Numerical weather models are indicating the potential for frost (29-32°F) and/or hard freeze conditions (below 28°F) to develop across the northern Corn Belt Sept. 14-16. The models have been consistent with this cold air push for the past three days which gives me a high degree of confidence that the event will occur.
Figures 1 and 2 are forecasted thickness levels for the surface to the 500 mb (jet stream) level for the mornings of Sept. 15 and Sept. 16. The 540 contour interval is usually associated with 32°F temperature at 4 feet above the surface, while the 534 contour typically indicates temperatures of 26°F.
These models do not account for heat stored in the soil surface or within crop canopies. At this time of year it is necessary to adjust for stored heat, so the 540 contour usually points to lows around 35°F and the 534 contour signals lows around 29°F.
For the morning of Sept. 15, Figure 1 indicates hard freeze conditions for northeastern Minnesota, the northern half of Wisconsin, and the northern peninsula and northern half of the southern peninsula of Michigan. Scattered frost to freeze conditions are currently projected for extreme eastern North Dakota southeastward through southern Michigan.
For the morning of Sept. 16, Figure 2 indicates scattered frost to freeze conditions for most of Michigan, northern Ohio, northern New York, and northern Pennsylvania. Hard freeze conditions are projected for southeastern Ontario — most of the agricultural grain crops are grown in this Canadian province.
Because of the spring planting delays due to excessive moisture and below normal May temperatures, a considerable amount of acreage in the frost/freeze forecast area has not reached physiological maturity. Many of these areas need to see normal temperatures into early October to escape significant damage.
Even if the freeze doesn’t occur, crop maturity will grind to a halt as daily highs from Sept. 14 to Sept. 22 will struggle to reach the lower 60s.
Source: Al Dutcher, Extension State Climatologist