Base 50ºF growing degree accumulations during the ten warmest Marches on record at Benton Harbor, Mich., plus the current month are shown in Figure 2. A projection of accumulations through the end of March 2012 data was obtained with temperature forecasts from latest short- and medium-range National Weather Service forecast guidance, including the assumption that mean temperatures during the last week of the month averaged 10ºF above normal. With these assumptions, the GDD total by the end of the month exceeds 200 units as well as the old historical record of 146 set in 1945 and the normal value of 45.
The abnormal warmth already has or will shortly bring most perennial crops and vegetation out of their protective dormant states. Given that this is taking place at least two full weeks or more ahead of normal, and that the relative susceptibility of plant tissue to frost and cold damage generally increases with increasing growth stage, there is an overall elevated risk of cold injury this spring season.
Figure 2. Observed (solid colored lines), normal (solid black line) and projected (dashed line) base 50°F growing degree accumulations for 10 warmest Marches on record at Benton Harbor, Mich. Projections for the 2012 data include latest short- and medium-range National Weather Service forecast guidance through March 31. Growing degree days are calculated with the Baskerville-Emin methodology.
What are the chances of a hard frost?
Relative risks of spring freezing temperatures across Michigan are given in Figure 3. Even though current long lead outlooks for April and the spring season call for more of the same warm pattern across Michigan, further cold weather at some point in the next several weeks is virtually a certainty (see Figure 3).
The climatological average number of daily spring freeze events (32ºF or less) after March in a given season ranges from about eight in the southwest and southeast corners of the state to more than 20 in interior northern sections of the state.
In terms of hard freezes, the average number of low temperature events of 28ºF or less for the same period ranges from four to more than 15. Very cold temperatures are still possible during the month of April, with lows below 10ºF in the south to the single digits below zero in the north.
Figure 3. Numbers of daily April-June freeze events (32ºF or lower) at selected locations in Michigan, 1941-2011. The top number (red) is the average number of individual freeze events during the three-month period, the second- number (blue) is the range of total freeze events in a given season, the third number (green) is the average minimum temperature (ºF) for all freeze events, and the fourth number (orange) is the range of minimum temperatures (ºF) for all freeze events during the period of record.