Early Frost: How common historically and did it end the growing season for some?

Mark Seeley
Extension Climatologist, University of Minnesota

On three consecutive mornings, August 19-21, record or near record low temperatures were reported around Minnesota. Some resulted in damaging ground frosts, while others resulted in a hard freeze, all but ending the growing season for some crops.

Some of the temperature reports included:

Thursday, August 19 th, new record low temperatures were reported from the following locations:

Friday morning, August 20th brought even more frost to many sections of northern Minnesota. Numerous new record lows were set all around the state. Those locations reporting freezing temperatures or nearby ground frost conditions included the following:

And finally, on Saturday, August 21 st yet more freezing temperatures, ground frosts and new low temperature records were reported from the following:

Yes, we have had frequent intrusions of high latitude arctic air masses this summer thanks to the persistent position and strength of a continental polar vortex. This makes us all nervous about early frost probabilities for a very slow developing crop. Find below a chronology of August frosts reported south to north along the Red River Valley...

• Ada, MN (3 years) 8/31/1895, 8/21/1920, and 8/30/1931
• Crookston, MN (3 years) 8/28/1893, 8/13/1964, and 8/28/1965
• Thief River Falls (3 years) 8/26/1915, 8/30/1930, and 8/27/1982
• Argyle, MN (six years) 8/25/1934, 8/31/1935, 8/13/1964, 8/28/1965, 8/27/1982, and 8/27/1986
• Hallock, MN (8 years) 8/29/1915, 8/21/1920, 8/24/1923, 8/23/1927, 8/28/1934, 8/31/1935, 8/18/1942, and 8/27/1982

So at least for northern counties there is certainly precedent for such temperatures in August, though they are unusual. In southern counties it is exceptionally rare to see freezing temperatures in August, though not entirely unseen in the climate record. Witness Pipestone had frost on August 11, 1902 and again on August 23, 1987.

Though the microclimate effect weighs heavily on the occurrence of frost, notice some common years for all.....the summers (May through August) of 1895, 1915, 1923, 1942, 1965, and 1982 were all in the colder end of the distribution historically, just as the summer of 2004 has been. Although we are expected to average a bit warmer than normal between now and the first week of September, the Climate Prediction Center forecasts a cooler than normal month of September for Minnesota. This translates to a high likelihood for immature crops of relatively high moisture content. The prospect of making up for lost Growing Degree Days (GDD) is dim. The table below summarizes the cumulative GDD for field corn (modified base 50/86 method) over a series of planting dates that is representative of the calendar window when most of the state's nearly 7 million acres of corn were sown this spring. The shortage of GDD is amplified by later planting dates since the growing season has essentially been consistently too cool to close the gap and draw GDD totals closer to normal. Many of the GDD totals remain over 20 percent behind normal.

Table 1. Modified Growing Degree Day Summary (Base 500/86 F) for the 2004 Crop Season, Covering Corn Planting Dates from April 20 to May 10.

The other feature of the September climate outlook worth mentioning is that Minnesota is expected to be wetter than normal. This may impinge on the ability of the crop to dry down before being harvested. So all the climatic indicators suggest that corn and soybean crops will not reach normal maturity, will likely be exposed to further frost or freeze damage, and will be of higher moisture content and require further drying. (Naturally, I hope I am wrong!)