDD calculator tool. This table presents the number of GDD units expected to be in excess (positive value) or in deficit (negative value) by that location’s mean freeze date.
I am presenting GDD surplus/deficit estimates in Table 2 to account for the fact that June emerging corn will grow faster in the beginning leaf stages when compared to early May emerged corn. This is due to the fact that daily highs and lows are 6-8 degrees (Fahrenheit) warmer in June than in May. It is entirely possible that the risk for hard freeze damage is above 50%, but GDD accumulations would indicate that the crop is within 100 units of the target GDD and yields may not be impacted like they would if the crop were several hundred units shy of black layer.
Table 3 shows the odds of a hard freeze occurring by a specific date for 134 sites in Nebraska. (In instances where the weather station is more than 5 miles from the town listed, that's indicated.)
It is readily apparent from Table 1 that corn that emerged during late May through mid-June carries the most significant freeze risk, especially for producers that stayed with their same spring planting varieties and didn’t adjust to an earlier season variety. The forecast going forward will determine whether these odds shrink or get larger. During August, for every degree the monthly average temperature deviates from normal, total monthly GDD accumulations will vary by approximately 30 units.
Of additional concern is the fact that the Climate Prediction Center updated their August forecast at the end of July and it shows a tendency for August temperatures to average below normal for Nebraska. If this occurs, the freeze risk presented in Table 1 will increase. Temperatures in August will likely need to average at least 2 degrees above or below normal to see the probabilities in Table 1 change more than 5%.
Of particular importance to our freeze prospects going forward will be the release of the preliminary Climate Prediction Center September forecast next Thursday (August 15), along with the 90-day forecast (September-November). If the 90-day outlook also shows a below normal temperature forecast, the markets may begin to consider the implication of an early freeze and how production might be impacted.